13th World Conference on Earthquake Engineering
Vancouver, B.C., Canada
August 1-6, 2004
Paper No. 905

ON RELATIONSHIP BETWEEN THE ESTIMATED STRONG MOTIONCHARACTERISTICS OF SURFACE LAYER
AND THE EARTHQUAKE DAMAGE- CASE STUDY AT INTRAMUROS, METRO MANILA Jun Saita 1, Maria Leonila P. Bautista 2, Yutaka Nakamura 3

SUMMARY
It is important for the countermeasure against earthquake damage not only to know the earthquake damage
history of a particular site and any structures built on it, but also to correlate this knowledge with dynamic
seismic characteristics. Recently, the use of the H/V spectral ratio technique has become a popularmethod to investigate site characteristics. The H/V technique has also been extended further to estimate
to the vulnerability of the ground. In this paper, these techniques are applied for Intramuros, the oldwalled city of Metro Manila, which has been repeatedly affected by earthquakes in the past. The surveys
were conducted at two different time periods and the results show stability within a long time interval.
More importantly, the results of microtremor measurement correspond to past earthquake damages.

INTRODUCTION
It is important for earthquake disaster prevention to understand the dynamic characteristics of urban
infrastructures as well as the ground. Recently, it has been a popular technique to use H/V spectral ratio[1] for ground has been a popular method all over the world. Later on, the use of vulnerability index, Kg
to relate ground amplification factor and predomination frequency has been developed to correlate withearthquake damage [2]. The term vulnerability index, Kg is defined as equal to Ag2/Fg, where Ag and Fg
are the amplification factor and the predominant frequency determined from microtremor, respectively.
This study conducted microtremor measurements in Metro Manila, Philippines to verify the use of
microtremor and vulnerability index to determine correlation with past earthquake damages. This study
selected the area of Intramuros, the old walled-city of the city of Manila. Previous researchers [3-5] had
also conducted microtremor measurement in this area in 500 m to 1 km meshes and their results roughly
1Research Engineer, System and Data Research, Tokyo, Japan. E-mail: jun@sdr.co.jp
Philippine Institute of Volcanology and Seismology, Quezon City, PHILIPPINES. E-mail:leyo@phivolcs.dost.gov.ph3
President, System and Data Research, Tokyo, Japan. E-mail: yutaka@sdr.co.jp
2determined the predominant frequency distribution in this part of the metropolis. However, it is stillnecessary to determine the detailed distribution of the dynamic characteristics to be able to compare theearthquake damages for ground and structures directly. This study further supplements previous studies
by conducting measurements within much closely survey area.

STUDY AREA
The study area of this research is Intramuros, the old walled city of Manila, which frequently sufferedearthquake damage in the past [6-8]. The 1990 Luzon earthquake although about 120 km away fromManila, caused considerably damage to some 15 RC-buildings in the metropolis. Omachi and Nakamura
conducted microtremor measurements to compare microtremor with damage [9]. The results of themicrotremor measurement in 1990 show that the low frequency values of around 1 Hz with theamplification factor as high as 3 to 8 predominated in the reclaimed area where the earthquake damages
were also rather serious. Meanwhile, to verify stability of microtremor through time, a series ofmicrotremor measurements were conducted by this study from 2002 - 2003.

ON MICROTREMOR MEASUREMENT
Overview of Measurement
Figure 1 shows the distribution of measurement points in the study area. In and around this area, there
were ten points surveyed in 1990 (shown by red circles), fourteen points in 2002 (shown by blue circles)
and thirty points in 2003 (shown by green circles). Some points were resurveyed in 2002 and 2003 for thepurpose of comparing results of the 1990 survey.
At each site, microtremor was recorded for 4,096 samples in 100 Hz sampling. This procedure wasrepeated three times for each site. This methodology was also adopted for the 1990, 2002 and 2003
measurements. There were also five sites done during the 1990 measurements and one site during the2002 measurement outside of the ancient wall.

Figure 1 Distribution of Measurement Points.

Method of analysis and results

Figure 2 Distribution of Fg.

Figure 3 Distribution of Ag.

Figure 4 Distribution of Kg Value and Damage History

Figure 2 and 3 shows the results of measurement in terms of the predominant frequency Fg (Hz) andamplification factor Ag derived from H/V spectral ratio of recorded microtremor.
H/V spectral ratio was derived from the averaged horizontal to vertical Fourier spectrum. Smoothing wasdone using a Hanning window of 80 times (bandwidth is 0.4 Hz). Figure 4 shows the comparison
between Kg values and major damages of past earthquakes on detailed map. In Figures 2, 3 and 4, thediameter of the circle indicates measurement point corresponds to the Fg, Ag and Kg values.
Although the predominant frequency is almost stable for the study area, amplification factors showvariation. Three sites (MG5, MG6 and MG17) show high Kg values. These are located on the reclaimed
land near the ditch. Meanwhile, MG1, MG3 and MG4 have relative low Kg value. It seems that it
depends on the age of reclaimed area.
Examination on Stability of Microtremor with a Long Time Difference
Three points MG9, MG10 and MG11 conducted in 1990 locate close to MG9n, MG10n and MG11n
conducted in 2002, respectively. By comparing with the results, MG9 and MG10 give approximately
similar frequency and amplification. However, in the case of MG11, the shapes of spectrum are almost
similar, but the peak values differ from each other. We investigated the possible cause and we found outthat a sewer has recently been constructed near this observation site. The operation of this structure may
have caused the change of site condition.

COMPARING WITH THE DETAILED DAMAGE AND MICROTREMOR
Figure 5 shows the overview and damage situation of San Agustin Church after the July 18, 1880
earthquake, located inside Intramuros and considered as one of the world heritage sites in the world. Thischurch repeatedly suffered earthquake damage. Figure 5(b) shows its picture in 1880 after this earthquake
caused one of the bell towers to fall.
1880

1898

1880

(a) Present Condition(b) Past Damage Condition

Figure 5 Overview of San Agustin Church

Around this church, microtremor was measured at points MG15, MG16, MG21 and MG22. Since depthof soil layer is another important criterion for determining earthquake damage, we tried to determine theestimated depth of soil layer using the following formula,

H = Vb / 4 Ag Fg
Where, H :Vb :Estimated depth of the surface layer (m),Shear wave velocity,

Ag :Amplification factor, andFg :Predominant frequency.

Here, Vb is assumed to be 600 m/sec. Figure 6 shows the estimated depth of the surface layer. Figure 7
shows the soil profile one block away from the church [10]. By comparing the depth with the soil profile,
it seems that the estimated depth of the surface layer corresponds to the fine-grained sand layer at 34mdepth.

M G 22

M G 21D epth of S urface G round(m )M G 1510m010M G 16

20304050

Figure 6 Estimated depth of surface groundFigure 7 Soil profile near San Agustin Church

Figure 6 shows that the base ground is leaning and the deepest site is MG15, near to the left (lost) tower ofthis church. The influence of a base ground inclination is considered to be a large factor for estimating thepotential damage of this structure. However, considering that other building abuts on the anothersurviving tower, it is supposed that the influence of an adjoining building is also large. In order to clarify
such a damage factor, a thorough understanding of the dynamic characteristic of the ground and thecharacteristic of the structures are necessary.
Damage Estimation from Microtremor on the Structure
To grasp the dynamic characteristics in detail, microtremor measurement was also conducted inside thechurch., both for the main building and the bell tower shown in figure 5. Here the height of the mainbuilding and the bell tower assumed to 12 m and 8 m, respectively. Measurement points were installed on
the grand floor and the top for the main building and on the bottom, middle and top of the bell tower.

From the microtremor on the ground, the predominant frequency and amplification factor is a little higher
than 1 Hz and 2-4 times, respectively, so Kg value is less than 20.
1414121210

108856644492587H L 2.9H z● RC● LC2510Tow er
4.8-5.0H z● HL● HT32HT● R C 3.2H z● LC 2.5H z

201515101055

1000020●H L 2.9H z●H T 3.2H z62025●H L 3.8H z●H T 3.9H z

05(a) The main building

1001020(b) Tower

00204002040(C) Unified of the main buildingand the tower

Figure 8 Mode diagrams of the San Agustin

Figure 8 is the mode diagram for the main building, the tower and unified of the main building and thetower.
The predominant frequency of the main building is around 3 Hz in general for both HL and HT direction.
Here HL and HT direction corresponds to the direction along the long and transverse axis of the mainbuilding, respectively. But in detail, in case of HL direction, predominant frequency is 2.9 Hz for both left
and right side but the amplification factor of left side is several times more than that of right side. In caseof HT direction, predominant frequency is 3.2 Hz and 2.5 Hz for left and right side, respectively, and theamplification factor of left side is 5 times or more than that of right side. Of course it is impossible toavoid the effect of dead load to keep the balance for the lost left tower, it seems to be damaged on the left
side. On the tower, there are two predominant frequencies, around 3 Hz and less than 4 Hz. Former
seems to be the characteristics consist of the main building and thew tower. Latter is thought as thecharacteristics effected by the tower. For both cases, HT component has high amplitude two times ormore than HL component, and the damage had been caused toward this direction.

CONCLUSIONSThis paper shows the microtremor measurement and vulnerability index can explain past earthquake
damages. The results also show that microtremor measurements provide almost stable results with longtime intervals. This technique can be used to identify possible areas or structures that may be damaged inthe future by earthquakes. At the same time, the technique can be used for monitoring a structure’s stateof health through time.

ACKNOWLEDGEMENT
The authors would like to thank Dr. Bartolome C. Bautista, Dr. Ishmael C. Narag and Dr. Esmeralda L.
Banganan of Philippine Institute of Volcanogy and Seismology (PHIVOLCS) for their coordination ofmeasurement in Intramuros.

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