The El Salvador earthquakes of January and February 2001...
Transcript of The El Salvador earthquakes of January and February 2001...
UNCORRECTED PROOF
The El Salvador earthquakes of January and February 2001:
context, characteristics and implications for seismic risk
J.J. Bommera,*, M.B. Benitob, M. Ciudad-Realc, A. Lemoined, M.A. Lopez-Menjıvare,R. Madariagad, J. Mankelowf, P. Mendez de Hasbung, W. Murphyh,
M. Nieto-Lovoe, C.E. Rodrıguez-Pinedai, H. Rosaj
aDepartment of Civil Engineering, Imperial College, London SW7 2BU, UKbUniversidad Politecnica de Madrid, Spain
cKinemetrics, 222 Vista Avenue, Pasadena, CA 91107, USAdEcole Normale Superieure, Paris, France
eEscuela de Ingenierıa Civil, Universidad de El Salvador, San Salvador, El SalvadorfBritish Geological Survey, Keyworth, UK
gDpto. Mecanica Estructural, Universidad Centroamericana “Jose Simeon Canas”, San Salvador, El SalvadorhSchool of Earth Sciences, University of Leeds, Leeds LS2 9JT, UK
iFacultad de Ingenierıa, Universidad Nacional de Colombia, Santafe de Bogota, Colombia.jFundacion PRISMA, San Salvador, El Salvador
Abstract
The small Central American republic of El Salvador has experienced, on average, one destructive earthquake per decade during the last
hundred years. The latest events occurred on 13 January and 13 February 2001, with magnitudes Mw 7.7 and 6.6, respectively. The two
events, which were of different tectonic origin, follow the patterns of the seismicity of the region although neither event has a known
precedent in the earthquake catalogue in terms of size and location. The earthquakes caused damage to thousands of traditionally built houses
and triggered hundreds of landslides, which were the main causes of fatalities. The earthquakes have clearly demonstrated trends of
increasing seismic risk in El Salvador due to rapid population expansion in areas of high shaking and landslide hazard, exacerbated by
deforestation and uncontrolled urbanisation. The institutional mechanisms required for the control of land use and building practice are very
weak and present a major obstacle to risk mitigation. q 2002 Elsevier Science Ltd. All rights reserved.
Keywords: El Salvador; Earthquakes; Strong motion; Landslides; Seismic risk; Vulnerability
1. Introduction
The earthquake of 13 January 2001 that struck El
Salvador was the first major seismic disaster of the third
millennium and the fifth destructive earthquake to affect the
small Central American republic in 50 years. The earth-
quake was followed exactly 1 month later by a second event,
of different tectonic origin, on 13 February, which
compounded the destruction. These two earthquakes
claimed almost 1200 lives. In addition, 20% of houses
were damaged, with 12% either completely destroyed or
declared uninhabitable. Economic losses were estimated by
the UN Economic Commission for Latin America (ECLA/-
CEPAL) at US$ 1.6 billion, which is equivalent to 12% of
the GDP of the previous year [1]. Estimates by the IMF and
the World Bank give a higher figure of US$ 1.9 billion [2].
This paper presents the context in which these earth-
quakes occurred, including their precedent in the seismic
history of El Salvador, and describes the characteristics of
the events and their impact on the built and natural
environments, and on the population. The primary objec-
tives of the paper are firstly to provide an overview of the
characteristics and effects of the earthquakes, and secondly
to assess the relative importance of the different factors,
physical and social, which have been demonstrated as
contributing to the high level of seismic risk in El Salvador.
2. Geophysical, geological and geographical contexts
With an area of just over 20,000 km2 El Salvador is the
smallest of the Central America republics, located on the
0267-7261/02/$ - see front matter q 2002 Elsevier Science Ltd. All rights reserved.
PII: S0 26 7 -7 26 1 (0 2) 00 0 24 -6
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* Corresponding author. Tel.: þ44-171-594-5984; fax: þ44-171-225-
2716.
E-mail address: [email protected] (J.J. Bommer).
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Pacific coast of the isthmus and bordered by Guatemala to
the west, and Honduras to the north and east (Fig. 1).
2.1. Tectonics, seismicity and seismic hazard
El Salvador is affected by earthquakes from two main
sources of seismicity. The largest earthquakes are generated
in the Benioff–Wadati zones of the subducted Cocos plate,
which is converging with the Caribbean plate in the Middle
America Trench (Fig. 2) at an estimated rate of 7 cm/year
[3]. The largest earthquake in this zone during the 20th
century, in the vicinity of El Salvador, occurred on 7
September 1915, with a reported magnitude of Ms 7.8 and a
focal depth between 45 and 60 km [4]. This earthquake
caused widespread destruction in western El Salvador,
affecting particularly the town of Juayua [5]. Large
subduction earthquakes on 28 March 1921 (Ms 7.4) and
21 May 1932 (Ms 7.1) caused minor and moderate damage
in eastern and central El Salvador, respectively; the
relatively small impact of these earthquakes was probably
the result of their focal depths of 170 and 150 km,
respectively [4]. An earthquake on 19 June 1982, offshore
from western El Salvador, did cause widespread damage in
the southwest of the country, mainly in adobe (sun-dried
clay brick) and bahareque (wattle-and-daub) houses, and
Fig. 1. El Salvador.
Fig. 2. Focal mechanisms determined for the earthquakes of 13 January and
13 February 2001. Stars are epicentral locations from NEIC. Red line is the
Middle America Trench and yellow triangles are volcanoes. Locations of
previous earthquakes are indicated by black stars.
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triggered many landslides [6,7]. This earthquake shared
many similarities with the earthquake of 13 January in terms
of mechanism and focal depth, although somewhat smaller
with a magnitude of Mw 7.3. The damage patterns were
similar to those of the January 2001 earthquake, but much
less severe with a total of just eight fatalities. The worst
destruction occurred in the town of Comasagua, which was
also very severely affected by the January 2001 earthquake.
The second source of seismicity affecting El Salvador is a
zone of upper-crustal earthquakes that coincide with the
Quaternary volcanoes that extend across the country from
west to east, forming part of a chain extending throughout
the isthmus from Guatemala to Panama. Due to their
shallow foci and their coincidence with main population
centres, these earthquakes (Fig. 2) have been responsible for
far more destruction in El Salvador, as in neighbouring
Nicaragua, than larger earthquakes in the subduction zone
[8]. During the 20th century, such shallow focus earth-
quakes caused destruction on at least seven occasions,
sometimes occurring in clusters of two or three similar
events separated by periods of minutes or hours. On 8 June
1917 an earthquake occurred west of the capital, San
Salvador, assigned a magnitude Ms 6.7 by Ambraseys and
Adams [4] and Ms 6.5 by White and Harlow [8], causing
destruction in Armenia, Ateos, Quetzaltepeque and other
towns. The earthquake was followed by an eruption of the
San Salvador volcano, which resulted in lava flows to the
north. White and Harlow [8] report a second event of Ms 6.4,
on the eastern side of San Salvador, less than an hour later,
but this is contested by Ambraseys and Adams [4]. On 28
April 1919 San Salvador was again damaged, this time by a
shallow earthquake of Ms 5.9. On 20 December 1936, an
earthquake of Ms 6.1 caused very heavy damage to the town
of San Vicente, 40 km east of San Salvador, with more than
100 deaths [9]. The 1936 earthquake is of particular interest
since the location was similar to that of the earthquake of 13
February 2001.
On 25 December 1937 an earthquake of Ms 5.8 near the
Salvadorian border with Guatemala caused damage and a
few deaths in the towns of Ahuachapan and Atiquizaya. A
series of three destructive earthquakes occurred in eastern El
Salvador on 6–7 May 1951, with magnitudes Ms 5.9, 6.0
and 5.5, destroying the towns of Jucuapa and Chinameca,
leaving about 400 dead [10].
The capital city of San Salvador, amongst the Latin
American city most frequently damaged by earthquakes,
was badly hit on 3 May 1965 (Ms 5.9) and on 10 October
1986 (Ms 5.4, Mw 5.7). The 1965 earthquake left about 120
dead [11] whereas the 1986, despite being of smaller
magnitude, resulted in 1500 deaths and more than 100,000
homeless [12–14]. Many engineered structures that col-
lapsed in 1986 had been damaged by the 1965 earthquake
and possibly further weakened by the 1982 subduction
event.
The shallow focus, moderate magnitude earthquakes that
occur along the volcanic chain are generally tectonic rather
than volcanic in origin, and are probably the result of a
right-lateral shear zone caused by an oblique component of
the Cocos–Caribbean collision [15]. However, swarms,
which may have volcanic origin, are also relatively
frequent. In March and April 1999 an important seismic
swarm occurred in an area close to the San Vicente
(Chichontepec) volcano, with almost 1000 small earth-
quakes registered, none exceeding M 4.5, registered, and as
many as 160 occurring per day. A similar swarm had
affected approximately the same area in July 1975. The
1999 swarm, despite the size of the individual events,
caused minor to moderate damage to a number of adobe
houses and also the church in Apastepeque. The same area
was also affected by the earthquakes of January and
February 2001; it is very likely that the level of damage
was exacerbated by the damage inflicted during the 1999
swarm.
Major earthquakes also occur on the Motagua and
Chixoy-Polochic faults that traverse Guatemala and mark
the boundary between the Caribbean and North American
plates, but they are sufficiently distant to not produce
damaging motions in El Salvador. The Ms 7.5 Guatemala
earthquake of 4 February 1976 caused shaking that did not
exceed MM intensity of V within El Salvador [16].
White [15] also describes a fourth source of seismicity as
a zone of tensional tectonics near the common borders of El
Salvador, Guatemala and Honduras, bounded by the
Motagua fault to the north, the volcanic chain to the south
and the Honduran Depression to the east. White [15] asserts
that an earthquake of Ms 7.5 occurred in this zone in June
1765. The largest earthquake during the 20th century in this
zone was that of 29 December 1915 (Ms 6.4), for which
Ambraseys and Adams [4] relate press reports alleging two
deaths in San Salvador due the collapse of walls, although
the effects in El Salvador were clearly not overly important.
There have been a number of probabilistic seismic
hazard assessments carried out for El Salvador [17–19] and
for Central America [20,21]. The hazard maps produced for
a 475-year return period, despite being based on generally
similar seismological and strong-motion data, differ signifi-
cantly in terms of the geographical distribution of the hazard
and by more than a factor of three in terms of the maximum
ground accelerations [22]. Following the San Salvador
earthquake of May 1965, Rosenblueth and Prince [23]
proposed two separate seismic zonations for El Salvador,
one for suduction earthquakes and one for upper-crustal
seismicity. Although the application of this proposal has
been explored [24,25], it has not been incorporated into
seismic design codes.
2.2. Geology, geomorphology and landslide hazard
El Salvador is made up of four morphological-geological
units, each of which forms an east–west strip across the
country parallel to the coast [26]. The northernmost unit,
along the border with Honduras, is a mountain range
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consisting mainly of plutonic rocks from the Tertiary. To the
south of these mountains is the Great Interior Valley that
forms the central area of the country; the southern part of the
valley includes the Salvadorian segment of the chain of
Quaternary volcanoes, six of which are active. To the south
of the valley are three coastal mountain ranges: Tacuba on
the western border with Guatemala; the Cordillera del
Balsamo to the south and west of the capital; and the
Jucuaran range bordering the Gulf of Fonseca to the east.
Between the coastal ranges are two coastal plains, the larger
one, in the centre and east of the country, including the
estuary of the Rıo Lempa, El Salvador’s main river.
The geology of El Salvador is entirely volcanic. The
youngest and most commonly encountered volcanic soil is
the tierra blanca ash, which originates from eruptions in the
Coatepeque and Ilopango calderas. The tierra blanca occurs
as silty sand or sandy silt, and is generally relatively well-
consolidated and stable only where it is thick [27]. The
strength of the tierra blanca, which permits it to stand in
near-vertical slopes of up to 15 m or more, derives from a
combination of weak cementation, probably due to silica
gels, and negative pore water pressure, resulting from partial
saturation, which have been measured in the range 400–
500 kPa [28].
Earthquake-induced landslides are common in Central
America although different mechanisms of slope failure
dominate in the northern countries of Guatemala and El
Salvador compared to southern countries of Costa Rica and
Panama [29]. In the south, the most abundant and most
damaging slope failures are translational soils in residual
soils, whereas in the north soil and rock slides on volcanic
slopes, and more commonly, soil falls and slides in steep
slopes of pumitic ash, dominate. Although these volcanic
ash deposits are able to form almost vertical slopes in
incised ravines (barrancas ) and in road cuts, they are
susceptible to sudden and catastrophic failure under
sustained or intense rainfall and under earthquake shaking.
The numbers of landslides triggered by earthquakes in these
ash deposits tend to be disproportionately high compared to
the numbers of landslides triggered by earthquakes of
similar magnitude in other parts of the world. In the global
database of Keefer [30], the 1976 Guatemalan earthquake
stands out as having caused an order of magnitude more
landslides than any other earthquake of the same size; the
1986 San Salvador stands out in the same way amongst the
cases in the database of Rodrıguez et al. [31], which extends
the Keefer [30] database from 1980 to 1997.
The record of landslides induced by earthquakes in El
Salvador dates back to 1576, when landslides in the Sierra
Los Texacuangos were reported to be triggered by an
earthquake [32]. Since then more than 20 earthquakes have
been found to cause widespread landsliding within the
Salvadorian territory [33]. Areas affected by earthquake-
induced landslides in El Salvador are much higher than
those affected by earthquakes of comparable magnitude that
occur in other geological, geomorphological and climatic
environments [29,31]. Historical evidences show that land-
slides triggered by earthquakes in El Salvador occur as soil
and rock slides on volcanic slopes but more abundantly as
soil falls and slides in slopes of pumitic volcanic ash [29,
33]. Subduction earthquakes generally trigger landslides
over areas that are large compared to crustal earthquakes,
which tend to concentrate landsliding around the epicentral
area. The 13 January and 13 February earthquakes have
confirmed these trends.
Rymer and White [34] reviewed topography, lithology,
rainfall, seismic hazard and historical cases of earthquake-
induced landslides, and concluded that landslide hazard in
El Salvador is high, the susceptible areas being the coastal
mountain ranges, the volcanic chain and the interior valley
areas. This evaluation has been confirmed by observations
during the 2001 earthquakes.
Fig. 3 shows a landslide hazard map prepared by the
Planning Office for the Metropolitan Area of San Salvador
(OPAMSS) as part of PLAMADUR in 1997. The map
identifies most of the metropolitan area of the capital as
being of medium landslide hazard with several areas
highlighted, in dark red, as being of high hazard: amongst
these is the area affected by the catastrophic landslide at Las
Colinas (Section 5.1).
2.3. Demographic and socio-economic conditions
The current population of El Salvador is about 6.3
million, very unevenly distributed throughout the national
territory. There has been a steady trend for the population to
concentrate in the south-western third of the country, which
was home to 53% of the population in 1971, a figure that had
risen to 64% by 1992 [35]. Probably three-quarters of the
population now live in the region west of Lake Ilopango and
south of Santa Ana, which is also the area of greatest seismic
hazard [36].
The main agricultural export of El Salvador is coffee,
having replaced anil (indigo) as the main cash crop at the
turn of the 20th century, following the introduction of
synthetic dyes in Europe. A large section of the rural
population depends directly or indirectly on the cultivation
of coffee for its livelihood, often in precarious conditions.
Even before the earthquakes of 2001, the coffee industry
was in a difficult situation as a result of low prices on the
international market, partly as a result of a bumper crop in
1999–2000, and a delay in the previous year’s harvest due
to particularly wet weather that affected Mexico, Guate-
mala, El Salvador and Honduras [37].
During recent years the relative importance of the coffee
industry in El Salvador has declined, with its contribution to
the GDP dropping from close to 10% in the early 1980s to
around 3% in recent years [38]. The main source of income
to the Salvadorian economy is now the dollars sent back to
relatives by Salvadorians living, often illegally, in the USA.
The migration of Salvadorians to the United States was
accelerated by the civil war that engulfed the country from
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1980 to 1992, but poverty and increasing crime rates have
maintained the exodus since the peace accords were signed
between the Salvadorian government and the FMLN
(Farabundo Martı National Liberation Front). Income
from remittances (known in Spanish as remesas ) reached
US$ 1751 million in 2000, almost six times the total value
of coffee exports (US$ 298 million) and 2.7 times the net
foreign exchange generated by the assembly (maquila )
industry (US$ 654 million).
El Salvador is classified as a lower middle-income
Fig. 3. OPAMSS map of landslide hazard in the Metropolitan Area of San Salvador.
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economy with an average GDP per capita of a little over
$2000, although the distribution of wealth is enormously
uneven. The economic fragility of the small republics of
Central America when subjected to natural disasters is well
established. Coburn and Spence [39] list the economic
losses inflicted by major earthquakes from 1972 to 1990; the
three highest losses, when expressed as a percentage of the
GNP for the year of the earthquake, of 40%, 18 and 31%
were caused by the 1972 Managua, 1976 Guatemala and
1986 San Salvador earthquakes, respectively.
3. Source characteristics of the 2001 earthquakes
The earthquake sequence that began on the 13 January
2001 lasted for a total of over 6 weeks. Within this sequence
were distinct and tectonically separate main shocks, on 13
January and 13 February, whose characteristics are
described in the next sections. A third event that occurred
on 17 February, with an epicentre on the western side of San
Salvador, was sufficiently remote from the two main shocks
not to be considered as an aftershock of either. However,
this was a small event, assigned a magnitude of ML 5.1 by
the Centre for Geotechnical Investigations (CIG) of the
Ministry of Public Works and mb 4.1 by NEIC, and caused
only very minor damage and caused less intense ground
shaking than many of the aftershocks of the 13 January
event; its impact was primarily psychological, owing to the
understandably agitated state of the population after 5 weeks
of tremors.
3.1. The 13 January 2001 earthquake
The first earthquake struck just after 11:30 am (local
time) on Saturday 13 January 2001. Table 1 gives the source
parameters determined by different agencies, which in terms
of size and depth of the earthquake are remarkably
consistent.
We determined fault mechanism, depth, source time
function and seismic moment of earthquakes of 13 January
and 13 February using very broadband digital data. In order
to avoid multi-pathing, upper mantle and core arrivals, we
only inverted body-waveforms from stations in the range
308 , D , 908. We modelled the earthquakes as single
point double-couple sources. The velocity structure near the
source and beneath the stations was approximated by a half
space with standard upper mantle wave velocities. We
modelled the direct waves (P and S) and the reflected phases
from free surface (pP, sP, sS, pS). In order to simulate
seismic attenuation; we assumed tp ¼ 11 s for P waves and
4 s for SH waves. We used a maximum likelihood principle
to obtain the source parameters that provide the best fit
between observed and synthetic waveforms [40,41]. During
the inversion, we solved simultaneously for focal mechan-
ism and source time function using the CMT solutions as a
priori models. We selected a set of teleseismic stations that
gave us the best azimuthal coverage as possible in order to
have a good constraint of the fault plane parameters. We
used displacement seismograms, deconvolving them from
their instrumental response and then reconvolving each
signal to a common instrumental response. Band-passed
filters were applied to the displacement records with a band-
pass Butterworth filter of order 3.
Fig. 4 shows body-waveform inversion results for the
earthquake of 13 January. For this event, P waves were
relatively well fitted by our simple point source model. For
P- and S-waves the first arrivals were not very well
modelled because the S-waves—which are much stron-
ger—dominate the inversion. The strong SH arrivals force
the source time function of the earthquake to start with a
very strong impulse. The peak appears to be much weaker
for P-waves. The depth we found was 50 km. The more
vertical fault plane was well constrained by the azimuthal
coverage. The source time function can be divided into two
sub-events: the first one had higher amplitude and a duration
of 22 s; the second sub-event occurred during 24 seconds.
The seismic moment was 5.54 £ 1020 N m; it is difficult to
observe any directivity effect. However, if we compare
signals from LBNH and KIP stations with their synthetics
(which were modelled with a point source), we observe a
possible directivity effect. If we assume that the fault plane
is the sub-vertical one, as is commonly observed for
intraplate events, there could be an upward rupture
propagation (Fig. 4); more data is required in order to
constrain this directivity. The Mw 7.7 earthquake of 13
January 2001 was an intermediate depth earthquake that
occurred inside the down-going Cocos plate; its tension axis
was sub-parallel to the dip direction of the descending slab.
Large magnitude, normal faulting earthquakes are not
unknown in subduction zones, indeed the El Salvador
earthquake of 19 June 1982 was of very similar rupture
mechanism. The highly destructive Peru earthquake (Ms
7.7) of 31 May 1970 was also associated with a normal
rupture, as was the large M 8.1 Chillan (Chile) earthquake in
1939. In the case of the Peruvian earthquake, the large-scale
extensional fracture in the underthrusting Nazca plate was
interpreted as being due to tensional stresses caused by the
denser descending plate [42]. In the case of the Cocos plate
in Central America, the cause of normal faulting may be
both extensional stresses due to slab pull and flexural
stresses induced as the slab begins to descend at a greater
dip angle inside the mantle [43].
Table 1
Source parameters for 13 January 2001 earthquake
Time
(UTC)
Epicentre Depth
(km)
Magnitudes Agency
N8 W8
17:33:32 13.049 88.660 60 Mw 7.7, Ms 7.8, mb 6.4 NEIC
17:33:46 12.97 89.13 56 Mw 7.7, Ms 7.8, mb 6.4 HRV
17:33:30 12.868 88.767 60 Mw 7.7 CASC
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UNCORRECTED PROOF
In view of the agreement that the focal depth was of
the order of 50–60 km, the earthquake would not have
been expected to generate tsunami, even though there
have been reports of a minor seismic sea wave [44].
Fig. 5 shows a tide gauge record from the port of
Acajutla in which it can be seen that no tsunami
occurred; the fluctuation in sea level at the time of the
earthquake was comparable with ambient noise levels,
and possibly due to the arrival of P-waves at the
surface.
The earthquake was felt from Mexico City in the north to
Colombia in the south. Our field observations from extensive
travel throughout the interior valley and the coastal areas of El
Salvador suggest that MM intensities throughout the southern
half of the country were between VI and VII with local pockets
of higher intensity between VII and VIII.
Fig. 4. Analysis of broadband body waves for the 13 January 2001 earthquake. Top: observed P-wave seismograms (solid lines) are in general fitted well by
synthetics (dashed lines) computed for a point source model with focal mechanism and source time function shown. Bottom: as above but for SH waves.
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UNCORRECTED PROOF
3.2. The 13 February 2001 earthquake
Aftershocks from the 13 January earthquake decayed
approximately according to Omori’s law in the period up to
13 February and were gradually dying out when a second
earthquake occurred. The parameters for Omori’s equation
for the aftershocks prior to the second major event were
obtained as follows:
log½NðtÞ� ¼ 2:7 2 0:7 logðtÞ R2 ¼ 0:9 ð1Þ
The source parameters for the second earthquake are listed
in Table 2. Wave-form modelling was also carried out for
this second event. Fig. 6 shows displacement seismograms
filtered between the same corner frequencies as the 13
January event. Signals were noisier but we managed to
constrain the mechanism using the envelope of the signal.
The depth was 14 km and the seismic moment was
6.05 £ 1018 N m. The total source time function duration
was 12 s. P and SH waves were very well fit even if at some
stations P waves were very noisy (PAS, KDAK). In spite of
these problems, the two fault planes were well determined.
The event of 13 February 2001 was totally different from
that of 13 January: it was a strike–slip event that took place
inside the upper continental plate, in the zone of weakness
of the volcanic axis. The fault plane must be the one sub-
parallel to the volcanic axis, i.e. sub-parallel to the
subduction trench, which is confirmed by the distribution
of aftershocks located by CIG.
The 13 February earthquake, despite its size and
relatively shallow focus, did not produce surface rupture,
although there are mapped faults to the east of Chichonte-
pec volcano whose rupture would be compatible with the
fault plane solution [45]. An important issue in the
interpretation of these earthquakes is the focal depth of
the 13 February earthquake, which appears to be of the
order of 15 km from our well-determined solution. Focal
depth is the most difficult seismic source parameter to
determine reliably and seismograph coverage in Central
America, although improved by recent regional collabor-
ations [46], is still limited, hence reported focal depths
Fig. 5. Tide gauge record from the port of La Libertad showing height above mean sea level (metres) from 12 to 15 January 2001.
Table 2
Source parameters for 13 February 2001 earthquake
Time
(UTC)
Epicentre Depth
(km)
Magnitudes Agency
N8 W8
14:22:06 13.671 88.938 10 Mw 6.5, Ms 6.5, mb 5.5 NEIC
14:22:16 13.98 88.97 15 Mw 6.6, Ms 6.5, mb 5.5 HRV
14:22:07 13.927 88.743 9.5 MC 5.9, ML 5.7 CASC
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UNCORRECTED PROOF
carry a considerable degree of uncertainty. A clear
example of this is the earthquake sequence of Jucuapa–
Chinameca on 6–7 May 1951; contemporary catalogues
list the earthquakes with focal depths between 80 and
100 km, and re-determinations using teleseismic data by
Ambraseys and Adams [4] confirms the intermediate focus
of the events. However, wave-form modelling, the
presence of well-developed surface waves on a seismo-
gram from Guatemala City, and the distribution on damage
and intensity, all point compellingly towards very shallow
focal depths, probably less than 10 km [10]. On the basis of
the very limited evidence available, there does appear to be
some correlation between magnitude and focal depth for
crustal earthquakes in the Central America region, with
events of this size occurring below the upper crust.
Ambrasyes and Adams [4] report that the 20 December
1936 earthquake in the region of San Vicente, one of the
towns most heavily affected by the 13 February event, was
of sub-crustal origin. The empirical relationship of Wells
and Coppersmith [47] for strike–slip faults yields a mean
value of 10.5 km for the rupture width of an earthquake of
this size; if the rupture did not advance more than 5–8 km
from the surface, this may at least partly explain why the
13 February earthquake was less destructive than may have
Fig. 6. Analysis of body waves for the 13 February 2001 earthquake. As for Fig. 4 except that only P-waves are considered.
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UNCORRECTED PROOF
been expected from an event of this size occurring so close
to population centres.
A preliminary isoseismal for the 13 February 2001
earthquake published by CIG reported a maximum MM
intensity of VII–VIII in the area from Lake Ilopango to San
Vicente and VI in San Salvador. Our field observations
suggest that these are overestimates and that the maximum
intensity generally did not exceed VII.
An obvious question to be addressed is whether the 13
February earthquake was in some way triggered by the
subduction event a month earlier. Stress transfer due to
relaxation of one crustal area leading to heightened tectonic
stresses in an adjacent area has been clearly observed, for
example, in the sequence of earthquakes from 1939 to 1999
along the North Anatolian fault in Turkey [48]. However,
the situation in Central America is much less clear because
the two earthquakes are of entirely different tectonic origin,
even though they are both ultimately the result of the same
general tectonic process. Lomnitz and Rodrıguez Elizar-
raras [44] report that normal faulting subduction earth-
quakes in Mexico tend to be followed by either large thrust
events or shallow intraplate events four or five years later. A
similar pattern may possibly exist in El Salvador, whereby
large magnitude subduction earthquakes in some way
trigger crustal events within the Caribbean plate within
similar, or in some cases much smaller, intervals. The large
subduction earthquake of 1915 in western El Salvador was
followed by crustal earthquakes in San Salvador and to the
west in 1917 and 1919; the subduction earthquake of 1932
offshore of central El Salvador was followed by the crustal
earthquake in San Vicente in 1936; the 1982 subduction
earthquake was followed in 1986 by the San Salvador
upper-crustal earthquake. Therefore, the events of January
and February 2001 may be simply a highly accelerated case
of a process that is characteristic of the region. Earlier
studies have alluded to relationships between Quaternary
faulting in the Caribbean plate and the nature of the
subducted Cocos plate [49] but the highly complex system
of stress transfer and the exact nature of the plate
interactions are not sufficiently well known to infer any
definitive model at this stage.
4. Strong ground-motion
Both the 13 January and 13 February 2001 earthquakes
were well recorded by three accelerograph networks in
operation in El Salvador: a network of SMA-1 analogue
instruments operated by the CIG, a network of digital and
analogue instruments operated at geothermal and hydro-
electric plants by GESAL, and the TALULIN network of
digital SSA-2 instruments operated by the Universidad
Centroamericana (UCA) ‘Jose Simeon Canas’ [50]. Records
were also obtained from the network of INETER in
Nicaragua. The records from the CIG network were
digitised and processed by the USGS.
4.1. Characteristics of accelerograms
Tables 3 and 4 list the main characteristics of the
accelerograph recordings of the 13 January and 13 February
earthquakes; the station locations are shown in Fig. 7. A
major difficulty in performing detailed analysis and
interpretation of the recorded accelerograms is the lack of
information about soil profiles at recording sites other than
the CIG stations in San Salvador for which investigations
were carried out as part of a microzonation study following
the 1986 earthquake [51]. Nearly all of the stations that
recorded the two earthquakes are located on pyroclastic
deposits such as tierra blanca and the older tobas color cafe.
Exceptions to this are the Presa 15 de Septiembre hydro-
electric dam site (alluvium), La Libertad (alluvium) and
Panchimaclo (volcanic rocks). It is very likely that the
ground motions at several of the recording sites are also
affected by topographical features: the Panchimalco station
of the UCA network is located within a N–S trending
valley, whereas the San Pedro Nonualco station sits atop an
E–W trending narrow ridge. Recordings of distant subduc-
tion events off the coast of Nicaragua have consistently
produced relatively strong recordings at the latter site,
whereas at Panchimalco recordings have generally been
weak, frequently below the instrument trigger level [50].
Regrettably several potential records were lost due to
malfunction of instruments. The CIG station at Santiago de
Marıa in eastern El Salvador did not trigger during the 13
January event; the instruments in the north-western towns of
Santa Ana and Metapan also failed to produce records,
although it is not clear whether this was due to malfunction
or due to accelerations not reaching the triggering level,
which may have been the case at Metapan at least. It is clear
from comparison of Tables 3 and 4 that the functioning of
the CIG instruments was not consistent. More important
cases of malfunction concerned the San Vicente instrument
of the UCA network, which did not record either of the
earthquakes, and the San Pedro Nonualco station that would
have produced the most important recording of the 13
February earthquake. The station operated by GESAL at the
Berlın geothermal energy plant also failed to yield an
accelerogram of the 13 February earthquake.
4.2. Comparisons of strong-motion parameters with
predictions
For earthquakes of magnitude greater than about 6, for
which the source dimensions are of the order of more than a
few kilometres, the use of epicentral distance can seriously
overestimate the separation of the site and the source of
energy release. For the 13 January earthquake, distances
have been measured from the assumed fault rupture, since
this is the distance measure proposed by Youngs et al. [52]
for subduction zone earthquakes. The actual location of the
fault rupture has been fixed by the angle of dip of the fault,
which coincides with the angle of dip of Cocos plate as
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UNCORRECTED PROOF
proposed by Burbach et al. [43]. Taking account of the focal
depth of the main shock, the seismic moment and the spatial
distribution of aftershocks, the fault plane is modelled as a
fault plane with a strike of 3008 dipping 558 to the NE,
which corresponds to a plane sub-parallel to the subduction
trench. The dimensions of the rupture plane were con-
strained by the distribution of aftershock hypocentres from
13 January until the end of August, concentrated at focal
depths between 20 and 40 km. The dimensions of the
inferred fault rupture plane are 65 km in length and 55 km in
width. The uppermost part of the rupture is assumed to
extend to a depth of 20 km and extends from (12.958N,
89.258W) in the west to (12.678N, 88.748W) in the east. The
distances from this assumed rupture are given in Table 3.
For the crustal earthquake of 13 February, a more
appropriate measure of the source-to-site distance is that
proposed originally by Joyner and Boore [53], namely the
shortest distance from the surface projection of the fault
rupture. The distances from the fault rupture for the 13
February earthquake were determined by modelling the
fault as a line striking N948E, extending from (13.668N,
89.08W) to (13.638N, 88.618W). This places the fault
rupture as extending eastwards from the western shore of
Lake Ilopango for about 42 km; this is longer than would be
expected from the relationships of Wells and Coppersmith
[47], which may indicate a narrow rupture and hence the
effective depth of the source that may explain the relatively
low amplitudes recorded. This rupture plane was con-
strained from aftershock distributions from 13 February
until the end of August with depths up to 15 km. Seismic
activity west of Ilopango has been reported after the 13
February earthquake but it is probably related to the 17
February event near San Salvador. The calculated distances
from this assumed source are presented in Table 4; since it is
possible that the length of the fault rupture has been
overestimated, there is the possibility that some of the
distances are underestimated. The uncertainty, however, lies
mainly in the eastward extension of the fault rupture, which
Table 3
Strong-motion records of 13 January 2001 earthquake
Network Station Location drupa (km) PGA (g) PGV (cm/s)
N8 W8 N–S E–W V N–S E–W V
GESAL Berlın Geoth. 13.50 88.53 54 0.459 0.370 0.235 21.3 24.0 12.3
UCA Armenia 13.744 89.501 93 0.601 0.454 0.223 28.8 29.4 19.6
UCA La Libertad 13.468 89.327 60 1.113 0.575 0.617 53.2 35.5 16.0
UCA Panchimalco 13.614 89.179 75 0.177 0.154 0.089 9.2 9.4 7.3
UCA San Bartolo 13.705 89.106 85 0.157 0.199 0.166 25.2 31.2 15.2
UCA S Pedro Nonualco 13.602 88.927 50 0.580 0.488 0.439 37.5 26.4 18.2
UCA San Salvador ESJb 13.707 89.201 85 0.301 0.278 0.154 25.4 17.4 11.9
UCA Santa Tecla 13.671 89.279 83 0.496 0.243 0.487 57.0 34.2 18.5
UCA Tonacatepeque 13.778 89.114 93 0.234 0.205 0.263 23.1 23.2 9.8
UCA Zacatecoluca 13.517 88.869 47 0.260 0.314 0.253 12.3 21.9 10.4
CIG Ahuachapan 13.925 89.805 123 0.146 0.214 0.124 14.9 16.6 10.8
CIG Acajutla 13.567 89.833 95 0.098 0.108 0.050 14.6 18.6 4.2
CIG Cutuco 13.333 87.817 125 0.078 0.079 0.063 13.8 8.6 4.0
CIG Presa 15 de Sept.c 13.616 88.550 66 0.152 0.187 0.122 23.5 16.0 10.2
CIG San Salvador DBd 13.733 89.150 84 0.225 0.250 0.160 23.2 19.2 11.3
CIG San Salvador REe 13.692 89.250 83 0.304 0.323 0.329 22.9 27.6 15.3
CIG San Miguel 13.475 88.183 107 0.136 0.120 0.089 12.8 12.1 6.0
CIG Sensuntepeque 13.867 88.663 81 0.082 0.061 0.058 8.5 9.1 6.2
INETER Boaco 12.473 85.658 336 0.004 0.003 0.002 0.5 0.5 0.4
INETER Chinandega 12.632 87.133 175 0.090 0.070 0.042 6.3 4.6 2.1
INETER DEC 12.124 86.267 276 0.045 0.044 0.028 3.1 3.3 1.7
INETER Estelı 13.092 86.355 263 0.014 0.011 0.009 2.3 2.5 0.9
INETER Granada 11.937 85.976 312 0.009 0.009 0.006 1.7 1.3 0.9
INETER Jinotega 13.086 85.995 302 0.006 0.005 0.004 0.7 0.9 0.5
INETER Juigalpa 12.107 85.372 371 0.003 0.003 0.002 0.6 0.6 0.5
INETER Leon 12.117 86.266 276 0.040 0.037 0.026 2.3 2.6 1.4
INETER Managua (ESSO)f 12.144 86.320 270 0.057 0.045 0.022 3.8 3.9 1.5
INETER Managua (INET)g 12.149 86.248 277 0.034 0.041 0.014 2.6 2.7 1.1
a Distance from fault rupture as defined by Youngs et al. [52].b Externado de San Jose.c Ground level instrument adjacent to dam.d Ciudadela Don Bosco.e Ministerio de Relaciones Exteriores; there are two accelerographs at the this site, the reported values are from the instrument at ground level, the other is at
the base of a well.f ESSO Refinery.g INETER.
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UNCORRECTED PROOF
would only affect the distances to the stations at Berlın and
the 15 de Septiembre dam.
Algermissen et al. [18] derived an attenuation relation-
ship from 82 recordings of strong-motion obtained in the
vicinity of San Salvador, without distinguishing between
subduction and crustal earthquakes. Taylor Castillo et al.
[54] derived an equation from 89 records from Costa Rica,
El Salvador and Nicaragua, again combining crustal and
subduction earthquakes. Dahle et al. [55] subsequently
produced attenuation equations for response spectral
ordinates, using a database of 280 records, including 157
from Costa Rica and more than 60 from Mexico, and
making no distinction between different sources of seismi-
city. Some other studies have separated subduction zone and
crustal earthquakes: Alfaro et al. [17] derived two separate
equations for PGA, but used only 20 records for each.
Schmidt et al. [56] have derived equations for spectral
ordinates from a database of 200 accelerograms recorded in
Costa Rica, presenting coefficients for the entire dataset and
for subduction and crustal sub-sets. Climent et al. [57]
derived spectral acceleration equations for Central America
using 280 records from Costa Rica, Mexico, Nicaragua and
El Salvador; these relationships also did not separate crustal
and subduction events.
There are shortcomings in all of the above attenuation
relationships in terms of applicability to El Salvador, either
because they do not discriminate between subduction and
crustal earthquakes, or because they are based on insuffi-
cient datasets. The equations of Schmidt et al. [56] are the
only exceptions, but there are important tectonic and
geologic differences between Costa Rica and El Salvador,
on the one hand, and on the other they make use of
epicentral and hypocentral distance, which are unsuitable
for large events as was noted previously. For these reasons,
comparisons have been made with predictions from
relationships derived for other regions. For the subduction
earthquake of 13 January, the most appropriate attenuation
relationships are those of Youngs et al. [52] derived from
regressions on almost 500 accelerograms from Alaska,
Chile, Cascadia, Japan, Mexico, Peru and the Solomon
Islands. These equations have been proposed for intra-slab
and interface subduction earthquakes, for events larger than
Mw 5 and distances from the fault rupture between 10 and
500 km, making them ideally suited to this situation. The
recorded PGA values are compared with those predicted by
the intra-slab equation of Youngs et al. [52] in Fig. 8;
ground conditions corresponding to more than 20 m of soil
overlying rock have been assumed. The equation appears to
Table 4
Strong-motion records of 13 February 2001 earthquake
Network Station Location drupa (km) PGA (g) PGV (cm/s)
N8 W8 N–S E–W V N–S E–W V
GESAL Berlın (town) 13.50 88.53 17 0.032 0.070 0.031 4.1 6.0 2.9
UCA Armenia 13.744 89.501 55 0.029 0.037 0.026 4.0 2.3 1.3
UCA La Libertad 13.468 89.327 41 0.091 0.093 0.037 4.7 4.5 3.1
UCA Panchimalco 13.614 89.179 20 0.185 0.106 0.045 9.4 4.6 2.0
UCA San Bartolo 13.705 89.106 13 0.106 0.141 0.123 25.6 22.3 6.9
UCA San Salvador ESJb 13.707 89.201 22 0.124 0.099 0.052 18.3 6.6 2.7
UCA Santa Tecla 13.671 89.279 30 0.047 0.040 0.023 6.4 4.8 2.0
UCA Tonacatepeque 13.778 89.114 18 0.345 0.251 0.240 30.0 24.7 10.5
UCA Zacatecoluca 13.517 88.869 18 0.408 0.305 0.262 20.1 20.4 9.6
CIG Presa 15 de Sept.c 13.616 88.550 7 0.020 0.026 0.017 6.4 5.0 2.4
CIG S. Salvador CIGd 13.698 89.173 19 0.138 0.071 0.059 19.9 8.4 3.8
CIG San Salvador DBe 13.733 89.150 18 0.100 0.094 0.055 14.8 12.2 4.6
CIG S. Salvador DUAf 13.737 89.209 24 0.077 0.059 0.046 8.2 8.7 3.5
CIG S. Salvador OBSg 13.681 89.198 22 0.107 0.104 0.068 6.7 13.9 3.3
CIG San Salvador REh 13.692 89.250 27 0.058 0.063 0.034 3.9 8.1 2.2
CIG S. Salvador SEMi 13.705 89.225 25 0.065 0.071 0.044 5.7 10.8 2.6
CIG S. Salvador UCAj 13.677 89.236 26 - 0.058 0.040 - 8.5 2.1
CIG Santa Tecla 13.675 89.300 32 0.039 0.042 0.019 6.4 7.4 2.2
a Distance from fault rupture as defined by Joyner and Boore [53].b Externado de San Jose.c Ground level site adjacent to dam.d Centro de Investigaciones Geotecnicas.e Ciudadela Don Bosco.f Viveros, Direccion de Urbanismo y Arquitectura; there are two accelerographs at the this site, the reported values are from the instrument at ground level,
the other is at the base of a well.g Observatorio Sismologico.h Ministerio de Relaciones Exteriores, ground-level instrument.i San Jose de la Montana Seminary, ground-level instrument.j Universidad Centroamericana.
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UNCORRECTED PROOF
fit the data well at distances of less than 300 km, with values
from greater distances being overestimated. It is worth
noting that in the distance range from 50 to 130 km, the
values obtained from the CIG network are consistently
lower than those from the UCA network.
One particularly interesting feature of the motions
recorded during the 13 January earthquake is the fact that
the response spectra are rich in high frequencies whereas for
such a large magnitude event greater energy at intermediate
and long periods would have been expected. Similar
features have been observed in accelerograms from large
subduction zone earthquakes in Japan and also in Peru [58,
59]. Since high-frequency ground motions were recorded in
both the 1966 and 1970 Peruvian earthquakes, which were,
respectively, associated with thrust and normal ruptures
[42], it would appear that this feature may not be exclusively
a function of source mechanism. Nonetheless, Prvance and
Anderson [60] identify normal faulting earthquakes in the
Mexican subduction zone as producing consistently more
high-frequency radiation than thrusting events. There is
evidence that the recording from La Libertad (Fig. 9), where
PGA exceeds 1 g, displays strong site effects at a period of
about 0.2 s (Fig. 10). This is visible on many recordings
from this station from previous smaller or more distant
earthquakes and would be consistent with a relatively thin
layer (,10 m) of alluvium overlying bedrock (lavas).
For the crustal earthquake of 13 February, one possibility
would be to use the relationships derived by Spudich et al.
[61] for zones of extensional tectonics, but the two
recordings from the 1986 San Salvador earthquake used in
that study were found to be outliers whose amplitudes were
significantly underestimated by the median predictions.
Other candidate equations derived from crustal recordingsFig. 8. Recorded PGA values from the 13 January earthquake compared
with predictions from the attenuation relationships of Youngs et al. [52].
Fig. 7. Location of strong-motion recording stations in El Salvador.
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UNCORRECTED PROOF
elsewhere in the world would include the western USA
relationships of Boore et al. [62], the global relationships of
Abrahamson and Silva [63], the European relationships of
Ambraseys et al. [64] and the Italian relationships of Sabetta
and Pugliese [65]; the latter may be particularly suitable
since large areas of Italy are also volcanic. Fig. 11a
compares the recorded PGA values with the median values
from these relationships, in which the equations seem to
consistently overestimate the observed values beyond about
20 km. Fig. 11b confirms that most of the recorded PGA
values are within the ^s values predicted by Ambraseys
et al. [64]. Worthy of particular note in this figure are the
low PGA values obtained at Berlın in this event, which
raises questions about the reliability of this particular
recording: GESAL operates digital accelerographs in the
Fig. 9. Accelerogram recorded at La Libertad during 13 January 2001 earthquake.
Fig. 10. Absolute acceleration response spectrum (5% damping) of record
in Fig. 9.
Fig. 11. (a) Recorded PGA values from the 13 February earthquake
compared with median values predicted by the equations of Abrahamson
and Silva [63], Boore et al. [62], Ambraseys et al. [64] and Sabetta and
Pugliese [65]. (b) Recorded values of PGA compared with the 16, 50 and
84% predictions from Ambraseys et al. [64].
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UNCORRECTED PROOF
town of Berlin and at the nearby geothermal energy plant,
with records having been obtained only from the latter
instrument on 13 January and reportedly from the former on
13 February. The even lower amplitudes recorded at the 15
de Septiembre dam may point to the fault rupture being
shorter than 42 km and not extending so far east as has been
assumed.
4.3. Implications of recorded motions for seismic design
code
The first seismic design code in El Salvador was
introduced in 1966 following the San Salvador earthquake
of the previous year; this code was adapted for El Salvador
from the code from Acapulco, Mexico [23]. A revised code
was drafted by the Salvadorian Association of Engineers
and Architects (ASIA) in 1989, issued as an emergency
regulation following the 1986 San Salvador; the design
spectra in this code took account of the nature of the ground
motions recorded in the earthquake. The current seismic
code, published in 1994, forms part of a comprehensive set
of regulations for building and civil works produced by the
Ministry of Public Works. The current code has several
merits, including the fact that it is the first to have involved a
probabilistic assessment of seismic hazard in El Salvador
[19]. Furthermore, the regulations cover a wide range of
practices, including geotechnical works, and also provides
guidance on construction using adobe despite initial
opposition from contractors who were concerned that
promotion of vernacular building techniques would be
detrimental to their business.
The elastic spectra in the current code appear to be
sufficient for most of the ground motions recorded in these
earthquakes. The somewhat exceptional record of 13
January at La Libertad exceeds the code spectrum
(Fig. 12), but it would not seem reasonable to increase the
code spectrum to a maximum level of 5g just to
accommodate the narrow-band amplification due to specific
site effects at this location. The strongest recording from the
13 February earthquake, obtained at Zacatecoluca, is
covered by the spectral ordinates specified in the code, as
shown in Fig. 13a. Fig. 13b shows the spectrum at the same
station from the 13 January earthquake, which is also
adequately covered by the code spectrum. In passing it can
be noticed that as at other stations [66], the shape of the
spectra from the two earthquakes are generally similar,
confirming the importance of site effects in determining the
nature of the ground motion.
5. Effects of the earthquakes
The impact of the January and February 2001 earth-
quakes was strong in many parts of the south of the country,
particularly the coastal cordilleras and locations around the
volcanic centres. The area around the San Vicente volcano,
where buildings had been weakened by the 1999 swarms,
and where both the 13 January and 13 February earthquakes
caused strong shaking, was particularly affected. The
patterns of damage, however, were very uneven and the
capital city, San Salvador, was largely unaffected. None-
theless, the overall impact was devastating to the fabric of
the country, with an estimated 40% of the health service and
30% of schools severely damaged.
The death tolls due to the two earthquakes have been
reported as 844 and 315, respectively, with the majority of
the casualties, particularly in the 13 January event, being
due to landslides. It is worth highlighting here that the loss
Fig. 12. Envelope of horizontal acceleration spectra from the 13 January
recording at La Libertad compared with the elastic spectra from the 1994
design code for soil class S3 and importance categories I and III.
Fig. 13. (a) Envelope of horizontal acceleration spectra from the 13
February recording at Zacatecoluca compared with the elastic spectra from
the 1994 design code for soil class S3 and importance categories I and III.
(b) Envelope of horizontal acceleration spectra from the 13 January
recording at Zacatecoluca compared with the elastic spectra from the 1994
design code for soil class S3 and importance categories I and III.
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UNCORRECTED PROOF
of life in these earthquakes underestimates their impact;
more people were killed by the Mw 5.7 San Salvador
earthquake of October 1986. The death toll is also small
compared with the 75,000 lives lost in the fratricidal war
from 1980 to 1992, and indeed when compared with the
numbers of victims of violent crime, which has grown to
epidemic proportions in recent years. The true impact of the
earthquakes is more accurately represented by the fact,
mentioned previously, that a significant proportion of the
population was either made homeless or suffered substantial
damage to their housing. Even before the earthquakes it had
been estimated that El Salvador had a housing deficit of
more than half a million dwellings.
5.1. Landslides and liquefaction
The 13 January earthquake triggered more than 500
landslides across in El Salvador and a further 70 occurred as
a result of the 13 February earthquake. Landslides were an
extensive secondary effect of the earthquake of 13 January
2001. The landslides could be divided into three broad types
according to the classification scheme proposed by Dikau
et al. [67]. These were rock and debris falls, slides and
debris flows. Many of the landslides occurred on the slopes
of volcanoes that are used for coffee cultivation; coinciding
with the time of the coffee harvest, many coffee pickers
were killed by these slides.
Rockfalls and debris falls were common throughout the
area and ranged from single block falls (some of which were
up to 3 m in diameter, Fig. 14) to the collapse of slopes cut
in pyroclastic ashfall deposits, which exist as a result of
weak cementation and high negative pore pressure [28,68].
Such failures were largely independent of lithology, but
occurred only on steep slopes. Individual block falls were
more common in the rocks of the Balsamo Formation
because of the prevalence of persistent discontinuities in the
form of bedding and cooling joints. Highly altered layers of
volcanic rock also acted as aquitards.
The occurrence of rock and debris falls in the tierra
blanca affected an extensive area around the Comasagua
Road on the Balsamo Ridge. Here the steep slopes which
were formed as part of the road construction and also by
quarrying failed in spectacular manner giving rise to an
extensive area of slope instability which extended from the
landslide at Las Barrioleras to the Jayaque–Comasagua
Junction (Fig. 15) and in Comasagua itself (Fig. 16). This
formed a classic shattered ridge. In many cases, it was
impossible to tell where one landslide ended and another
began.
Large landslides were observed along roads to Comasa-
gua, Talnique, Jayaque, Tepecoyo and Sacacoyo. The
principal cases were reported along the road between
Nueva San Salvador and Comasagua on slopes of volcanic
ashes mainly tierra blanca. The Pan-American Highway
was blocked between Los Chorros and Colon by landslides
to the west of San Salvador. At the Las Leonas location, to
Fig. 14. Single block fall, triggered by an aftershock of the 13 January earthquake, on the coastal motorway near La Libertad.
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UNCORRECTED PROOF
the east of San Salvador, this road was blocked by a large
slide of approximately 500,000 to 700,000 m3 of rock and
soil debris. Roads to San Agustın, Santiago de Marıa and
Berlın were also blocked. Several landslides were also
observed along the road between Cojutepeque and Santiago
Texacuangos, and around Lake Ilopango.
The most important group of landslides, which were
triggered by the earthquake, were the debris flows. These
landslides were responsible for more than half the deaths
during the 13 of January earthquake. The two most
important of these slides were at Las Colinas (Fig. 17)
and Las Barioleras. These landslides, which occurred in the
Tierra Blanca showed significant travel distances of 735 m
and c. 1140 m, respectively. The former of these two slides
destroyed part of the residential area at Las Colinas, while
the latter killed many people who were working on the
coffee plantations and travelled onto the Pan-American
Highway. The long travel distances of these landslides
indicate low coefficients of internal friction of between 6
and 98, inferred from the ratio of slope height to run-out
length of the slide [69]. This indicates a significant drop in
frictional strength from the undisturbed state, which may be
as high as 388. Debris flows were common throughout the
Cordillera del Balsamo resulting from the steep terrain
mantled with weak volcanic debris and the presence of
aquitards in the underlying Balsamo Formation.
The Las Colinas landslide in Santa Tecla was the most
notorious slide triggered by the earthquakes due to its
devastating impact on population. This slope failure buried
as many as 500 people. This slide was approximately 790 m
long, 150 m wide and left a scarp 50 m high. The total
volume of the slide was approximately 200,000 m3. The
slide affected a part of the northern flank of the Balsamo
Ridge composed of the Balsamo Formation. This formation
is formed mainly of andesitic cinders and some interbedded
tephra. Extensive cracking was observed on the ridge crest
in areas that did not slide, which was cause for additional
concern. Some authors attributed this slide to liquefaction of
saturated tierra blanca deposits [70–72], however, a
rotational slope failure of the upper part of the slope has
also been attributed as initial failure mechanism [73].
Failure has been found to be related to high water content of
the lower part of the slope, which has been attributed to
natural drainage blockage by a retaining wall observed on
the bottom of the slope [72], although a perched aquifer on
the slope due to the impermeable nature of the Balsamo
Formation has also been proposed as the cause of this high
water content. Soil saturation was observed only locally,
due mainly to the earthquake occurring after 5 months of
dry season. The destructiveness of the landslide may have
been due to its high mobility, which may have been the
result of an unfavourable combination of high water content
and material brittleness.
Harp and Wilson [74] have identified Arias intensity
(sum of the two horizontal components) as a useful indicator
of the capacity of the ground shaking to trigger landslides.
Fig. 15. Debris fall from the Tierra Blanca near Comasagua.
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UNCORRECTED PROOF
Fig. 17. The Las Colinas landslide, Santa Tecla.
Fig. 16. Landslide damage in Comasagua.
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UNCORRECTED PROOF
From studies of the 1987 Superstition Hills and Whittier
Narrows earthquakes in California, Harp and Wilson [74]
proposed thresholds of about 0.25–0.3 m/s for landslide
triggering. The values of Arias intensity for the records of
the 13 January earthquake are as high as 14 m/s and in all
cases (excluding the Nicaraguan records), exceed this
threshold [75].
In terms of geographical distribution, landslides were
triggered across most of the southern half of El Salvador,
with a particular high concentration in the Cordillera del
Balsamo to the southwest of San Salvador, between Nueva
San Salvador and Armenia (Fig. 18), affecting a much larger
area than in previous earthquakes [29]. In a general way, the
geographical distribution of landslides roughly corresponds
to the distribution of young ash, tuff, and tephra deposits on
steep slopes, incised valley walls and river channels.
Landslides were also reported to occur in Guatemala [76].
Slides blocked roads between Quesada and Monte Verde
and between Moyuta and El Obraje in the Jutiapa District.
Landslides were also reported along the Guatemala-Mexico
and Quetzaltenango–Retalhuleu roads, and along the road
to Ixtahuacan, Solola.
The 13 February earthquake triggered additional land-
slides to those reported by the 13 January event. Along the
Pan-American Highway new landslides were observed at
Las Leonas and adjacent locations. A large landslide was
reported in the water head part of the Rio Jiboa; it was
estimated that volume of sediments yielded in this area
reaches between 10 and 15 £ 106 m3 of debris, mainly
tierra blanca [77]. This landslide blocked the river course
for 600–700 m causing an artificial lake to be formed.
Another large landslide blocked the course of Rio El
Desague; in this case a volume between 1 and 2 £ 106 m3
was mobilised, consisting of andesitic breccia blocks of
around 0.5–2 m in diameter embedded into a tierra blanca
matrix [77].
On the slopes of the San Vicente volcano landslides were
reported along the El Muerto and El Blanco creeks. The El
Muerto landslide was estimated to have mobilised around
700,000–800,000 m3 of andesitic rock blocks, whereas the
El Blanco landslide mobilised silty and sandy gravels and
blocks coming from pyroclastic flows. This slide becomes a
latent hazard against the Tepetitan town, which was flooded
in 1930 by a mudflow resulting in four deaths. New
landslides were also reported around the Lake Ilopango
[77].
Fig. 18. Distribution of areas affected by landslides (solid line), by concentrated landslides (small dashes) and by liquefaction (large dashes).
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UNCORRECTED PROOF
Analysis of SPOT image data after the earthquake of 13
of January 2001 with 10 m ground resolution, reveals many
flowslides in the Balsamo Cordillera. A similar scene
collected after the earthquake of 13 February shows that
many of these landslides have expanded in size either as a
result of aftershocks from the first earthquake, or from the
effects of the second, much closer, event.
The susceptibility of slopes to earthquake-induced
instability has been shown to be strongly dependent on the
rainfall in the months and weeks prior to the seismic event
[33]. Although ACPC [37] reports that the 2000/2001 coffee
harvest was delayed due to wet weather, average annual
precipitations reported by the Meteorlogy Department of the
Salvadorian Ministry of Agriculture (MAG) indicate that
rainfalls for the year 2000 were in fact slightly low in many
parts of the country, at least compared to the previous 2
years (Table 5), although it should be noted that 1998 was an
exceptional year because of Hurricane Mitch.
The hazard of rainfall-induced landslides in the rainy
season (normally starting in April or May) following the
earthquakes became a major concern. On 19 September
1982, after a rainfall of 223 mm in less than 2 days, a
landslide began to move on the slopes of San Salvador
Volcano (El Picacho) and then descended rapidly into the
densely populated neighbourhood of Montebello. The slide
had an estimated volume of 200,000 m3 and killed an
estimated 500 people, leaving another 2400 homeless
(CEPRODE, 1994). This slide happened exactly three
months after an Mw 7.3 subduction earthquake, which is
reported to have caused extensive cracking on slopes.
Extensive cracking along ridges, especially along the road
to Comasagua in the Cordillera del Balsamo, caused by the
13 January earthquake led to concerns that a similar
sequence of events might follow in the 2001 rainy season.
However, the hazard did not materialise during the first
months of the rainy season since rainfall levels were
exceptionally low, to the point of creating drought and
consequently severe problems with water supply and
agriculture. Nonetheless, heavy rainfalls have occurred
since the earthquakes and a large mud and debris flow was
triggered on the lower slopes of the San Vicente
(Chichontepec) volcano on 15 September 2001.
Liquefaction was observed at various locations along the
coast in central and eastern El Salvador, accompanied by
lateral spreading and consequent damage to some houses.
Similar observations were made on the shores of Lake
Ilopango, where lateral spreading was significant and some
houses were rendered uninhabitable due to foundation
damage. The most serious effects of lateral spreading
occurred on the banks of the Lempa River at San Nicolas
Lempa that resulted in collapse of a railway bridge (Figs. 19
and 20).
5.2. Damage to housing
The initial estimates by the Committee for National
Emergency (COEN) of the Salvadorian government of the
number of homes destroyed by the earthquake was about
150,000, with another 185,000 damaged. There has been
some debate regarding the damage statistics, with reports
that the COEN figures are overestimated [2], but all sources
agree that more than one million people were made
homeless by the earthquakes. The overwhelming majority
of the damaged houses were of adobe and bahareque, with
the former being the most susceptible type of housing.
Timber frames and reinforced masonry houses performed
significantly better and it was not uncommon to visit
locations where most adobe houses were in a state of at least
partial collapse whereas reinforced masonry houses were
practically unscathed.
In the rural area of El Salvador the dwelling construction
types mostly used are adobe, bahareque, reinforced brick
masonry (mixto ), wood frames cover by thin metal sheets
(lamina ), and wood frames cover by palm fronds (ranchos ).
Other building practices, which are less widely used,
include concrete and soil-cement block masonry using
soil-cement blocks, and steel frames cover by precast walls
(Fig. 21).
Roofs of adobe houses may be of metal sheets and/or
clay tiles supported by wood trusses or thatched roof
supported on wood timber purlins. Load transfer between
the roof and walls, or between walls, is often not effective.
This building system has high mass and stiffness but low
strength.
Bahareque consists of timber vertical elements and
horizontal timber, cane or bamboo elements, infilled with
mud and finished with plaster. The seismic resistance of
bahareque depends primarily on the condition of the timber
and cane elements, having low vulnerability when carefully
maintained. Bahareque is a more expensive building system
than adobe. Roofs are similar to those for adobe and show the
same problems. Mixto is composed of fired clay bricks with
mortar and slender elements of concrete with thin steel
reinforcement, or the same thickness as the wall, which are not
properly reinforced concrete and are known as nervios (nerves
or tendons). This system, in which the load bearing system is
Table 5
Annual average rainfalls (mm) at selected meteorological stations
Year Ilopango Santiago de Marıa La Union San Miguel Ahuachapan Acajutla Puente Cuscutlan
1998 1958 2338 2123 1648 1623 2280 2037
1999 1504 1902 1859 1470 1554 1953 1303
2000 1454 1890 1783 1543 1052 1761 1637
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UNCORRECTED PROOF
Fig. 19. Lateral spreading on river bank at San Nicolas Lempa.
Fig. 20. Collapse of railway bridge due to lateral spreading on banks of the Lempa River.
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UNCORRECTED PROOF
provided by the masonry walls, has relatively good seismic
resistance but is considerably more expensive than both adobe
and bahareque. Lamina is the name given to buildings of
timber or metal frames covered by thin metal sheets, usually
founded on a 50 cm high block wall. Lamina has good seismic
resistance due to its low weight and mass. Wood frames
covered by palm fronds have excellent seismic response
characteristics but this building system is rapidly disappearing
due to scarcity of materials. The construction systems most
severely affected by both the January and February earth-
quakes were adobe and bahareque.
The damage patterns clearly revealed the social vulner-
ability of poor forced to live in susceptible locations and
vulnerable houses. Small towns such as San Agustın (Fig.
20), where 80% of the houses were made from adobe, were
particularly hard hit. The same pattern was visible in small
hamlets and villages, where adobe was even more dominant
and where the quality of construction was generally poor.
Even in San Salvador, where damage to engineered
structures was very limited, extensive damage was observed
in shanty dwellings such as in the Jose Cecilio del Valle area
adjacent to the exclusive Escalon neighbourhood, due to
both shaking and to movements on slopes.
5.3. Damage to engineered structures
The majority of engineered structures in El Salvador are
located in the metropolitan area of San Salvador, and mainly
in the cities of San Salvador and Nueva San Salvador (Santa
Tecla). When seismic design has been considered at all, the
approach has generally been focused on the ultimate limit
state. Most damage in engineered structures due to the 13
January earthquake was non-structural, with damage to
partition walls, ceilings, A/C ducts and windows. Many
major hospitals were put out of service because they lost
their functionality and non-structural damage needed to be
repaired before they could be used again. Several buildings
that suffered light damage in the 1986 earthquake and were
not adequately repaired suffered some damage due to the
2001 earthquakes. One structure that had been badly
damaged in 1986, a building housing a hair salon next to
the Externado de San Jose High School (itself destroyed in
the 1986 earthquake and subsequently rebuilt), withstood
the shock without collapse, although the owners have since
taken the decision to demolish.
There are only a few buildings in San Salvador that stand
more than 70 m height, most of which behaved very well
during the earthquakes. One of these, the Torre Cuscutlan
(formlerly known as Torre de Democracia), located in the
south west of San Salvador, is an irregular tower with
external glass walls; none of these were broken. One high-
rise structure did suffer some internal damage, the
Ministerio del Interior building in the centre of San
Salvador, which was also damaged by the 1986 earthquake.
Fig. 21. Collapsed adobe house in San Agustın, of which only the door remains standing after the 13 January earthquake; behind are houses of mixto and
lamina, which have survived the earthquake.
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UNCORRECTED PROOF
There are a few other examples of structures having suffered
severe damage, such the Regis Condominium in the San
Jacinto area of south San Salvador. Most structures that
suffered some damage are of reinforced concrete; steel
structures behaved well.
Outside the capital there are few engineered structures
compared to San Salvador. The hospitals in Usulutan and
San Miguel were left non-operational due to non-structure
damage. Public buildings erected in the 1960’s were
severely damaged in Santiago de Marıa. Another case of
damage outside of the capital city was El Salvador’s
International Airport located near Comalapa on the coastal
plain. The airport suffered important non-structural damage
including cracking of infill walls, breakage of windows and
collapse of ceilings, as well as some minor cracking in
columns and beams in the older sections of the airport
buildings.
The 13 February earthquake damaged some engineered
structures mainly in the central area of El Salvador. The city
that shows most of this kind of destruction is San Vicente,
where at least two schools, one of them built in the early
1970s, have suffered severed damage; one branch of the
Universidad de El Salvador suffered non-structural damage
that limited its function. In addition, this earthquake
damaged the Zacatecoluca hospital, which is a twin
structure of the Usulutan Hospital. However, in general
damage levels were low for the size and location of the
earthquake. In the towns of Guadalupe (Fig. 22), Verapaz
and Santa Cruz Analquito, which are located very close to
the assumed fault rupture, there was total collapse of many
houses built from adobe and bahareque, but mixto
(reinforced masonry) constructions generally survived
intact. Even structures that had been weakened by both
the 1999 seismic swarm and the 13 January earthquake,
such as the church in the town of San Esteban Catarina, did
not suffer as much damage as might have been expected. In
the town of Apastepeque, close to the source of the 13
February earthquake and badly affected by the 1999 swarm,
residents reported that the most severe effect of the
earthquake was to dislodge roof tiles. That a crustal
earthquake of Mw 6.6 did not cause greater levels of
damage in reinforced masonry nor in some cases in
weakened adobe buildings, suggests that the earthquake
was less superficial than is typical of volcanic chain
earthquakes such as those in Jucuapa–Chinameca in 1951
and in San Salvador in 1986.
In terms of the cultural heritage of El Salvador, there are
relatively few examples of colonial architecture surviving in
many parts of the country, indeed in San Salvador all
buildings from the colonial period have been destroyed by
fire or earthquake. The earthquakes caused damage to more
than 400 churches in El Salvador.
An important question that immediately presents itself is
why the damage to engineered structures, particularly
Fig. 22. Guadelupe following the 13 February earthquake, contrasting total collapse of adobe houses with the almost undamaged state of the reinforced
masonry building to the right.
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UNCORRECTED PROOF
during the 13 January earthquake, was so limited? Even at
the Health Centre in La Libertad, where the maximum 5%
damped spectral acceleration exceeded 4.5g, damage was
limited to the fall of part of the ceiling (non-structural) and
minor cracks in an external wall. Comparison of accel-
erograms obtained in San Salvador during the 1982
subduction-zone and 1986 upper-crustal earthquakes pro-
vides insight into possible reasons, since the latter event
caused significantly higher levels of damage in engineered
structures, despite the fact that the response spectral shapes
were not very dissimilar, hence the frequency content of the
motions is unlikely to provide the explanation. The 1982
and 1986 accelerograms were found, however, to contain
almost identical levels of energy, as measured by the Arias
intensity but with very different durations, so that the rate of
energy input was an order of magnitude greater in the 1986
earthquake [50]. The total energy input, which was actually
higher in the January 2001 earthquake than for the 1982 and
1986 records, is a good indicator of the damage potential in
brittle and degrading materials such as adobe and volcanic
soils. It would appear that for damage to be inflicted on
engineered structures it is necessary that the motion has both
a high energy content and a high rate of energy input, as
indicated by the root-mean-square acceleration.
5.4. Performance of lifelines
The performance of lifelines in the two earthquakes has
been reported in detail by Lund [79] and EERI [80].
Telecommunications were not seriously affected and service
was fully restored in the capital within one day of the first
event. Electricity generation was not seriously affected but
the distribution system was affected by a large number of
transmission lines broken by landslides. There are no gas
distribution lines in El Salvador since all household use
imported propane distributed in canisters. The diesel and
petrol refinery in the port of Acajutla was not damaged and
production was not interrupted by the earthquakes.
The distribution of potable and waste water in El
Salvador is managed by the state-owned company ANDA.
The earthquakes caused disruption to the water distribution
system but breakage of pipes was limited; for example, only
three repairs in the northern area supply line in San Salvador
were reported by ANDA. The most serious disruption to the
water distribution system was the damage caused by the 13
February earthquake to the treatment plant at Cacahuatal
that supplies the San Vicente area. Although the disruption
to the water distribution system by the earthquakes was
limited, it is worth noting that even under normal conditions
there are problems with water distribution in El Salvador,
with chronic shortages and few households, even in urban
areas, have uninterrupted water supply 24 h a day.
The most seriously affected lifelines were transport lines.
There are three railway lines in El Salvador, connecting the
ports of Acajutla and Cutuco (La Union) and the cement
production plants in Metapan in the northwest of the
country, used predominantly for transportation of cargo
rather than passengers. The eastern line connecting Cutuco
has not been operational for many years. The only damage
to the railway system was the collapse of the steel arch truss
bridge at San Nicolas Lempa due to lateral spreading.
The two main highways in El Salvador run across the
country from east to west. The Panamerican Highway
(CA-1) runs along the Great Interior Valley; it was
originally constructed to serve the coffee industry. The
second major artery is the coast road (CA-2) whose original
purpose was to serve the cotton plantations that previously
occupied the coastal plains. Transport on both roads was
severely disrupted by landslides. The coast road between the
ports of La Libertad and Acajutla in the west was partially
blocked by a number of rock falls and relatively small
landslides; the five tunnels on this section of motorway were
undamaged apart from minor cracks in their lining. The
Panamerican Highway was completely blocked by major
landslides both east and west of San Salvador for several
weeks. To the west, major slides at Los Chorros blocked the
road and even after several weeks traffic was only able to
circulate in one direction, with vehicles entering the capital
in the morning and leaving in the afternoon. East of San
Salvador the highway was completely blocked in both
directions by the huge slide, re-activated by the 13 February
earthquake, at Las Leonas, obliging traffic to use the old and
practically abandoned road running approximately parallel
to the north.
The motorway joining San Salvador and the international
airport at Comalapa on the coastal plain was damaged by
cracks at several locations and during several weeks traffic
was reduced to a single lane in each direction over part of
the road. The airport was closed for one day following the
13 January earthquake to allow clearing up of debris and
inspections of buildings and runways.
6. Implications for seismic risk: physical, social and
institutional vulnerability
The 2001 earthquakes have revealed the extreme levels
of vulnerability to natural hazards that exist in El Salvador.
Moreover, the failure to mitigate earthquake risk in El
Salvador is a reflection of institutional vulnerabilities that
have not been addressed; chief among these are the
capacities for emergency response, monitoring of natural
hazards, land-use planning, and seismic design and its
enforcement.
6.1. Emergency response
Some observers have claimed that the government
response to the disaster in El Salvador has been poorly
organised and in particular that the lessons from Hurricane
Mitch were clearly not learnt [81]. Although this study is not
primarily concerned with emergency aid following the
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UNCORRECTED PROOF
earthquakes, there were some obvious shortcomings, at least
in the initial phases of the response. For example, most aid
arriving at Comalapa International Airport, located on the
coastal plain, was transported almost 30 km to the Feria
Internacional in San Salvador for centralised logging and
thence distributed to affected areas, several of which were
within 1 hour’s drive of the airport. Our visits to badly
affected rural areas generally indicated that government
assistance, in the first few weeks of the crisis, was not
getting through to many of the earthquake victims,
particularly in more remote rural areas.
The main response to the emergency seems to have been
provided by the affected people themselves, although
important contributions by NGOs and others, including
contingents of the Venezuelan Armed Forces and, changing
their historical role, the Salvadorian Armed Forces as well,
should not be overlooked. Despite the huge numbers of
people made homeless by the earthquakes, there were very
few examples of victims living in temporary shelters in the
streets of the cities, as there were after the 1986 earthquake.
Most rural communities, except where affected by land-
slides, appear to have remained to rebuild their homes and
continue with their lives. Middle class people made
homeless, such as those from Las Colinas and adjacent
neighbourhoods, were either absorbed by relatives or added
to the exodus to the USA. May 2001 saw the highest ever
influx of remesas into El Salvador, with a monthly total of
US$ 197.1 millions.
6.2. Seismic design of buildings
Although damage to engineered structures was limited,
at least in terms of structural collapse, there is still a
significant danger of many large engineered structures
having been weakened by the earthquakes and therefore
urgently requiring intervention. This is, in the majority of
cases, which may be unlikely to happen given that seismic
design requirements are not imposed even for new
buildings. Lara [82] reports that prior to the 1986 earthquake
in San Salvador, the seismic design code was rarely applied,
and there is little evidence to suggest that the codes of 1989
and 1994 have been more widely implemented. Indeed,
although it has many technical merits, there is no effective
mechanism for the imposition of the current code for
earthquake-resistant design in El Salvador [83].
The current seismic design code in El Salvador has many
technical merits but the lack of a credible system for its
enforcement severely limits its effectiveness in mitigating
seismic risk. There are almost many aspects of seismic risk in
buildings that fall outside the remit of the code, one being
repair and strengthening. As noted previously, the code does
include an appendix of guidelines for the improved earth-
quake-resistant construction of adobe although this, logically,
does not form part of the actual regulations. These guidelines,
and other publications [84], affirm that adobe buildings can be
constructed with a degree of earthquake resistance, with
minimal requirements in terms of additional costs and
building skills. There is clearly a need, however, for a
transfer of this knowledge to the most isolated and vulnerable
rural communities where these forms of housing are most
abundant and also where they are built with the highest levels
of susceptibility. Amongst the many obstacles to this effective
mitigation are the relatively high rate of illiteracy in rural
areas and the lack of confidence in adobe construction
following its poor performance in the 2001 earthquakes.
6.3. Land use planning
The high level of landslide hazard in El Salvador makes
land-use planning an issue of great importance. The high
population density of El Salvador and the housing deficit
also makes it a sensitive and controversial issue. There is
currently almost no effective control over land develop-
ment. It is interesting to note that the landslide hazard map
shown in Fig. 2 clearly indicates that the area affected by the
catastrophic landslide at Las Colinas was identified as being
of high hazard. The hazard map was prepared some years
after Las Colinas was developed in 1985, but nonetheless no
remedial action was taken to stabilise the slopes or to protect
the area from landslides. The irrelevance of geohazards in
planning decisions is very clearly demonstrated by a recent
housing development to the north of San Salvador, called
Santısima Trinidad (Fig. 23). The development consists of
several rows of four-storey apartment blocks built on
terraces on a natural slope with an inclination of about
358, above which three huge water tanks have been
constructed. The constructors apparently did not face any
serious obstacles in obtaining permission to build.
Despite the apparent lack of control and accountability in
land use planning, the earthquakes may lead to important
changes in this area. Following the earthquakes, 200
survivors from Las Colinas, supported by the Salvadorian
Foundation for the Application of Law (FESPAD) brought a
case against the State to the Supreme Court of Justice for
their failure to prevent or mitigate the risk of landslides on
the slopes of Cerro La Gloria, which were well known and
identified in the PLAMADUR hazard map (Fig. 3). The case
was unsuccessful, being dismissed by the Supreme Court;
similarly, the attempt by the Santa Tecla municipality to
prosecute the developers also failed [81].
More generally, after the earthquake there was a renewed
interest in addressing hazard-related land use issues, not
only in urban areas but in the country as a whole. However,
it is not yet clear how this process will evolve. Certainly,
both public and private sectors in El Salvador will need to
make rapid advances in their risk management and
evaluation practices if large-scale losses are to be avoided
in future due to earthquakes or other natural hazards.
6.4. Seismic monitoring
The monitoring of earthquakes, volcanoes and landslides
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UNCORRECTED PROOF
Fig. 23. Santısima Trinidad residential area north of San Salvador built on terraces along a steep slope (a) above which are three large water tanks (b).
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UNCORRECTED PROOF
has traditionally been the responsibility of the Centre for
Geotechnical Investigations (CIG), which is part of the
Ministry of Public Works. Monitoring capacities for natural
hazards were severely weakened in the 1980s due to the war
and efforts to re-build and re-generate these activities since
have been limited. Some responses to this situation have
taken the form of independent initiatives by private
institutions, a clear example of which is the digital
accelerograph network established in 1996 by the UCA.
The government of El Salvador has now responded to this
situation by forming, in October 2001, SNET (National
Service for Territorial Studies), which will unify and
strengthen current natural hazards monitoring capacities.
The structure of SNET includes four different national
services: Geological Service (including earthquake, volca-
noes and landslides); Meteorological Service; Hydrological
Service; and Risk Management Service. The United States
and Japan are expected to provide equipment and technical
assistance for SNET. The Spanish government has approved
a project to expand and upgrade the existing strong-motion
network previously managed by CIG and a convention has
been agreed for collaboration amongst the three strong-
motion networks in El Salvador.
7. Discussion and conclusions
The El Salvador earthquake of 13 January 2001 was the
first major earthquake disaster of the new millennium and
serves perhaps as a warning that in many countries of the
Third World seismic risk is growing. The combination of
population expansion and increasing urbanisation, in the
case of El Salvador with cities expanding in the zones of
highest seismic hazard, together with the increasing
susceptibility of the terrain to landslides, has led to
increased levels of risk both to lives and to the livelihood
of the country.
The impact of the earthquake of 13 January was
compounded by the second event on 13 February, which
came as the aftershocks of the former event were beginning
to diminish in frequency and intensity. The 13 February
event was followed by many aftershocks, both around the
crustal source of this earthquake but also offshore in the
subduction zone. This would tend to indicate interaction
between the two earthquakes, a topic that will be the focus
of future research. Interactions between earthquakes are
well recognised, with stress release in one location causing,
by transfer, stress increase in adjacent zones and hence
inducing or accelerating rupture on adjacent faults or fault
segments. The clearest example of such interaction is the
progression of earthquakes from 1939 to 1999 along the
North Anatolian fault in Turkey [48,85,86]. Interactions
between different earthquakes has also been identified
within subduction zones, as for example in the 1997–1998
sequence in central Chile [87]. That there is interaction
between subduction and crustal earthquakes in Central
America seems probable: it has been noted that the
subduction zone from central El Salvador to the northern
Nicaragua has a far lower rate of moment release than the
zones offshore from Guatemala and Nicaragua either side
[88]. There is also evidence that destructive shallow-focus
earthquakes along the volcanic chain opposite the El
Salvadorian section of the Middle America Trench are
more frequent than in Guatemala and Nicaragua. The exact
nature of the interaction and the mechanism of stress
transfer between the two seismogenic sources is, however,
far from clear at this stage.
The large numbers of accelerograms recorded during the
two earthquakes provide a very useful basis for the
characterisation of strong ground-motion in Central Amer-
ica, although the lack of any near-source recordings of the
13 February earthquake—due to malfunction of the San
Vicente and San Pedro Nonualco stations of the TALULIN
network—is an unfortunate gap in the data set. This is
particularly the case because the indications from the
recorded motions of the second earthquake, and the
observed levels of damage, are that the ground motions
generated were less intense than would be expected from a
shallow earthquake of magnitude Mw 6.6, indicating either
very high attenuation with distance or a focus within the
lower part of the crust. Macroseismic observations and the
limited strong-motion recordings from other earthquakes
point towards high rates of attenuation in the volcanic chain
zone, as has been found elsewhere including the volcanic
region of the North Island of New Zealand [89].
Notwithstanding this observation, the 13 February earth-
quakes appears not to have been as shallow as other slightly
smaller but more destructive events along the volcanic chain
in El Salvador and neighbouring countries. There are
several features of the ground motion that warrant further
research:
† The differences between ground motions from crustal
and subduction events in Central America, and the
development of separate predictive relationships for the
two sources of seismicity.
† The influence of site effects due to both surface geology
and topographical features; the apparent predominance
of these influences suggests that microzonation is a
potentially very useful tool in El Salvador.
† The specification of earthquake loads for seismic design,
taking account of both the different geographical
distributions of the hazard from crustal and subduction
earthquakes and the different natures of the resulting
ground motions.
† The relationship between the nature of the recorded
motion and its capacity to produce damage: it is
abundantly clear that PGA is of very little significance
in this respect, and to some extent this is also true for
spectral accelerations (whence the current trend towards
displacement-based approaches to assessment and
design).
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UNCORRECTED PROOF
The lack of extensive structural damage in reinforced
concrete buildings due to these earthquakes should not be
interpreted as a vindication of the success of the current
seismic design code in El Salvador nor its predecessors,
since regardless of their technical merits these codes have
generally been applied only sporadically. The lack of major
structural damages and collapse of large buildings appears
to be more closely related to the nature of the ground
motions generated than the quality of engineering design or
construction. Particularly in San Salvador there is now a real
danger of complacency regarding the capacity of existing
buildings, despite the fact that it is widely known that many
buildings have been left damaged by the 10 October 1986
earthquake and these may have been further weakened by
the 2001 earthquakes. Destructive moderate magnitude
earthquakes occur in San Salvador on average every 20–25
years [14] and the next event could cause terrible damage
and loss of life in the overcrowded and expanding capital.
The most devastating impact of the 2001 earthquakes has
been the triggering of hundreds of landslides in volcanic soils,
which have buried houses and blocked roads, causing most of
the deaths in these earthquakes and bringing massive
disruption: the Pan-American Highway remained closed for
more than 10 months due to the landslide at Las Leonas. The
number of landslides triggered by these earthquakes, the size
of the slides and their geographical distribution, all indicate
increasing susceptibility of the terrain when compared to
patterns in previous earthquakes, with no indication that this
was due to precedent rainfall. The hazard of earthquake- and
rainfall-induced landslides in the volcanic soils that dominate
much of El Salvador, and particularly the most densely
populated areas, urgently requires attention. The identification
of zones of high landslide hazard is an important component of
any programme of mitigation, but relocation to lower hazard
zones will often not be an option in this densely populated
country with a long history of conflicts over land ownership.
Stabilisation measures cannot necessarily be imported from
regions of the world with entirely different soil characteristics,
since one of the distinguishing features of volcanic soils such
as tierra blanca is the complete loss of cementation at small
strains, followed by the collapse of its matrix structure and a
drastic loss of strength [68]. One of the most important fields
of research in El Salvador is the engineering characterisation
of the tierra blanca, in order to model its behaviour in slopes
subjected to rainfall and due to earthquake shaking, and
similarly to model its modified behaviour after the application
of different stabilisation techniques.
Seismic risk in El Salvador clearly cannot be viewed in
complete isolation from other risks, including those due to
other natural hazards such as floods and volcanic eruption but
also anthropogenic risks such as pollution, deforestation,
crime, poverty, disease and social conflict. The failure to
tackle the challenges of seismic risk, or even to hold back its
increasing levels, is not due to any lack of awareness amongst
Salvadorians of the very high earthquake hazard that affects
their country. Rather the lack of effective measures against
earthquake risk reflects the fact that there are many urgently
pressing needs on limited resources, exacerbated by the
weakness of central and local government. A pessimistic
view of the situation may conclude that earthquake risk
mitigation will only be possible following the solution of
other major social problems in El Salvador. An alternative
view holds that recognition of the interaction of seismic
vulnerability with other features of vulnerability, including
institutional vulnerability, means that concerted programmes
of seismic risk mitigation could provide a vehicle and a
stimulus to the solution of many other issues, including the
current concentration of more than half of the population in
one-third of the national territory. El Salvador will need
external assistance, both in terms of material resources and
technology transfer, to make this vision a reality.
8. Uncited reference
[78].
Acknowledgments
Field investigations of the 13 January earthquake were
funded by the Natural Environment Research Council
(NERC), the Royal Academy of Engineering and the
Universidad Nacional de Colombia. Records from GESAL
instrument in Berlın were provided by Javier Rivas,
Salvador Handal Candray and Jose Antonio Rivas; Griselda
Marroquın kindly provided the records from Nicaragua.
Jose Mauricio Cepeda of the Universidad Centroamericana
provided useful assistance in deciphering incorrect time
codes on records from the TALULIN network and in
providing additional information about the performance of
the network during the earthquakes. We are grateful to
Dominic Dowling of the University of Technology, Sydney,
for his very thorough review of the manuscript. We also
wish to express our sincere thanks to all the people in El
Salvador who, despite the situation into which they had been
thrown, generously assisted us in our field studies of the
earthquakes: their fortitude, resilience and vitality are El
Salvador’s most valuable resources in tackling the problems
posed by natural hazards.
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