s-wave velocity based on microtremor array
TRANSCRIPT
S-wave Velocity Structure of Mexico City Obtained from Three-component
Microtremor Measurements and Microtremor Array Measurements
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Koichi Hayashi*, GeometricsAtsushi Nozu, Port and Airport Research Institute,
Masanori Tanaka, Port and Airport Research Institute Haruhiko Suzuki, OYO Corporation
Efraín Ovando Shelley, Universidad Nacional Autonomade Mexico
Outline
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• Introduction
• Investigation site
• Data acquisition EquipmentH/V spectrum Dispersion curve
• Analysis results
• Comparison with United States
• Conclusions
Introduction
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• The earthquake that struck Mexico on 19 September1985 caused severe damage in Mexico City althoughthe city is located 400km away from the epicenter.
• The main reason for this damage is that the city islocated on a basin filled with very soft sediments.
• Distribution of these soft sediments has beendelineated by drillings and microtremor measurements.
• A small number of attempts have been made to image the S-wave velocity structure of the basin using downhole seismic loggings.
• In order to delineate S-wave velocity structure of thebasin down to depth of approximately 200m, we have performed three-component micro-tremormeasurements and microtremor array measurements.
1985 Mexico Earthquake
Mexico City
400km
Mw=8.3
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1985 Mexico Earthquake
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• Mw=8.3
• Most-often cited number of deaths is an estimated 10,000 people but experts agreed that it could be up to 40,000.
• Damage area corresponds to the western partof the lake zone within 2 to 4 kilometers of theAlameda Central.
• 6 to 15 story buildings are mainly damaged in the city due to a frequency range of 0.25 to 0.5Hz (period of 2 to 4seconds)
> 2sec
Natural Period (H/V) of Mexico Basin
Lermo and Chavez-Garcia (1994)
Central Mexico City
Airport
Damage Area
> 4sec
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Investigation Site
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• Investigation site is placed at the downtown of Mexico City.
• 30km length survey line crosses the basin with awest-southwest to east-northeast direction.
• 3 component microtremor measurements were performed at more than 10 sites on the line.
• Microtremor array measurements wereperformed at 6 sites on the line.
• Microtremor array measurements used 25 to650m equilateral triangular arrays.
Chapultepec
AragonTexcoco No.7
Almeda
Texcoco No.8
Texcoco TXC
Investigation Site
Central Mexico City
Airport
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Example of Array Configuration
19.422938
-99.182564
19.423838
99.18416319.423818
99.180783
19.42138
99.182296
330m19.46228399.067562
19.46396299.066875
19.462999.063903
19.46473199.069911
19.46181899.06931
19.46098899.066317
19.45923899.068806
650m
Chapultepec
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Aragon
Data Acquisition
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• Data acquisition was carried out during the daytime inDecember 2008 and December 2009.
• Microtremor measurement systems (JU210) made by Hakusan Corporation and data loggers (GPL-6A3P) made by Mitsutoyo Corporation were mainly used for dataacquisition.
• Both systems use accelerometers for the sensors.• In order to verify applicability of the accelerometers, servo-
type velocity meters made by Katsujima Corporation (SD-110) and Tokyo Sokushin Corporation (VSE11F, VSE12F)were also used in the 3 component microtremormeasurements.
• H/V spectra obtained through the accelerometers and thevelocity meters were compared.
• 30 min. to 1 hour of microtremors were recorded for eachthree component measurement or array measurement.
Equipment(accelerometer)
Hakusan
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Mitsutoyo
Comparison of H/V
It is clear that peak frequency of H/V decrease from west to east(from edge to center of basin) 12
600
500
,-...(/')
.'..E....., 4oo>-
.+-'( .)
0 300(J)
>I(J)
200
a..
100
0
0.1
• Texcoco Main No.7(2008)
• Texcoco North No.8(2008)
• Aragon(2009)
• Almeda (2009)
<>Chapult epec(2009)
\ c
1
Frequency(Hz)
10 100
Sites where the peak frequency of H/V spectra is higher,
the phase velocity of the dispersion curve is also higher 13
\,•
9
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:
-
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.o.c.
=
Analysis (1)
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• A joint inversion was applied to the observed H/V spectra and dispersion curves, and S-wave velocity models were analyzed for six sites.
• In the inversion, phase velocities of the dispersion curves and peak frequencies of the H/V spectra were used as the observation data.
• Unknown parameters were layer thickness andS-wave velocity.
• A Genetic algorithm was used foroptimization.
Analysis (2)
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• Initial models were created by a simple wavelength transformation in which wavelength calculated from phase velocity and frequency is divided by three and plotted at depth.
• Theoretical H/V spectra and phase velocities are generated by calculating the weighted average of the fundamental mode and higher modes (up to the 4th modes) based on medium response.
• Rayleigh-Love ratio (R/L) is fixed as 0.7
S-wave Velocity Model(East)Texcoco No.7
Aragon 0
50
100
150
200
250
300
350
400
450
500
0 0.2 0.4 0.6Vs(km/ s)
De
pth
(m
)
S-wave Velocity Model(West)Almeda
Chapultepec
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Chapultepec
AragonTexcoco No.7
Almeda
Texcoco No.8
Texcoco TXC
Peak Frequency of H/Vand S-wave Velocity Model
0.7s
90m/s
1.5s
80m/s
3.9s
60m/s
3.3s
30m/s
1.1s
60m/s
>400m/s450m
<100m/s
≒200m/s
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Comparison with United StatesH/V Spectra Dispersion Curve S-wave Velocity Model
S-wave velocity of Mexico is extremely lowcompare with San Jose and Redwood City.
Mexico CitySan Jose (William St. Park), CARedwood City, CA
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0
50
100
150
200
250
300
350
0.1 1 10 100
Phas
e-vel
oci
ty(m
/sec)
Frequency(Hz)
Mexico
San Jose
Redwood City
0.1
1
10
0.1 1 10
H/V
spec
tra
Period(sec)
Mexico
San Jose
Redwood City
0
20
40
60
80
100
120
140
160
0S-wave velocity (m/sec)
100 200 300 400 500
Dep
th(m
)
Mexico
San Jose
Redwood City
Conclusions• We have performed the three-component
microtremor measurements and microtremorarray measurements in the Mexico basin and estimated the S-wave velocity models down to a depth of 200m.
• S-wave velocity in the middle of the Mexico basin is lower than 150m/s to a depth of 70m and much lower than typical alluvial plains in Japan and United States.
• Peak frequencies of the H/V spectra in Mexico City vary from 0.25 to 1Hz and it seems that these peak frequencies are mainly due to thelow-velocity layer shallower than a depth of 100m.
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