13th World Conference on Earthquake Engineering
Vancouver, B.C., Canada
August 1-6, 2004
Paper No. 862

EVALUATION OF THE AMPLIFICATION CHARACTERISTICS OFSUBSURFACE USING MICROTREMOR AND STRONG MOTION- THE STUDIES AT MEXICO CITY Tsutomu SATO1, Yutaka NAKAMURA2 and Jun SAITA3

SUMMARY
This paper examined the relationship between the H/V spectral ratio of strong motion and theamplification characteristics or the H/V spectral ratio of microtremor. Both the data of strong motion andmicrotremor have been recorded at the lake, transition and hill zones in Mexico City. As a result, in caseof strong motion, the amplification characteristic agreed with the H/V spectral ratio approximately.
Moreover the H/V spectral ratio of strong motion agreed with that of microtremor at the same site.
However, at some sites of soft ground on irregular basement, the shape of the H/V spectral ratio ofmicrotremor and the strong motion are almost similar for each other but the H/V spectral ratio ofmicrotremor is less than the other. At such a site, microtremor contains Rayleight wave component andalso strong motion contains relatively less Rayleigh wave component. It suggests that Rayleigh wave has
influenced to this situation.
INTRODUCTION
In recent years, microtremor is measured as a useful tool to estimate easily the vibration characteristics ofsurface ground in the various places. And H/V spectral ratio made use of the analysis of microtremor andstrong motion is noticed as an analysis method to describe simply and dynamic characteristics of theground or the structure accurately. H/V spectral ratio was developed from an analysis of strong motion
data on the various ground conditions.
It is supposed that because the body wave mainly contributes the seismic damage, ground amplification
occurred by multi reflection of SH wave in the superficial layer. Spectral ratio of the surface and basement(Hs/Hb) is large value generally disturbed by the influence of surface wave. And so, Nakamura considered
Vb/Vs as revised spectra to remove the influence of Rayleigh wave, predominant from mixed variouswaves. It is expected that if Rayleigh wave doesn’t exist, the Vb/Vs of microtremor will be 1 in thefrequency range of horizontal vibration predominating and be smaller than 1 in the frequency range ofRayleigh wave energy predominating. Nakamura et al. suggested that more certain amplification
1

Research engineer, System and Data Research Co., Ltd., Tokyo, Japan. Email: tsato@sdr.co.jp
Dr., President, System and Data Research Co., Ltd., Tokyo, Japan. Email: yutaka@sdr.co.jp
3
Research engineer, System and Data Research Co., Ltd., Tokyo, Japan. Email: jun@sdr.co.jp
2

Fig.1 The target observation points for strong motion in Mexico City

characteristic estimated by Hs/Hb spectral ratio multiplied by Vb/Vs. However, Hb/Vb actually is almost 1 in
the wide range of frequency, so strong motion characteristics of surface ground can be estimated only
from the Hs/Vs ratio on surface. This is the H/V spectral ratio.
This paper examined that it can be estimated the amplification characteristics of strong motion from H/Vspectral ratio of microtremor, and is pointed an attention to use microtremor for estimating strong motion
characteristics.
STUDY AREA FOR THE INVESTIGATION OF GROUND MOTION CHARACTERISTIC
The investigated area is central Mexico City as shown in Fig.1. Mexico City has developed on a reclaimed
lake since 16th century. Therefore, almost all the central city locates on soft sedimentary ground. The
represented earthquake (1957, 1979 and 1985 Michoacan) caused a sever damage to this area. And also,
although it didn’t cause heavy damage, an earthquake (2003, Mw7.8) occurred near Colima City, 400
kilometers away from Mexico City. At this time the maximum acceleration about 50 Gal on the soft
ground was recorded in Mexico City.
ANALYSES AT THE EACH OBSERVATION POINTS FOR STRONG MOTION
Microtremor measurement was conducted to confirm whether it was possible to estimate a dynamic
characteristic of surface ground from microtremor at several strong motion sites in Mexico City, gathered
the strong motion data recorded before, and analyzed with measured microtremor data as follow.
1. Comparing the horizontal to vertical spectral ratio (H/V spectral ratio) of strong motion with theamplification characteristics of strong motion.
2. Comparing the H/V spectral ratio of strong motion with the H/V spectral ratio of microtremor.
3. Confirming to be able to estimate an amplification characteristic of strong motion from H/V spectral
ratio of microtremor, based on the similarity of the H/V spectral ratio of strong motion and H/Vspectral ratio of microtremor at the same site.

Fig.1 shows some observation points of strong motion we measured microtremor in Mexico City. Table 1
shows maximum acceleration of each observation points at Colima earthquake (Jan. 22, 2004). The
analyzed points in this study were total 17 points located typical grounds (Lake, Transition and Hill) inMexico City. These points have been installed and managed by CIRES (Centro de Instrumentacion yRegistro Sismico). Totally 84 observation points are installed in Mexico City.
Microtremor was recorded 3 times at about 41 seconds (4096 data in 100Hz sampling) on each site. A
three component (NS, EW and UD direction) sensor installed on the basement for seismometer. Aftertaking H/V spectral ratio on each site, it was compared with H/V spectral ratio of strong motion recordedat same site. On strong motion records, besides of taking H/V spectral ratio, amplification characteristics
was estimated using CS78 site at Hill zone for a reference site.
Table1 The maximum acceleration of Colima earthquake (Mw7.8, 2003) ateach observation points for strong motion in Mexico City(Gal)observatio
n point
UI21
CS78
IM40
TP13
CE18
EO30
DX37
CI05
CO56
BL45XO06
HJ72
CA59
DM12
AE02
CJ03
JA43UD

NS

EW

division

2.9
2.8
2.7
2.1
2.1
2.4
2.7
4.6
5.0
3.9
5.9
7.8
7.5
4.1
7.1
3.5
3.6

4.8
5.8
4.7
5.4
3.7
4.9
10.7
14.8
22.4
14.4
24.8
19.8
19.7
24.5
28.3
11.7
14.0

4.5
2.7
3.6
3.6
2.3
5.3
12.4
24.9
22.8
17.1
23.4
31.6
35.7
15.7
17.6
18.9
17.4

hill
hill
hill
hill
hill
transitiontransition
lake
lake
lake
lake
lake
lake
lake
lake
lake
lake

COMPARISON OF EACH SPECTRA
Fig.2 shows the results of comparison of the amplification characteristics (reference site: CS78) with theH/V spectral ratio of strong motion of Colima earthquake. The amplification characteristics of strongmotion and the H/V spectral ratio at all site are similar on the shape and amplitude value approximately.
Four sites at Hill zone show small amplification characteristics on flat spectral form, and the spectral
forms at Lake and Transition zone represent high amplification at low frequency as the depth of surface
ground become deeper. It agrees with the maximum acceleration shown in Table1. More specifically,Fig.2 represents that an amplification characteristic of strong motion at surface ground is estimated wellby the H/V spectral ratio of strong motion without reference site.
Fig.3 shows the result of comparison of strong motion of Colima earthquake and a microtremor at eachsite using H/V spectral ratio. From Fig.3, the H/V spectral ratio of strong motion and the microtremor
agreed with the shape and amplitude approximately. However, the peak value of H/V spectral ratio at Lakezone is smaller than the one in Transition zone. Although the shapes of H/V spectral ratio at double

Hill zone

U I2110N S (H /V )EW (H /V )N S (A m p.)
EW (A m p.)

0.10.1110

C E1810TP 13101110.1

IM 40

10N S (H /V )EW (H /V )N S (A m p.)
EW (A m p.)

0.1

1100.1N S(H /V )EW (H /V )N S(A m p.)
EW (A m p.)

0.1

C S78

101

Transition zone
EO 30

1010.1

10

D X37

101
N S(H /V )EW (H /V )N S(A m p.)
EW (A m p.)

1

N S (H /V )EW (H /V )0.1

1100.1

0.1

1

100.1

Lake zone
C I05

10C O 56

1001N S(H /V )EW (H /V )N S(A m p.)
EW (A m p.)

110B L45

100

100.1

N S(H /V )EW (H /V )N S(A m p.)
EW (A m p.)

100.1

N S(H /V )EW (H /V )N S(A m p.)
EW (A m p.)

0.1

110XO 06

100

N S(H /V )EW (H /V )N S(A m p.)
EW (A m p.)

101
N S(H /V )EW (H /V )N S(A m p.)
EW (A m p.)

0.10.1

110

H J72

100

N S(H /V )EW (H /V )N S(A m p.)
EW (A m p.)

100.10.11110C A 59

100

100.10.1110C J03

100.1

10.1

110100.10.10.1

1

N S(H /V )EW (H /V )N S(A m p.)
EW (A m p.)

10N S(H /V )EW (H /V )N S(A m p.)
EW (A m p.)

0.1

10A E02100

1010.1

1

D M 12100

N S(H /V )EW (H /V )N S(A m p.)
EW (A m p.)

110.1

N S(H /V )EW (H /V )N S(A m p.)
EW (A m p.)

111100.10.1

1JA 43100

N S(H /V )EW (H /V )N S(A m p.)
EW (A m p.)

101
N S(H /V )EW (H /V )N S(A m p.)
EW (A m p.)

0.10.1

11

100.1

0.1

1

10Fig.2 The comparison with the amplification characteristics and the H/V spectral ratio of microtremor
on each observation points for strong motion10Hill zone

U I21 (H /V )

101

0.1

IM 40 (H /V )101
S （強震）NN
S (strong)（強震）
E WEW(strong)NN
S(mi微動）cro)
S（EWEW(m（icro)
微動）

0.1

1

C E18 (H /V )101S （強震）NNS(strong)
EW(strong)（強震）
EW
NNS(micro)
S （微動）
EWicro)EW(m（微動）

S （強震）
NNS(strong)
EW (strong)（強震）
EW
S （微動）
NNS(micro)
EWicro)EW (m（微動）0.1

10

TP 13 (H /V )

100.1

10.1

10

C S78 (H /V )

100.1

0.1

EO 30 (H /V )

0.1

10.1

100.1

C I05 (H /V )10N S （強震）N S(strong)
（強震）
EWEW
(strong)
S（微動）N SN(micro)
E WEW(m （icro)微動）

1

N S （強震）N S(strong)
（強震）
EWEW
(strong)
N S(micro)
NS（微動）EWEW(m（icro)微動）

100.1

10.1
10 0.1

1

B L45 (H /V )S （強震) 10
N NS(strong)（強震)
EWEW(strong)
S（N NS(mi微動）cro)
EWEW(m（i微動）cro)

11

10.1
1

10

H J72 (H /V )100.1

110C A 59 (H /V )10N S(強震)N S(strong)
(強震)
EWEW
(strong)
N S(micro)
N S(微動)EWEW(m (微動)icro)

0.1

110XP 06 (H /V )101N S (強震)N S(strong)
(強震)
EWEW
(strong)
N S(micro)
N S (微動)
EWEW(m (微動)icro)

0.1

S （強震）
NN
S(strong)（強震）
EWEW(strong)
S（NNS(mi微動）cro)
EWEW(m（icro)
微動）1

C O 56 (H /V )10

D X37 (H /V )10

S （強震）
NNS(strong)
EW
EW(strong)（強震）
NNS(mi微動）
cro)
S（EW(mi微動）cro)
EW （Lake zone

0.1

N S (強震)N S(strong)
(強震)
EWEW
(strong)
N S(mcro)
N S i(微動)EWEW(m i(微動)cro)1

D M 12 (H /V )100

1010N S(強震)N S(strong)
EW (強震)EW (strong)
N S(micro)
N S(微動)EW (mcro)
EWi(微動)

0.1

0.1

1

10

A E02 (H /V )100

N S(強震)N S(strong)
(強震)
EW EW
(strong)
N S(micro)
N S(微動)EW EW(m i(微動)cro)

1011
N S(strong)
N S (強震)(強震)
EWEW(strong)
N S(micro)
N S (微動)
EWEW(m (微動)
icro)

0.1

100

1（強震）
NNSS(strong)
EEW
W (strong)（強震）
NNS(micro)
S （微動）
EW(micro)
EW （微動）0.10.1

10Transition zone
10110.1

11100.1

C J03 (H /V )100.1

1100.1

10.1

1100.10.1

JA 43 (H /V )100

S (強震)N SN(strong)
E WEW(strong)(強震)
N SN(mS i(微動)
cro)
E WEW
(m i(微動)
cro)

1010.1

1

S （強震）N NS(strong)
EW
EW(strong)（強震）
N
S（N S(mi微動）
cro)
EWi微動）cro)
EW(m（0.11

100.1

0.1

110Fig.3 The comparison with the H/V spectral ratio of strong motion and the H/V spectral ratio of
microtremor on each observation points for strong motion

110M icrotrem or

1000

1000

H ill
T ransition
Lake

100

S trong m otion
H ill
T ransition
Lake

100

10101

10.1

0.1
0.1

1100.1

1

10Fig.4 The amplification characteristics of vertical component at each observation points forstrong motion

logarithmic chart are similar, the values of amplitude are different. It indicates that the UD component of
microtremor has large value.
Using microtremor and strong motion records at basement ground, it is able to estimate the influence ofRayleigh wave at the observation points. In this study, the behavior of Rayleigh wave in sedimentaryground inferred from the microtremor and the strong motion of reference site instead of basement groundof each observation site.
Fig.4 shows the amplitude ratio of UD-component at each site as reference site of CS78. In Fig.4, both
figures of a microtremor and a strong motion separated Hill, Transition and Lake zone. From the figure ofstrong motion, the amplitude of the ratio is almost same value compared with CS78 for all frequency
range though the amplitude is slight high or low value at Transition and Hill zone. However, at Lake zone,
the amplitude is almost similar and 3-6 times compared with CS78 around predominant frequency F0(0.4Hz-0.8Hz) and 2F0, respectively. If CS78 and the basement of each site have a same vertical motion
characteristic, it is considered to exist the potent influence of Rayleigh wave around about 0.8Hz-1.6Hz at
Lake zone though influence of Rayleigh wave is slight at Transition and Hill zone. Rayleigh waves wayaffect for microtremor at upper frequency range because of high amplitude at Transition zone. In a soft
sedimentary ground like Lake zone, the amplitude of UD component is 10 times more than CS78 on
almost all the frequency range, the contrast is extremely high compared with that of strong motion. The
microtremor of sedimentary ground is thought to be consisted of Rayleigh wave occurred from traffic orother noise. Though the H/V technique aims to reduce the influence of Rayleigh wave, the thick and soft
sedimentary ground in Mexico City has high amplification of vertical component around predominantfrequency of horizontal component. And it is considered that it is sometimes difficult to estimate anamplification characteristic of surface ground effectually with ordinarily H/V technique. It way occurs at
not only Mexico City but also the other place like this ground condition.
CONCLUSION

This paper examined the relationship between the H/V spectral ratio of strong motion and theamplification characteristics of strong motion, and between the H/V spectral ratio of strong motion andthat of microtremor recorded at Mexico City. Mexico City has been built on very soft sedimentary groundfilled on a lake. It emerged as follows from the results of comparing with microtremor and strong motion
recorded on Hill, Transition and Lake zone in the city.

1. In case of strong motion, the H/V spectral ratio as reference site agreed with estimated amplification
characteristic from hard ground approximately.
2. The H/V spectral ratio of strong motion is similar to that of microtremor without the site Rayleighwave exceeding extremely.
3. Therefore, the amplification of strong motion around first predominant frequency could be estimatedby the H/V spectral ratio of microtremor approximately.
4. However, in a case that of large amplitude of vertical component caused by Rayleigh wave exceedingextremely, it is difficult to estimate an amplification characteristic of surface ground because of lower
shifted H/V spectral ratio.
It is necessary to consider the techniques to eliminate influence of Rayleigh wave. And henceforth, thestudy aims to find the technique to estimate an amplification characteristic in case of 4.
ACKNOWLEDGMENT

Microtremor measurement in Mexico City was preformed with Centro de Instrumentacion y RegistroSismico (CIRES). Authors thank you Mr. Espinosa Aranda (Director of CIRES) and other CIRES staffs
for their cooperation and help.
REFERENCES

1.

2.
3.
4.

Nakamura, Y. “A Method for Dynamic Characteristics Estimation of Subsurface using Microtremor
on the Ground Surface.” Quarterly Report of RTRI, Railway Technical Research (RTRI), Vol.30,
No.1, 1989
Nakamura, Y. “Clear identification of fundamental idea of Nakamura’s technique and its
applications.” 12WCEE, 2000.
Nakamura, Y., T. Sato and Jun Saita “Development of vulnerability assessment models using
microtremor.” 5th EQTAP Workshop in Bangkok, Thailand, 2002.
Aranda, J. Manuel Espinosa. “Earthquake early warning service in Mexico City.” 2nd multilateralworkshop on development of earthquake and tsunami disaster mitigation technologies and theirintegration for the Asia-pacific region, 2000.