1) The study performed three-component microtremor measurements and microtremor array measurements at sites across Mexico City to delineate the S-wave velocity structure down to 200m depth. 2) Analysis of the H/V spectra and dispersion curves using a joint inversion technique produced S-wave velocity models for six sites showing significantly lower velocities in Mexico City compared to sites in the US, with velocities less than 150m/s down to 70m depth. 3) The peak frequencies of the H/V spectra across Mexico City from 0.25-1Hz correspond to a low velocity layer shallower than 100m depth, which helps explain the amplification and damage from earthquakes.

1. S-wave Velocity Structure of Mexico
City Obtained from Three-component
Microtremor Measurements and
Microtremor Array Measurements
1
Koichi Hayashi*, Geometrics
Atsushi Nozu, Port and Airport Research Institute,
Masanori Tanaka, Port and Airport Research Institute
Haruhiko Suzuki, OYO Corporation
Efraín Ovando Shelley, Universidad Nacional Autonoma
de Mexico

2. Outline
2
• Introduction
• Investigation site
• Data acquisition
Equipment
H/V spectrum
Dispersion curve
• Analysis results
• Comparison with United States
• Conclusions

3. Introduction
3
• The earthquake that struck Mexico on 19 September
1985 caused severe damage in Mexico City although
the city is located 400km away from the epicenter.
• The main reason for this damage is that the city is
located on a basin filled with very soft sediments.
• Distribution of these soft sediments has been
delineated 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 the
basin down to depth of approximately 200m, we have
performed three-component micro-tremor
measurements and microtremor array measurements.

4. 1985 Mexico Earthquake
Mexico City
400km
Mw=8.3
4

5. 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 part
of the lake zone within 2 to 4 kilometers of the
Alameda 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)

6. > 2sec
Natural Period (H/V) of Mexico Basin
Lermo and Chavez-
Garcia (1994)
Central Mexico City
Airport
Damage Area
> 4sec
6

7. Investigation Site
7
• Investigation site is placed at the downtown of
Mexico City.
• 30km length survey line crosses the basin with a
west-southwest to east-northeast direction.
• 3 component microtremor measurements were
performed at more than 10 sites on the line.
• Microtremor array measurements were
performed at 6 sites on the line.
• Microtremor array measurements used 25 to
650m equilateral triangular arrays.

8. Chapultepec
Aragon
Texcoco No.7
Almeda
Texcoco No.8
Texcoco TXC
Investigation Site
Central Mexico City
Airport
8

9. Example of Array Configuration
19.422938
-99.182564
19.423838
99.184163
19.423818
99.180783
19.42138
99.182296
330m 19.462283
99.067562
19.463962
99.066875
19.4629
99.063903
19.464731
99.069911
19.461818
99.06931
19.460988
99.066317
19.459238
99.068806
650m
Chapultepec
9
Aragon

10. Data Acquisition
10
• Data acquisition was carried out during the daytime in
December 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 data
acquisition.
• 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 microtremor
measurements.
• H/V spectra obtained through the accelerometers and the
velocity meters were compared.
• 30 min. to 1 hour of microtremors were recorded for each
three component measurement or array measurement.

11. Equipment(accelerometer)
Hakusan
11
Mitsutoyo

12. 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

13. 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)
<>Chapultepec(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
<>
:
-
·l.llt>
.o.c.

14. Analysis (1)
14
• 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 and
S-wave velocity.
• A Genetic algorithm was used for
optimization.

15. Analysis (2)
15
• 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

16. 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 0.6
Vs(km/ s)
Depth(m
)

17. S-wave Velocity Model（West）
Almeda
Chapultepec
17

18. Chapultepec
Aragon
Texcoco No.7
Almeda
Texcoco No.8
Texcoco TXC
Peak Frequency of H/V
and 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
18

19. Comparison with United States
H/V Spectra Dispersion Curve S-wave Velocity Model
S-wave velocity of Mexico is extremely low
compare with San Jose and Redwood City.
Mexico City
San Jose (William St. Park), CA
Redwood City, CA
19
0
50
100
150
200
250
300
350
0.1 1 10 100
Phase-velocity(m/sec)
Frequency(Hz)
Mexico
San Jose
Redwood City
0.1
1
10
0.1 1 10
H/Vspectra
Period(sec)
Mexico
San Jose
Redwood City
0
20
40
60
80
100
120
140
160
0
S-wave velocity (m/sec)
100 200 300 400 500
Depth(m)
Mexico
San Jose
Redwood City

20. Conclusions
• We have performed the three-component
microtremor measurements and microtremor
array 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 the
low-velocity layer shallower than a depth of
100m.
20