Post on 13-Jul-2020
Revisiting the 1960 Chilean Earthquake
-For the 50th Anniversary –
Hiroo KanamoriSeismological LaboratoryCalifornia Institute of Technology
In collaboration with Luis Rivera, and with help from Stewart Smith, and Frank Press.
1960 Chilean Earthquake (Mw=9.5)
Believed to be the largest in the last century.
cf. 1964 Alaskan Earthquake, Mw=9.22004 Sumatra-Andaman Is. Earthquake, Mw=9.21952 Kamchatka Earthquake, Mw=9.0
Is it really the largest ?
Is something special ?
What have we learnt from it?
Dextral (right-lateral) Rotation of the Pacific Benioff (1958)
San Andreas
KamchatkaHodgson’s (1956) mechanism
Background, e.g., Benioff’s Idea
Japan, Right-lateral (Median Tectonic line)
Saint-Amand, 1961
200 km
Aftershocks immediately after the 1960 earthquake
Static (uplift, subsidence, strain etc)
Plafker and Savage (1970)
Plafker and Savage (1970)Plafker (1972)
up
lift
up
lift
sub
sid
ence
Vertical Displacement (subsidence and uplift)
up
lift
Plafker and Savage (1970) Uplift-Subsidence data and Models
Model B
Moreno et al. (2009)Barrientos and Ward (1990)
Mw=9.26
Linde and Silver (1989)
deep slip
Larger Mw (=9.45)
Seismology
Normal modeSurface waves
1960 Chile, PAS =83 , =135
One-day seismogram of the 1960 Chilean earthquake (Mw=9.5)
Interpretation of Pasadena strain record (Kanamori and Cipar, 1974)
Mw=9.55
Uncertainties: dip angle, source finiteness, instrument response
Mw=9.35
1960 Chile, Palisade R4
Synthetic HK M0=2.7x1023 N-m sf=l80
4 c
m
500 s
Obs.
Syn.
Mw=9.55
1990
2010
2000
9.0 9.1 9.2 9.3 9.4 9.5 9.6 9.7 9.81960
1970
1980
Mw
static
seismic
“precursor”
History of the 1960 Chilean Earthquake Mw estimates
PSP
BW
B
B
S
KC
LSCS
OT
BZ
“relative”
MBKM
1990
2010
2000
9.0 9.1 9.2 9.3 9.4 9.51960
1970
1980
Mw
static
seismic
History of the 1964 Alaskan Earthquake Mw estimates
PHS
MM
KBRI
O
A
P
An important record is the strain seismogram recorded at Isabella, California.
This record is of historical importance because it provided the first observations of the Earth’s free oscillations.
Bullen (An Introduction to the Theory of Seismology, 1963)
Then in 1960 at the Helsinki meeting of the I.A.S.P.E.I., there occurred one of the most dramatic scientific sessions this author has witnessed.
Press announced that Benioff had once again observed long-period waves ….
Slichter announced ….
Lamont result ……
1960 Chilean Earthquake, Isabella Benioff Strain Record
Computed Earthtide
Chile
ISA Back Azimuth to Chile, N145.7 W
Benioff et al. (1961) N32 Wα=2.3
N
Angle (α) between the ISA strain rod and the great circle to Chile
α
L(G) to R sensitivity ratio
L
R
Strain rod
Incident wave
2
sin(2 )
cos
Smith (1966)
54 min 7.5 min
Normal-mode spectrum (from 7.5 min to 54 min) of the 1960 Chilean and the 1964 Alaskan earthquakes recorded with the ISA strainmeter.
Smith (1966)
54 min 7.5 min
Toroidal Modes on the ISA Spectrum (Red)
Observed Mode Energy Ratio of Chile to Alaska earthquakes, Smith(1966)
Normal mode spectral amplitudes of the 1960 Chilean earthquake are about 2 to 5 times larger than those of the 1964 Alaskan earthquake.
This does not necessarily mean that the Chilean earthquake is correspondingly larger (in M0) than the Alaskan earthquake.
Unknown in 1966Can be computed now
0
0
0Chile
0Ala
unitChile
unitAlask
Chile
Alaska ska
a
Excitaobserved amp
observ
tion
Excitationed amp
M
M
M
M
Determined in 1966
116
134
N N
ISA
ISA
Alaska Chile
Rupture-Station Geometry
Ru u
eR R
From 285 min to 8139 min (about 5.45 days)
For Chile K&C (1974) s=10°, =10°, =90°, d=35 km
modified s=10°, =17°, =90°, d=20 km
For Alaska K (1970) s=246°, =20°, =90°, d=75 km
modified s=246°, =10°, =90°, d=20 km
Computation of Strain Spectrum
0
0.05
0.1
0.15
0.2
0.25
0.3
-200 0 200 400 600 800 1000 1200
Data 3
Chile (uniform)
Chile (B-W)
Ch
ile (
un
ifo
rm)
Distance, km
0
0.05
0.1
0.15
0.2
0.25
0.3
-200 0 200 400 600 800 1000 1200
Data 3
Alaska (uniform) (uniform)Alaska (Tapered)
Ala
ska
(u
nifo
rm)
(un
ifo
rm)
Distance, km
Slip distribution
Chile
Alaska
Barrientos and Ward (1990)
Tapered
uniform
uniform
1960 Chile, ISA strain spectra (285 to 8139 min) Unit Moment (1x1023 N-m, s/d/r=10/17/90, d=20km)
Nan
ost
rain
-sec
23
4
56
7 8 9 10 1112
13
14
2
3
4
0 0.5 1.0 1.5 2.0 2.5Frequency, mHz
Av. 2-14 15003
average
Finite source (B-W)
5
6 8 910
1964 Alaska, ISA strain spectra (285 to 8139 min) Unit Moment (1x1023 N-m, s/d/r=245/10/90, d=20 km)
Nan
ost
rain
-sec
0 0.5 1.0 1.5 2.0 2.5Frequency, mHz
2
3
4 5 6 7 8
9
1011 12
1314
2 3
4
Av. 2-14 15003
average
Finite source(Tapered)
5
6
8 109
Angular order, l(7.5 min)(54 min)
Ch
ile/A
lask
a M
om
ent
Rat
ioChile/Alaska Moment Ratio
1
10
2 3 4 5 6 7 8 9 10 11 12 13 14
Data 2 3:32:13 PM 9/24/2009
uncorrected Smith (1966)Excitatio-corrected Point sourceExcitation-corrected finite (variable)Excitation-corrected finite (uniform)
Sq
rt o
f F
ig.
5
AMode #
1990
2010
2000
9.0 9.1 9.2 9.3 9.4 9.5 9.6 9.7 9.81960
1970
1980
Mw
static
seismic
“precursor”
History of the 1960 Chilean Earthquake Mw estimates
PSP
BW
B
B
S
KC
LSCS
OT
BZ
“relative”
MBKM
Revisiting the Toroidal modes
Comparison of the 1960 Chile (Valdivia) and the 2010 Chile (Maule) Earthquakes
Top: The strainmeter record of the 1960 earthquake at ISA (NW component). This is one of the most importanthistorical records in seismology from which the first observation of the Earth’s free oscillations was made.
Bottom: The strainmeter record of the 2010 earthquake at PFO (NW component).
Note the large even-order G waves (Love waves and toroidal modes) on the 1960 record and the almostcomplete absence of them on the 2010 record, suggesting significantly different mechanisms for the twoearthquakes.
Chile
PFO Strain rodN45 W α=10.5
N
Angle (α) between strain rod and the great circle to Chile
Sensitivity to transverse motion sin(2α) Back Azimuth to Chile,
N145.7 W
ISA Strain rodN38.8 W α=4.5
34.3
Smith (1966)
54 min 7.5 min
Toroidal Modes on the ISA Spectrum (Red)
T 3 4 5 6 8 9 10
To increase G/R (or T/S) ratio, increasing the strike-slip component is most effective.
No thrust mechanism can explain the observed ratio.
Free oscillation patterns
Spheroidal mode
Toroidal mode
Courtesy of Dr. Michel Van CampAnimation from Hein Haakhttp://www.knmi.nl/kenniscentrum/eigentrillingen-sumatra.html
earthquakes.
Isabella strain seismograph worked normally (i.e., no anomalous L/R ratio) for the 1964 Alaska earthquake (after) and the 1957 Mongolian earthquake (before).
1964 Alaska ISA NW strain L/R sensitivity=0.42
R2G2 R3
U=4.4 km/s for G2
U-3.5 km/s for R2
U=3.5 km/s for R3
overtone
1964 Alaska ISA Strain NE M0=7.5x1022 N-m (s/d/r=245/10/20) (Network, sf=l50s)L/R sensitivity=9.5
G2 G4
stra
in
10000s
G1
R1
R2G2
1957 Mongolian earthquake recorded at ISA
time, s
1957 Mongolian earthquake recorded at ISA(strain meter + network + galvanometer)
Integrated strain
Comparison between spheroidal and toroidal mode excitation (l=2 to 10)
spheroidal toroidal
=90
130
140
150
Observed (FFT)
Precursor?About 15 min before the mainshock.Large (comparable to the mainshock)
Kanamori and Cipar (1974)Pasadena strain meter
Kanamori and Anderson (1975)Normal mode
Cifuentes and Silver (1989)Normal mode (more complete than K&A)
Pasadena Spectrum
Kanamori and Cipar (1974)
1000 s
Mainshock PForeshock P S
1960 Chilean earthquake, PAS Benioff Strain (NS) 180-4-10“Precursor”
G2 R2
P, S, G1,R1
Mainshock P
3000 s
G2 and R2U=7 km/s
G3 and R3U=7 km/s
G4 and R4U=7 km/s
G2 R2
P
observed
syntheticwithout precursor
1960 Chilean earthquake, PAS Benioff Strain (NS) 180-4-10
bp n 4 c 0.00125 0.01 p 2
P, S, G1, R1
Precursor to (1) Precursor to (2)
Cifuentes (1989)
Foreshock-Mainshock sequence
8.3 7.8
Mainshock
Where does the precursory deformation occur?
Cascadia Current model (Geological Survey of Canada)
Moment-rate Spectrum(with ω2 reference spectra)
Mo
me
nt,
N-m
Frequency, Hz
Mw=9.5
9.0
8.5
8.0
7.5
2004 Sumatra Mw=9.2
2010 Chile Mw=8.8
1960 Chile Mw=9.5
Hartzell and Heaton, 1985
Also:mB_hat= 7.6 for both 1960 Chile and 1964 AlaskaHouston and Kanamori (1986)
mB_hat=7.2 for the 2004 Sumatra Kanamori( (2006)
Estimated from ISA strain Smith (1966),also tsunami Abe (1979, 2010)
Estimated from PFO strain for the 2010 event
Conclusion
1. The 1960 Chilean earthquake is probably 2 to 5 times (in M0) larger than the 1964 Alaska earthquake. (super-cycle event?)
2. The existence and mechanism of precursor are still inconclusive.
3. Slip or deformation may have to be invoked in somewhere other than the mega-thrust boundary. ( e.g., deep slip or deformation)
4. “Super-cycle event” may involve a different deformation pattern.Most likely, half thrust and half right-lateral.
5. The super-cycle event can be different from other “average” great earthquakes. Strike slip strain is not released in every great
earthquake, and only when it accumulates over several events it triggers a super-cycle event.
Caveat:
Old data are inevitably incomplete and uncertain.
End