Effects of Surface Topography on Seismic Ground Response in the Egion

7/30/2019 Effects of Surface Topography on Seismic Ground Response in the Egion

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Effects of surface topography on seismic ground response in the Egion(Greece) 15 June 1995 earthquake

G.A. Athanasopoulos*, P.C. Pelekis, E.A. Leonidou

Department of Civil Engineering, University of Patras, GR-26500Patras, Greece

Received 23 January 1998; received in revised form 24 July 1998; accepted 12 August 1998

Abstract

The Greek coastal town of Egion on 15 June 1995 was shaken by a strong, small epicentral distance, earthquake that caused heavy

damages to buildings and loss of life. The damages were concentrated in the central elevated part of the town whereas the at coastal regionremained almost intact. This non-uniform distribution of damage is studied in this article in terms of surface topography effects by

conducting seismic response analyses of a simplied 2-D prole of the town. A dynamic nite element code implementing the equiva-

lent-linear soil behavior (FLUSHPLUS) was used for the analyses and it was found that the step-like topography amplied greatly the

intensity of motion without affecting its frequency content. The analyses showed that the motion recorded by an accelerograph installed at

the center of the town is in agreement with the computed values; they also indicated a particularly intense amplication close to the crest of

the steep slope, where a multi-story RC residential building partially collapsed. In contrast, the level of motion was found to be low at the at

coastal zone of the town where the earthquake damages were insignicant. It is concluded that the characteristic surface topography of the

town played an important role in modifying the intensity of base motion. q 1998 Elsevier Science Ltd. All rights reserved.

Keywords: Seismic ground response; Topography effects; Site effects; Finite element method; Dynamic soil properties

1. Introduction

On 15 June 1995 a strong earthquake occurred in the

vicinity of the western end of the Gulf of Corinth in Central

Greece, Fig. 1. The epicenter of the earthquake was located

in the sea between the coastal towns of Egion (in Northern

Peloponnese) and Eratini (in Southern Sterea Hellas).

Although the earthquake damages were spread in a rather

extended area, the hardest hit town was that of Egion and for

this reason the particular earthquake has since been known

as the `Egion 1995 earthquake'. The occurrence of the

Egion 1995 earthquake was followed by the appearances

of almost all the phenomena usually studied under the

general heading of `seismic ground response' i.e. amplica-tion and attenuation of base motion, effects of surface topo-

graphy, ground ruptures, liquefaction and landslides. The

main shock as well as the major events of the aftershock

activity were recorded by an accelerograph installed at the

center of the town and by similar instruments installed in a

number of cities and towns surrounding the epicentral area.

The damage pattern in Egion was not uniform and included

partial collapse of buildings and loss of life.

Following the destructive Egion 1995 earthquake severalresearchers [15,4,16] expressed the suspicion that the

presence of the fault escarpment that runs through the

town might be responsible for some amplication of the

ground motion in the central part of the town. These suspi-

cions were based on the existing knowledge on the subject,

which is briey reviewed in the following.

The effect of surface topography to the seismic ground

response has been the subject of numerous studies during

the last 25 years [17]. These studies have examined the

cases of ridge-or valley- type surface irregularities in a 2-

D form whereas only a limited number of results are avail-

able for 3-D congurations of the problem (e.g. Shanchez-

Sesma et al. [18]). Pioneering work on the subject wasaccomplished by Aki and Larner [19] who introduced a

numerical method based on a discrete superposition of

plane waves; this method was later extended by other inves-

tigators [2022]. Useful results have also been reported by

Wong and Trifunac [23], Wong [24] and Sanchez-Sesma et

al. [25].

Aki [26] used a simple structure of a wedge-shaped

medium to illustrate the effects of topography, Fig. 2(a).

An exact solution exists for SH waves propagating normal

to the ridge and polarized parallel to the ridge axis, which

predicts a displacement amplication at the vertex equal to

Soil Dynamics and Earthquake Engineering 18 (1999) 135149

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0267-7261/99/$ - see front matter q 1998 Elsevier Science Ltd. All rights reserved.

PII: S0267-7261(98) 00041-4

* Tel.: 1 61-997677; Fax: 1 61-997274;; e-mail: geolab.gaa

@upatras.gr.


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2/v, where the ridge angle is np (0 , n, 2). Faccioli [27]

used this triangular wedge structure to model approximatelyridge-valley topography, as shown in Fig. 2(b). This simple

model predicts an amplication at the crest relative to the

base equal to v1/v2 and may be used for rough numerical

estimates of amplications at the crest of ridges or deam-

plications at the bottom of valleys or canyons.

The nite element method, which offers the advantage

of being able to model irregularities of arbitrary shape

G.A. Athanasopoulos et al. / Soil Dynamics and Earthquake Engineering 18 (1999) 135149136

Fig. 1. Map of the western part of the Gulf of Corinth (Greece) with the locations of Egion and other coastal towns and communities, the surface traces of the

ve major normal faults of the region, the routes of the four major rivers (or streams) of the area and the approximate position of the 15 June 1995 epicenter.

Fig. 2. Approximation of ridge/valley topography by triangular wedges

[27].

Fig. 3. Relative distribution of peak horizontal accelerations along a ridge

from Matsuzaki area in Japan [33].


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involving inhomogeneous and non-linear soil materials, has

also been used in studies of surface topography effects

[28,29,17]. Some hybrid methods, combining a particlemodel with the nite element model have also been used

for studying the surface topography effects [30]. In terms of

physical modeling a photographic recording of particle

motion in a 3-D foam rubber physical model has been

used for studying the effects of topography at the Pacoima

Dam site [31]. The results of the studies mentioned above

(and of many others) indicate that the theoretically predicted

values of amplication of motion in steep topography

depend on the relative size of the irregularity (compared

to the incident wave-length), the angle of incidence and

the type of incident wave, i.e. SV vs. SH. Amplication

values range from 3 to 4 in the spectral domain and areless than 2 in the time domain [32].

In addition to the theoretical predictions, the amplica-

tion of surface motion in ridge-or steep slope-type topo-

graphy has also been veried from measurements during

natural earthquake events. The diagram of Fig. 3 depicts

the variation of normalized peak recorded horizontal accel-

erations from ve earthquakes in Japan as a function of

elevation across a ridge [33]. The normalization in this

diagram is referred to the crest motion and in addition to

the mean values, the standard error bars are also included in

the graph. The measurements indicate an amplication at

the crest (relatively to the base) varying from 1.8 to 5.5 with

a mean value of 2.5. In terms of damage patterns, increasingdamages have been reported [34] along the slope and the top

of hills after the Chile 1985 earthquake. A characteristic

example of increased earthquake damages close to the

crest of a step-like topography has been reported by Castel-

lani et al. [28] for the case of the Irpinia 1980 earthquake

and is illustrated in Fig. 4. In this case the damages of an

Italian village sitting at the top of a hill, were concentrated

close to the crest of a steep slope whereas they were insig-

nicant in the direction away from the crest.

It is worth mentioning that when comparing observed and

theoretically predicted amplications of surface motions

due to surface topography, it is usually found that the

observed values are much greater than the predicted ones.

Thus, the observed amplications range from 2 to 20 in thespectral domain and from 2 to 5 in the time domain. The

difference between predicted and observed values is attrib-

uted to the inuence of 3-D effects but it may also be due to

the fact that the measured amplications are actually rela-

tive amplications between points with amplied and

diamplied motion [22].

The article presents the results of a study regarding the

possible effects of surface topography on the seismic ground

response of the central part of the town of Egion. The

ground motion was analysed by using a 2-D nite element

code capable of modeling the surface relief and the strati-

graphy of the area with the aim of explaining the contrast inearthquake damages between the central elevated part of

town and the low and at waterfront area. Before proceed-

ing to the presentation of the main subject, however, some

information is given on seismological, geological, tectonic

and geotechnical data as well as on earthquake damages.

2. Seismological data

The Egion earthquake occurred on 15 June 1995 at 3:16

a.m. local time with a magnitude Ms 6.1 (or 6.2) and an

epicenter lying in the Gulf of Corinth northeast of Egion,

Fig. 1. There are minor deviations in the location of theepicenter of the main shock as reported by different sources

[1 3]. Thus the value of the main shock-epicentral distance

for Egion ranges from 8 to 26 km depending on the reported

location of the epicenter. Also, the reported values of the

focal depth range from 14 to 26 km [3,2]. The strongest

aftershock of the sequence (ML 5.4) occurred 15 min

after the main shock with a much shallower focus and

smaller distance from the town of Egion, whereas the post

earthquake activity was continued for several weeks with a

trend of epicenters moving toward the Peloponnese coast-

line [2]. The analysis of seismic data indicated the presence

G.A. Athanasopoulos et al. / Soil Dynamics and Earthquake Engineering 18 (1999) 135149 137

Fig. 4. Effect of surface topography on damage distribution in the Irpinia (Italy) 1980 earthquake [28].


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of a normal causative fault with the following source para-

meters: seismic moment M0 3.6 1025 dyn cm, fault

length L 13.4 km, stress drop Ds 53 bars and average

displacement u 0.85 m (Chouliaras and Stavrakakis [1]).

An analog accelerograph (SMA-1) installed at the ground

oor of a two-storey reinforced concrete building (with a

basement) in the center of the town, (herein denoted as OTE

site) recorded the strong motion of the main event. The time

histories of the three components of motion (transverse

longitudinalvertical) are shown in Fig. 5. The diagrams

of Fig. 5, in addition to the measured values of acceleration,

also include the calculated time histories of velocity and

displacement as well as the acceleration response spectra

(for 5% critical damping). The orientation of the horizontal


Fig. 5. The acceleration time histories (Transverse, Longitudinal and Vertical components) recorded at the accelerograph station in Egion (OTE site), the

calculated time histories of velocity and displacement, the corresponding acceleration response spectra (for 5% damping) and the orientation of the horizontal

components of the accelerograph.


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components of the accelerograph is shown at the bottom of

Fig. 5. The high intensity of the recorded horizontal ground

motion at the OTE site is remarkable: a peak acceleration

equal to 0.54 g in the T-component and 0.49 g in the L-

component (probably the highest recorded values in

Greece). However, the values of vertical acceleration

remained lower than 0.20 g. According to the time history

of recorded accelerations, the motion actually consisted of

only one or two cycles of strong motion. In terms of hori-zontal ground velocity, it may be seen from the calculated

time histories that the peak values were particularly high:

for both horizontal components they approached the value

of 50 cm/s. It may be further observed that the time history

of horizontal ground velocities consist of a high intensity

pulse involving an increment equal to 70 cm/s. It is worth

mentioning at this point that high ground velocity (and velo-

city increment) values are generated from shallow and near-

eld earthquakes. In such cases the time history of ground

motion bears the characteristics of the source and usually

reveals directivity effects [4]. Fig. 6 shows the in-plan

trajectory of accelerograph motion at the OTE site which

was obtained by combining the T- and L-components of thecalculated displacements. It may be seen that the horizontal

motion involved two major pulses: the rst in an approxi-

mate EW direction and the second in an approximate NS

direction.

According to the acceleration response spectra of hori-

zontal and vertical motion at the OTE site, shown in Fig. 5,

the peak spectral acceleration of the T-component reached a

value of 1.5 g with a predominant period of 0.5 s. It is

believed that this value of period reects the characteristics

of the source mechanism [4], whereas the period of a

secondary spectral peak ( , 0.25 s) is most probably

associated with the soil conditions at the site of accelero-

graph station being equal to the fundamental period of

ground at the OTE site. This value of the fundamental

period may also be derived from the stratigraphy of the

site (Fig. 13) as Ts (4 25)/400 0.25 s. The effective

horizontal acceleration determined in accordance with the

ATC (1978) provisions was found to be equal to 0.43 g

(Lekidis et al. [3]).

As mentioned in the introduction, the main shock of the

Egion earthquake was also recorded in a number of loca-

tions surrounding the epicentral region. Lekidis et al. [3]

have studied the accelerograms from nine accelerograph

stations installed at epicentral distances ranging from 18

to 84 km. They concluded that the earthquake energy was

anisotropically radiated from the source at a mean

frequency of 2 Hz and with attenuation rate depending on

the azimuth of the direction of propagation.

3. Geology and tectonics of the area

The area that was shaken by the Egion 1995 earthquakes

lies at the western end of the asymmetric Corinth graben.

This graben together with the Rio and Patras grabens form

an 140 km long and 40 km wide rift which separates the Pre-

Neogene folded basement of the Sterea Hellas and Pelepon-

nese [57]. A simplied geologic prole of the region is

shown in Fig. 7. The Pre-Neogene basement of the area

consists of Mesozoic carbonates (limestones) and ysch

and it is overlain by thick layers of marls deposited from

Upper-Pliocene to Lower-Pleistocene. The marl deposits

are in turn overlain by Quaternary alluvial fan depositsand fan delta deposits of considerable thickness (mostly

conglomerates). Finally, the surcial layers consist of Holo-

cene beach and river mouth deposits of variable thickness

(gravel, sand, silt and clays).

A number of north-facing WNW-trending active normal

faults have been mapped in NW Peloponnese by Doutsos

and Poulimenos [5]. Five of these faults are crossing the

wider Egion area and their surface traces are shown in the

map of Fig. 1 (Koukouvelas and Doutsos [8]). As shown in

the simplied NS cross-section passing through the town

of Egion, in Fig. 8, these faults have a curved listric geo-

metry and reach depths of about 10 km. The segmented

Egion fault has a total length of 12 km and its position ismarked by an escarpment 40100 m high along a 2 km long

segment which runs through the town of Egion [9]. This

escarpment forms a characteristic hill-front morphology

along the coast of Egion Bay as shown in the photograph

of Fig. 9. Some disagreement seems to exist regarding the

causative fault of the Egion 1995 earthquake. Tselentis et al.

[2] suggested that the main shock was probably generated in

a normal fault at the north side of the Gulf of Corinth,

dipping towards SSE (see Fig. 8) with a dip angle approxi-

mately 708. The main aftershock was then generated in

another normal fault (probably the Egion fault) dipping


Fig. 6. The in-plan trajectory of the horizontal ground motion at the OTE

site during the main shock of 15 June 1995 Egion earthquake.


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towards NNE to the southern side of the Gulf. Koukouvelas

and Doutsos [8] have reported the results of eld observa-tions of surface ruptures (with sizes of a few centimeters)

and of geodetic measurements and suggest that the Egion

fault was reactivated during the 1995 earthquakes. It has also

been suggested that the focus of the earthquakes was hosted

by a low angle normal fault (like the one depicted with dotted

line in Fig. 8) cutting under the Gulf of Corinth and the town

of Egion at depths ranging from 10 to 25 km [3,4,10].

4. Geotechnical data

It was mentioned in the previous section that a segment ofthe fault of Egion runs through the town of Egion in

approximately EW direction and forms a characteristic

escarpment. The map of Fig. 10 shows the Egion town limits

and the horizontal extent of the escarpment. The major part

of the town has been built south to the escarpment on the

elevated region comprising the footwall of the fault. A

representative soil prole of this elevated region and

measured values of Standard Penetration Test blowcount

(NSPT) are shown in Fig. 11, based on borings at the OTE

site [11]. It may be seen that the depth to the conglomerates

at this site is 22 m, whereas the overlying soil layers are

stiff/dense clays, silts and gravels characterized by high

values of NSPT. It should be noted that the water table was

not encountered up to the explored depth of 45 m in this site;

however some perched water tables have been found to exist

in some areas of the elevated region of the town. Regarding

the thickness of the conglomerates it is believed that at the

OTE site might be greater than 150 m.

As shown in Fig. 10 a relatively small part of the town has

been built on the soft deposits of the coastal area lying to the

north of the escarpment. The soil prole of this low-elevation at region is rather variable but the stratigraphy

at the BH10 site [12] depicted in Fig. 12 may be considered

as a representative case. The characteristic feature of this

prole is the existence of a very soft clay layer character-

ized by very low NSPT values encountered at a depth of

10 m and having a thickness of 7 m. Silty sand-gravels are

encountered above and below this soft layer and they are

characterized by rather high values of NSPT. The explored

depth at this site reached only 30 m from the ground surface

so the depth to the conglomerates could not be established

with certainty. It is believed, however, that this depth may

range from 50 m to 60 m from the ground surface. Thewater table in this region of the town is high and it is


Fig. 7. Geologic prole of the Egion area.

Fig. 8. Simplied NS cross-section passing through the town of Egion and showing the geologic prole and the geometry of major faults of the area [5].


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encountered approximately 1.0 m below the surface of the

ground.

5. Earthquake damages

It was mentioned in the introductory section that thedamages from the Egion 1995 earthquake were spread in

an extended area around the epicenter of the earthquake. At

least 20 communities located close to the north coast of

Peloponesse to the east, south and west of Egion were

affected, whereas some damages also occurred in the

towns of Eratini and Itea located at the north coast of Gulf

of Corinth across Egion (see map of Fig. 1). The hardest hit

area, though, was that of Egion. The buildings of Egion area

are either reinforced concrete structures with one to nine

oors or bearing masonry structures with one to three oors.

Fardis [13] has presented preliminary information regarding

the distribution of damages among buildings with different

age, type of construction and number of oors. A correlation

of damage pattern with the local soil conditions has not yetbeen reported with the exception of the observation of a

strong contrast in damages between the waterfront and the

central area of the town. The absence of damages in the

waterfront area of the town becomes more impressive

when it is noted that in this area the buildings are very old

(some already ruined) and without any seismic resistance

provisions [13]. However, a large number of buildings


Fig. 9. Photographic view of the coastal region of Egion looking southward.

Fig. 10. A simplied map of Egion showing the town limits, the extent of the elevated region and the location of points of interest and of the cross-section

A AH

.


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sitting in the elevated region of Egion, suffered heavy

damages including the partial collapse of a six-storey rein-

forced concrete residential building located at a small

distance behind the fringe of the Egion fault escarpment

(DESP site in Fig. 10). Partial collapses of reinforced

concrete buildings also occurred in the village of Valimitika

(four-storey Hotel ELIKI), about 7 km to the east of Egion

and at a site 2 km west of Egion (three-storey Administra-

tion Building of Hellenic Weapons Industry and a nearby

two-storey residential building). Twenty-eight people lost

their life in the earthquake whereas the total cost of damages

was estimated to be $600 million [14]. It is worth mention-

ing that the Egion 1995 earthquake did not result in damages

to the infrastructure of the wider area (roadways, bridges,

retaining walls, port facilities, etc.) It did result, however,

limited soil liquefaction at several coastal sites of Egion

Bay, along the banks of the Selinuntas and Kerynitis river

and along the coastal zone of Rizomylos, Fig. 1. The conse-

quences of soil liquefaction to buildings and other structures

were not signicant and no further mention to soil liquefac-

tion will be made in this article.

6. Surface topography effects

As was mentioned in the introduction, the main objective

of this study was to investigate the possibility of explaining

the differentiation of motion between the coastal area and

the elevated region of the town of Egion by the effects of

surface topography. The authors would like to make clear,

though, that surface topography is only one of the factors

that may be responsible for this differentiation of motion.

According to the seismological data presented in a previous

section the town of Egion is located in the near-eld of the

event and the radiation pattern and directivity of motion

have certainly affected the characteristics of the ground

motions. However, it should be taken into considerationthat the Egion fault was (most probably), not the causative

fault of the earthquake. Any attempt, then, to explain the

differentiation of motion in the two regions of the town in

terms of up-thrown and down-thrown blocks, may be ques-

tionable. In view of the above uncertainties the authors

believe that an investigation of surface topography effects

is worthwhile and could provide some useful insights in the

phenomenon.

7. Surface topography at the town of Egion

In order to study the effects of surface topography on theground seismic response at the town of Egion it was decided

to use a 2-D ground prole which was established by

considering a cross-section of the northern part of the

town along the NS direction. This cross-section was

made along the line AAH

shown in the map of Fig. 10.

The line AAH

was selected in such a way as to intersect

in a right angle the trace of the escarpment and to pass close

to the accelerograph station (OTE site), close to the site of

partial collapse of the RC building (DESP site) and through

a site of the port area where the earthquake caused no

damage (PORT site). The free surface prole along the


Fig. 11. Geotechnical soil prole and corresponding values ofNSPT at the

OTE site.

Fig. 12. Geotechnical soil prole and corresponding values ofNSPT at the

BH10 site.


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line A AH

was constructed by reading distances and surface

elevations from an 1:5000 topographic map of the regionand is shown in a simplied manner in the plot of Fig. 13.

The soil stratigraphy along the 2-D prole was established

by utilizing the geotechnical data presented in a previous

section. As shown in Fig. 13 the established 2-D prole has

a length of about 500 m and a height of 140 m below the

OTE site and 70 m below the PORT site.

7.1. Dynamic properties of soil materials

The dynamic soil properties that are needed in an equiva-

lent-linear type ground response analysis are the low-ampli-

tude shear wave velocity, Vs0 and the G/G0gc and Dgccurves describing the degradation of soil shear stiffness

with increasing amplitude of cycle shear strain, gc (G0

low-amplitude shear modulus, i.e. for gc # 1025, G

higher amplitude shear modulus). Values of Vs0 vs. depth

at the DESP site were obtained by applying the Spectral

Analysis of Surface Waves (SASW) method [35]. As

shown in the diagram of Fig. 14, a great depth of penetration

of surface waves was achieved in this site by utilizing the

drop of a heavy weight (5 kN) on the ground surface.

According to the diagram of Fig. 14, the measured shearwave velocities are remarkably high indicating the great

shear stiffness of soil formations at the elevated region of

the town. For comparison purposes, the diagram of Fig. 14

includes also plots of Vs0 versus depth from crosshole

measurements conducted at the OTE site by the Central

Laboratory of Public Works (CLPW) of the Ministry of

Public Works and Environment [11]. Although the DESP

and OTE sites are about 150 m apart, a good agreement

seems to exist between the two Vs0 vs. depth proles. To

check the reliability of an empirical Vs0NSPT correlation

established by Athanasopoulos [36,37], the diagram of

Fig. 14 includes also a plot ofVs0 vs. depth curve, estimatedby entering the values ofNSPT taken from Fig. 8 into Eq.(1)

Vsom=s 107:6NSPT0:36

: 1

The comparison shows that the agreement is good, espe-

cially when considering the empirical nature of Eq. (1).

Based on the Vs0 vs. depth curves of Fig. 14 the soil strati-

graphy and corresponding Vs0 values for the elevated region

of the town of Egion were established as shown in the 2-D

prole of Fig. 13.

To establish the Vs0 vs. depth variation for the low-eleva-

tion at coastal region of the town SASW measurements


Fig. 14. Vs0 versus depth prole at the OTE site with results of SASW and

crosshole measurements compared with values obtained from Eq. (1).

Fig. 13. Two-dimensional soil prole along the direction A AH

with the Vs0 values used in the seismic ground response analyses.


10/15

were conducted at the PORT site whose location is indicated

in the map of Fig. 10. The diagram of Fig. 15 depicts the

results of measurements. The penetration of surface waveswas smaller in this site and reached a depth of approxi-

mately 90 m. It may be observed from Fig. 15 that the

surcial soil layers at PORT site are characterized by low

Vs0 values. Beyond a depth of 10 m, however, the values of

Vs0 show an increasing trend with depth, whereas at a depth

of about 50 m, an abrupt increase of Vs0 is observed (Vs0 .

1000 m/s). Based on this abrupt increase, it may be assumed

that the depth to conglomerates in this site is equal to 50 m.

For comparison purposes the diagram of Fig. 15, also

includes a Vs0 versus depth curve estimated from Eq. (1),

by using the NSPT values taken from Fig. 12 for the BH10

site, which is about 700 m to the west of PORT site. The

rather large distance between the PORT and BH10 sitesseems to explain the deviations between the two curves.

Based on the results of measurements depicted in Fig. 15

the soil stratigraphy and corresponding Vs0 values for the

coastal region of the town of Egion were established as

shown in the 2-D prole of Fig. 13.

As mentioned previously the description of the non-linear

behavior of the soil materials of the 2-D model requires the

knowledge of the G/G0gc and Dgc curves which are

usually obtained through laboratory cyclic loading tests.

No such experimental data are, however, available for the

soils of Egion area and for this reason it was decided to

resort to the empirical relations reported recently by Ishiba-

shi and Zhang [38]. These relations allow the determination

of G/G0gc and Dgc curves in terms of the plasticity

index, Pl, and the mean effective normal stress, s0H

, of a

soil element. By taking into consideration two different

mean depth levels, two sets of curves were determined for

the PORT site and two more sets for the OTE site. The

determinations were accomplished by using the recently

developed PC program NOLISM [39]. All sets of curves

(OTE1, OTE2 and PORT1, PORT2) are shown in graphic

form in Fig. 16. It should be noted that the OTE2 set of

curves refers actually to rock material and was established

by utilizing the values frequently used for rock material in

the 1-D seismic response program SHAKE91 [40].

7.2. Seismic response analyses

The seismic response analyses of the 2-D ground prole

of Fig. 13 were performed by the 2-D dynamic nite

element program FLUSHPLUS [41]. This PC programmodels soil as linear viscoelastic material and simulates

the non-linear aspect of behavior by the iterative equivalent-

linear method. It should be mentioned that one of the objec-

tives of this study was to take into consideration the non-

linear behavior of surcial soil layers which affects signi-

cantly the ground response in the case of strong motion (as

in the case of Egion earthquake). The input motion is

applied at the rigid base of the model in the form of a

time history of acceleration; this means that the program

can only analyze the vertical propagation of shear waves

from the seismic bedrock to the surface of the ground.

The consideration of vertically propagating SV wavesconstitutes a simplication of the actual phenomenon, espe-

cially in the case of near-eld events involving complex

wave elds. In the case of Egion earthquake, though, the

focal region of the main event seems to lie at a horizontal

distance of about 20 km and at a depth of a similar magni-

tude. By applying Snell's law and utilizing the shear wave

velocities of soil strata (Fig. 13), it may be shown that at

least a portion of the seismic waves arrived at the site inves-

tigated herein following an approximately vertical direction.

One of the advantages of the FLUSHPLUS code is its

capability to use viscous dampers at the lateral boundaries

of the mesh and thus avoid undesirable reections of

outgoing waves. This feature makes possible the use ofnite element meshes with smaller lateral dimensions. The

discretisation of the 2-D ground prole into nite elements

is shown in Fig. 17. In the mesh of Fig. 17 the size of the

nite elements (in particular the vertical dimension) was

selected following the maximum size criteria suggested in

the manual of the program. By using these criteria, the size

of the nite elements remains (appropriately) smaller than

the wavelengths which are expected to be developed during

the passage of seismic waves through the particular soil

formations comprising the 2-D prole of Fig. 13. It should

be mentioned, however, that even in the case of conformance


Fig. 15. Vs0 versus depth prole at the PORT site with results of SASW

measurements compared with values obtained by applying Eq. (1) with

NSPT values of the BH10 site.


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to the above mentioned maximum size criteria, the results of

the analyses may still be affected by the density of the nite

element mesh. The best way to determine the inuence of

the nite element discretisation on the results of the analyses

would be to conduct a convergence type of study in which

the mesh is progressively rened until the results of the

analyses do not change appreciably. A limited convergence

study of this type was conducted herein by using two differ-

ent F.E. meshes. It was found that a two-fold increase in the

number of elements resulted in a 15% change in the


Fig. 16. G/G0gc and Dgc curves used in the seismic ground response analyses of the 2-D soil prole of Fig. 15.

Fig. 17. Finite element discretization of the 2-D soil prole.


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response values without affecting their relative magnitudes.

This change was considered as acceptable for the needs of

the present investigation and it was therefore concluded that

the nite element mesh of Fig. 17 provides reliable results.

The nodes corresponding to the positions of PORT, DESP

and OTE sites are also indicated in the mesh of Fig. 17.

It should be mentioned that the surface topography effects

were investigated in this study in terms of a horizonal base

excitation only, although the FLUSHPLUS code allows the

input of both horizontal and vertical components of base

motion. This simplication was based on the fact that the

peak value of vertical component of motion recorded at

OTE site was a small fraction (37%) of the peak value

corresponding to the vertical direction. Further, the majority

of studies on surface topography effects are conducted in

terms of horizontal base motions.

One of the objectives of the study was to establish a base

motion that when propagated through the 2-D model gener-

ates a response at the OTE site similar to the one recorded

during the main shock of 15 June 1995. In the case of 1-Danalyses this task (deconvolution) can be conveniently

accomplished by using the SHAKE91 code (or other similar

codes) which takes into account the non-linear behavior of

soil materials. In the case of 2-D analysis of the present

study the task of determining the base motion from the

recorded surface motion at OTE site should be accom-

plished by using a 2-D code offering the capabilities of

the SHAKE91 code. Since such a code was not available

to the authors at the time of writing the article the objective

was accomplished by trial-and-error i.e. by providing at the

base downscaled time histories at the recorded horizontal

acceleration (T-component) at the OTE site. The frequencycontent of the input motion was kept identical to the

frequency content of the recorded motion on the premise

that the recorded accelerogram at the OTE site reects the

characteristics of the source and was not modied in

terms of frequency content by the soil stratigraphy and

topography. By following this trial-and-error procedure it

was found that a downscaled base motion with a peak hori-

zontal acceleration equal to 0.14 g, was necessary in order to

produce a response at the OTE site that approximately

matched the recorded motion (T-component). A comparison

of the computed and actually recorded time histories of

accelerations at OTE site is shown in the diagram of Fig.

18. This comparison indicates a good agreement betweenthe two time histories. A further comparison of the corre-

sponding acceleration response spectra, however, shows a

deviation of the computed value of peak spectral accelera-

tion which is approximately 50% higher than the recorded

one. There is no deviation, however, regarding the values of

predominant period at computed and recorded horizontal

motion. It should be noted that despite a large number of

trials in some of which not only the intensity but also the

frequency content of input motion was varied it was not

possible to establish a horizontal base motion that produced

a surface motion matching the recorded motion at the OTE


Fig. 18. Computed seismic response at the OTE site from 2-D analyses

compared to the recorded values in the 15 June 1995 Egion earthquake.

Fig. 19. Computed seismic response at the DESP site from 2-D analyses.


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site in terms of both the time history and the response spec-

trum. It was, therefore, decided to accept and use as an input

motion the recorded horizontal acceleration time history at

the OTE site (T-component) downscaled to a peak value of

0.14 g. Based on the above results it is concluded that the

motion at OTE site was greatly amplied with respect to thebase motion (amplication factor < 3.80). This amplica-

tion of horizontal motion cannot be attributed to 1-D reso-

nance, since the fundamental period of soil prole at OTE

site is approximately equal to 0.25 s, whereas the predomi-

nant period of input motion is 0.5 s. It seems, therefore,

appropriate to assume that the 2-D topography of the area

has resulted in the great differentiation of motion.

By using the base input motion established above, the

response at the DESP site was estimated and the results

are shown in graphical form in Fig. 19. A very strong ampli-

cation of motion is indicated at this site: the peak horizon-

tal acceleration is equal to 0.79 g, whereas the spectral

acceleration approached the value of 5 g with a predominantperiod equal to 0.5 s. Although these results may have been

amplied by articial resonance effect an inherent

problem in equivalent-linear seismic ground response

analyses they still indicate a tendency for a strong ampli-

cation of horizontal ground motion close to the crest of the

slope. This trend seems to be in agreement with the results

of eld observations reported in the Introduction (Fig. 4) and

with the damage pattern observed in this region of the town

partial collapse and heavy damages of RC buildings and

discussed in a previous section.

The response at the PORT site is shown in graphical form

in Fig. 20 The results indicate a much lower level of hori-

zontal motion: the peak horizontal acceleration is equal to

0.25 g and the maximum spectral acceleration is equal to

0.70 g with a predominant period at 0.5 s. These results are

again in agreement with the impressive lack of damages in

the waterfront area (including the undamaged Port facilities)

of the town which was mentioned in a previous section. It is

worth mentioning that the fundamental period at the PORT

site may be estimated (from the stratigraphy shown in Fig.

13) to be approximately equal to 0.5 s a value that almost

coincides with the predominant period of the base motion.

In case the wave eld was governed by a 1-D propagation, a

resonance should have been developed and the surface

motion at the PORT site could have been amplied (with

respect to the base motion) to a greater degree compared to

the OTE site. The fact that this trend was not observed in the

results of response analyses of this study seems to indicate,

again, that the surface response is governed by a 2-D propa-

gation of seismic waves i.e. the surface topography has

played a rather signicant role in modifying the groundmotion of the area.

The results of the 2-D seismic response analyses that were

presented above, seem to indicate a signicant surface topo-

graphy effect on the ground response during the Egion 1995

earthquake. Before reaching the nal conclusions, however,

it would be appropriate to examine the capability of 1-D

response analysis to duplicate the results obtained by the

2-D analyses. To accomplish this check the program

SHAKE91 was used to estimate the 1-D ground response

at the sites of OTE and PORT. The dynamic soil properties,

soil stratigraphy and depth to the base of each site were

identical to the ones used in the 2-D analysis. By usingthis approach the peak horizontal acceleration at the OTE

site was found to be equal to 0.69 g, a value that could be

accepted as not being too far from the results of the 2-D

analysis. At the PORT site, however, the surface accelera-

tion reached a value equal to 0.53 g. This value is more than

twice the value obtained by the 2-D analysis and it demon-

strates the inability of 1-D analyses to predict the surface

motion in this part of the town. It becomes, therefore, possi-

ble at this point to summarize the ndings of this study and

state the following conclusions.

8. Conclusions

The effects of surface topography in the Egion 1995

earthquakes was studied by establishing a 2-D ground

prole along a cross-section of the town. This section cuts

through the central part of the town, has a NS direction and

crosses in a right angle the fault escarpment that runs

through the town in the EW direction. The response of

the ground surface along the direction of the cross-section,

was analysed by a nite element code and implementing the

equivalent-linear method. It was found that the peak hori-

zontal acceleration of the seismic base of the area does not


Fig. 20. Computed seismic response at the PORT site from 2-D analyses.


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need to be greater than 0.14 g in order to generate the

recorded surface motion at the OTE site. This base motion

was greatly amplied (290%) at the elevated region of thetown as shown schematically in Fig. 21(a) whereas at sites

close to the fringe of the slope (DESP site) the amplication

was even greater: 460%. This behavior seems to be in agree-

ment with both theoretical results and eld measurements

and observations presented in reviewing the existing knowl-

edge on the subject as well as with the damage pattern in the

town. By normalizing the surface motion with respect to the

recorded motion at the OTE site Fig. 21(b), it may be seen

that close to the fringe of the elevated region of the town the

motion is amplied by 47% whereas it is deamplied by

57%67% at the waterfront low-elevation at region. It is

therefore concluded that the characteristic topography of the

town played an important role in modifying the intensity ofbase motion.

Acknowledgements

The authors express their thanks to Drs G. Stavrakakis

and I. Kalogeras of the Geodynamics Institute of the

National Observatory of Athens, Greece, for providing

accelerograph records for the Egion area in digital form.

Thanks are also expressed to the Hellenic Earthquake Plan-

ning and Protection Organization for the nancial assistance

during the in situ measurement of dynamic soil properties in

Egion area as well as to Mr P. Theodorou and to the Civil

Engineering students at the University of Patras, D.

Kostouros and N. Roumeliotis for their assistance during

the in situ tests and in collecting maps of the Egion area.

Finally, thanks are expressed to the anonymous reviewers of

the article whose comments helped the authors to address

some important issues and clarify some aspects of their

approach of examining the surface topography effects.

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Effects of Surface Topography on Seismic Ground Response in the Egion

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