S-wave Velocity Structure of Mexico City Obtained from Three-component

Microtremor Measurements and Microtremor Array Measurements

1

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

2

• 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

5

• 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

7

• 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

9

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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:

-

·l.llt>

.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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