earthquake design

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1082

Bulletin of the Seismological Society of America, Vol. 92, No. 3, pp. 10821094, April 2002

Estimation of Peak Ground Accelerations of the Malay Peninsula

due to Distant Sumatra Earthquakes

by Tso-Chien Pan and Kusnowidjaja Megawati

Abstract The low-seismicity Malay Peninsula has never experienced any earth-quake damage. Thus, earthquake-resistant design has not been specifically required

in the building codes. However, it has been realized that urban areas located rather

distantly from earthquake sources may also be affected by tremors. In this article,

the potential ground motion in terms of the peak ground accelerations (PGAs) due

to long-distance Sumatra earthquakes is investigated for Singapore and Kuala Lum-

pur, following a probabilistic seismic hazard assessment approach. Earthquakes that

have occurred in Sumatra in the last 37 yr are used, for which Ms and Mw catalogs

are constructed from the availablembcatalog. The analysis is then carried out using

the Mw catalog. Based on the PGAs of 52 recent Sumatra earthquakes recorded in

Singapore, the attenuation relationship of Fukushima and Tanaka (1992) is found to

correlate well with the high-rate attenuation characteristic of the region. The pre-dicted design-basis PGAs (i.e., PGA with 10% probability of being exceeded in a

50-yr exposure time) on rock outcrop sites are 12.7 and 29.5 gal for Singapore and

Kuala Lumpur, respectively. The predicted maximum credible PGAs (i.e., PGA with

2% probability of being exceeded in 50 yr) are 24.3 and 55.1 gal for the two cities.

Current building design codes in the region require that buildings be capable of

resisting a notional ultimate horizontal design load equal to 1.5% of the characteristic

dead weight, applied at each floor simultaneously to ensure structural robustness.

The base shear forces resulting from the predicted design-basis PGAs and the max-

imum credible PGAs at rock site in Singapore and Kuala Lumpur are therefore

comparable to or higher than the capacity required by the current building codes.

Introduction

Under some circumstances, distant earthquakes, occur-

ring several hundred kilometers away, are capable of causing

considerable damages. The Michoacan earthquake of 1985

(Ms8.1) is a good example. The earthquake caused serious

damage in some areas of Mexico City, 300450 km from

the epicenter, because the incoming earthquake waves were

amplified by the soft soil on the ground surface (Seed et al.,

1987). This may be a peculiar case, but obviously soft-soil

amplification effects are to some extent present in many

places. For example, in February 1994, some buildings inthe densely populated areas of Singapore responded to an

earthquake of Ms 7.0 that occurred near Liwa in southern

Sumatra, more than 700 km away (Pan, 1995). Hundreds of

people were awakened and rushed out of their high-rise flats

in panic. In May 1994, tremors from an earthquake near

Siberut Island, 570 km away, which measured only 6.2 on

the Richter scale, were felt in Singapore and Kuala Lumpur,

Malaysia (Pan and Sun, 1996). The shaking of some build-

ings in Singapore again caused panic, and some office work-

ers rushed out of their high-rise offices. In both incidents,

the buildings that responded to the remote earthquakes were

located in the southeastern part of the island, where they are

underlain by Quaternary deposits of the Kallang Formation,

which consists of Holocene sediments of marine, alluvial,

littoral, and estuarine origin. Buildings in other areas of Sin-

gapore had no apparent response. It appears that the Qua-

ternary deposits amplified the incoming earthquake waves

in both incidents. In October 1995, even stronger and more

extensive tremors were caused in Singapore by an Ms 7.0

earthquake occurring 450 km away. This earthquake alsogenerated ground tremors in Kuala Lumpur and in the south-

ern state of Johor in Malaysia. The recent Bengkulu earth-

quake of 4 June 2000, which had an Mw of 7.7 and an epi-

center 700 km south-southwest of Singapore, produced the

strongest tremors felt in the city in the past 40 years (Pan et

al., 2001). Many high-rise buildings across virtually the

whole island were shaken, regardless of the local ground

conditions. So far, 32 earthquakes have reportedly been felt

in Singapore since the British settlement in 1891; 27 events

were reported in Pan and Sun (1996), one in October 1996,


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Estimation of Peak Ground Accelerations of the Malay Peninsula due to Distant Sumatra Earthquakes 1083

one in April 1998, and the remaining three tremors during

the recent Bengkulu earthquake. The frequency of the felt

events seems to rise as the country develops.

The Malay Peninsula is of low seismicity. Historically,

earthquakes have never posed any real problems in the Ma-

lay Peninsula, and earthquake-resistant design has thus not

been specifically required in the current building codes. This

is understandable since the nearest earthquake belt, com-prising the Sumatra subduction zone and the Sumatra fault,

is more than 300 km away. However, the increasing number

of felt tremors described previously demonstrates that, al-

though there has never been any earthquake damage in the

Malay Peninsula and Singapore, the seismic hazard may not

be negligible, especially the damage potential to high-rise

buildings on soft sedimentary deposits or reclaimed land. It

is therefore appropriate to investigate the magnitude of likely

ground motions in the major cities on the Malay Peninsula

under such circumstances. The present study therefore fo-

cuses on Singapore and Kuala Lumpur, the capital of Ma-

laysia, where there are the largest concentrations of high-rise

buildings, complex infrastructure systems, and population.The results are presented in the form of predicted peak

ground accelerations for various levels of probability of ex-

ceedance in a 50-yr exposure time. In this study, only the

ground motions at rock outcrop sites (bedrock) will be in-

vestigated. The ground motion at soft-soil sites may then be

estimated separately based on site response analysis when

required.

Probabilistic Seismic Hazard Analysis

There are generally two approaches in earthquake haz-

ard analysis: one is deterministic and the other is probabi-

listic. In the first approach, an event with a certain magnitude

at a certain distance is chosen as the design earthquake.

Buildings are then required to be designed against the

ground motions caused by the design earthquake. The de-

terministic approach is straightforward and easy for the pub-

lic and policymakers to understand, but the choice of the

design earthquake is difficult. In the probabilistic approach,

structures are required to withstand a certain level of ground

motion that has a certain probability of being exceeded

within an exposure time period. For example, in the Uniform

Building Code (International Conference of Building Offi-

cials, 1991), the minimum ground motion is taken as one

that has a 10% probability of being exceeded in 50 yr. Itrecognizes the possibilities of stronger ground motion and

accepts the risk. The probabilistic approach tells how much

risk one is taking when designing a structure against a certain

level of ground motion. This exploits the trade-off between

safety and economy. The probabilistic approach also incor-

porates the uncertainty in seismic activity and the random-

ness in ground motion predicted by an attenuation function.

However, it is more difficult for the public to understand

how the results have been obtained. Despite the difficulty,

the probabilistic approach is chosen for this study.

In the probabilistic approach, a ground motion value i

is selected, and the probability of ground motion Iexceeding

the value i, FI(i), for an earthquake occurring in the region

studied can be calculated as follows:

M rmax max

F(i) P[I i |m, r] f (m)h(r)drdm , (1)I M rmi n min

whereP[Ii |m,r] is the probability ofIgreater thanigiven

a magnitudemand distancer,f(m) is the probability density

function for an earthquake of magnitude m to occur,h(r) is

the probability density function of an earthquake occurring

at distancer,Mminis the minimum magnitude of earthquake

in the sample, Mmax is the maximum possible earthquake

magnitude in the area studied, rminis the minimum distance

of earthquake in the sample, and rmax is the maximum dis-

tance of earthquake that may still affect a site concerned.

Assuming that earthquakes have a Poisson distribution,

given an exposure timet, ifNearthquakes are expected per

year in the region, the probability of ground motion i being

exceeded in tyears,PE, will be (Lomnitz, 1974)

F (i)NtIP 1 e . (2)E

Hence, the probabilistic method of earthquake hazard anal-

ysis consists of the following four steps:

1. identifying the seismic source areas;

2. determining seismicity statistics (i.e., the probability den-

sity functions for magnitude and epicentral distance,f(m)

andh(r));

3. defining an attenuation law (i.e.,P

[I i

|m

,r

]); and4. computing probabilities of a given ground motion being

exceeded at a particular site for a given exposure time,

using equations (1) and (2).

Seismotectonics of Sumatra

Sumatra is located adjacent to the Sunda trench (Fig.

1), where the IndianAustralian Plate subducts beneath the

Eurasian Plate at a rate of about 67 7 mm/yr, toward

N11E 4 (Demets et al., 1990; Tregoning et al., 1994).

Sumatra and Java Islands lie on the overriding plate, a few

hundred kilometers from the trench. Convergence is nearlyorthogonal to the trench axis near Java, but it is highly

oblique near Sumatra, where strain is strongly partitioned

between dip slip on the subduction zone interface and right-

lateral slip on the Sumatra fault along the western coast of

the island (Fitch, 1972; McCaffrey, 1991). The earthquake

focal mechanisms and hypocentral distributions indicate that

the subducting plate in Sumatra dips less than 15 beneath

the outer arc ridge and becomes steeper to about 50below

the volcanic arc (Newcomb and McCann, 1987; Fauziet al.,

1996). The relatively shallow dip angle gives a strong cou-


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1084 T.-C. Pan and K. Megawati

Figure1. Epicenters of earthquake data used in the present study. The selected dataare earthquakes in Sumatra that have occurred within a radius of 1250 km from Sin-gapore, from 1963 to 1999. The trace of the Sumatra fault is adopted from Sieh andNatawidjaja (2000).

pling between the overriding and the subducting plates, and

large earthquakes have been generated in the region.

A great earthquake of moment magnitude (Mw) esti-

mated between 8.8 and 9.2 occurred in 1833, and it wasbelieved to have caused a 500-km-long rupture along the

interface extending from the southern island of Enggano to

Batu Island. Another great earthquake of Mw between 8.3

and 8.5 occurred near the Nias Island in 1861 (Newcomb

and McCann, 1987; Zachariasenet al., 1999). The locations

of the islands are shown in Figure 1. Both earthquakes oc-

curred in the subduction zone.

On the land side, a dextral strike-slip fault, the great

Sumatra fault, constitutes yet another source of numerous

earthquakes (Katili and Hehuwat, 1967; Sieh and Natawid-

jaja, 2000). The fault is about 1650 km long and runs

through the entire length of Sumatra, coinciding with the

Bukit Barisan Mountain belt. Unlike many other great strike-

slip faults, the Sumatra fault is highly segmented, so that ithas a limited capability to generate very large earthquakes.

The fault is about 350 km away from the major cities, such

as Kuala Lumpur, Penang, Ipoh, and Melaka, on the western

coast of Malay Peninsula.

Earthquake Data of Sumatra

The earthquake data used in the present study are taken

from the Earthquake Data Base System (EDBS) managed by

the National Earthquake Information Center (NEIC), United


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Figure2. Empirical relationship between surface-

wave magnitude (Ms) and body-wave magnitude (mb)constructed using records that have both magnitudes.

Figure3. Empirical relationship between momentmagnitude (Mw) and surface-wave magnitude (Ms)constructed using records that have both magnitudes.

States Geological Survey (USGS).EDBSis a collation of 54

worldwide or regional catalog, some of which are directly

relevant to this investigation (NEIC, 1994). The data sets

selected for the present study are earthquakes in the Sumatra

region that occurred within a radius of 1250 km from Sin-

gapore, from 1963 to 1999. This time period is chosen be-

cause, unlike the pre-1963 catalogs, the post-1963 PDE cat-

alog covers small and large earthquakes and is considerablymore complete. The older catalogs (the ABE, NOAA, and

Pacheco and Sykes [1992] catalogs) record only large events

with magnitude greater than 6.5. ABE refers to global cat-

alog of large earthquakes occurring from 1897 to 1980,

taken from Abe (1981, 1982, 1984), Abe and Kanamori

(1979), and Abe and Noguchi (1983a,b). NOAA refers to

the listing of large or destructive worldwide historical earth-

quakes occurring from 2150 BC to 1991 (Dunbar et al.,

1992). There are 3896 earthquakes that satisfy these criteria,

and the epicenters of these events are shown in Figure 1.

The majority of the earthquakes (3541 events) has body-

wave magnitude (mb) assigned. There are 460 events with

surface-wave magnitude (Ms) assigned, and only 34 eventshave moment magnitude (Mw). In equation (1), m should

always be of the same type of magnitude, and most of the

recent PGA attenuation relationships (Fukushima and Ta-

naka, 1992; Abrahamson and Shedlock, 1997) were derived

as functions of moment magnitude,Mw, and distance. There-

fore, a catalog in Mw is needed for the present study. How-

ever, Mw is normally determined only for earthquakes of

very large size or of special interest.

It is known thatmb, which is defined based on the mag-

nitude of theP-wave, begins to saturate at a low magnitude

of about 6.0. Below this magnitude, mbwould have a linear

correlation withMsor Mw. Out of the 3541 events that have

mb, only 32 events have mb greater than 6.0, which means

that the majority of the mbvalues can be converted linearly

to either Msor Mw. There are 455 events that have both Msand mb, but only a few large events have both Mw and mb,

where the mbis almost saturated. Therefore, the events that

have both Ms and mb are used to establish a relationship

between Ms and mb. The data are plotted in Figure 2, and

the linear regression gives

M 1.361m 2.125, (3)s b

with a coefficient of correlation of 0.783. Using the empir-

ical relationship obtained for Msand mb, theMsvalues werecalculated for those records initially having mbonly. In this

way, a surface-wave magnitude catalog is constructed for

earthquakes between January 1963 and December 1999.

Most earthquakes in Indonesia are not assigned Mwval-

ues, but there were 34 events in Sumatra having both Mwand Ms. The data are shown in Figure 3, and they are used

to establish a relationship between Mw and Ms. It is known

thatMsstarts to saturate at 7.5, and is fully saturated at 8.0.

Below the saturation range,Mshas a linear correlation with

Mw. TheMscatalog that we derived has values ranging from

4.0 to 7.5; thus, it can be converted linearly to form an Mwcatalog. The linear regression of the 32 events that have both

Mw and Msgives

M 1.250 0.834M. (4)w s

The pattern of the regression is similar to that obtained by

Heaton et al. (1986). Equation (4) is then used to convert


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Table 1Mmin,Mmax,M

, and k in Regions 1, 2, and 3for Singapore and Kuala Lumpur

Singapore Kuala Lumpur

Region 1 Region 2 Region 3 Region 1 Region 2

Mmin 4.5 4.5 4.5 4.5 4.5

Mmax 7.8 8.5 9.0 7.8 8.5

M 5.10 5.14 5.19 4.96 5.11

k 1.6245 1.5425 1.4446 2.1842 1.6095

Figure4. Moment magnitude and distance distri-bution of earthquakes occurring within 700 km ofSingapore.

Msinto Mwto create an Mwcatalog of Sumatra earthquakes

forMw 4.5.

Probability Density Function f(m)

In this section, the probability density function f(m) of

an earthquake of magnitude m to occur is computed for Su-

matra earthquakes. The magnituderecurrence relationshipis typically in the form of the Gutenberg and Richter (1949)

formula, if there is no limit to earthquake size. In reality,

however, there exists a limit to the size of earthquakes phys-

ically possible in the region studied. There have been several

suggestions for a better description of the magnitude

recurrence relationship. Donget al.(1984) proposed that the

probability density function f(m) should be

kmke

f(m) (M m M ), (5)min maxkM kMe emin max

whereMmaxis the maximum possible earthquake magnitudefor the region studied, Mminis the minimum magnitude of

the given sample, and the minimum biased estimate ofk can

be obtained from the following equation:

kM kMM e M e1 min min max max M, (6)kM kM

k e emin max

in which M is the mean value of the earthquake magnitudes

in the sample. Therefore,f(m) depends on the predetermined

Mmin, the assumed Mmax, and the mean value M of the mag-

nitudes in the region.

The probability density function f(m) is determined us-

ing the past earthquake distribution shown in Figure 1. Thesefunctions are different for Singapore and Kuala Lumpur, de-

pending on the spatial and magnitude distributions of the

earthquakes with respect to these cities.

f(m) for Singapore

Figure 4 shows the moment magnitude and distance dis-

tribution of earthquakes occurring within 700 km from Sin-

gapore, from January 1963 to December 1999. The solid

circles denote the 923 events with Mw 4.5 recorded in the

37-yr period. The maximum possible magnitude varies spa-

tially, depending on the past earthquake distribution and the

tectonic settings. To determine the maximum magnitude,older earthquake catalogs are used. These catalogs (ABE,

NOAA, Pacheco and Sykes [1992]) cover a longer time span

from 1900 to 1962 but do not include small earthquakes.

There are eight such events, plotted as solid triangles in Fig-

ure 4.

The Sumatra fault is composed of 19 major segments,

with cross-strike width of step-overs between adjacent seg-

ments of about 5 to 12 km (Sieh and Natawidjaja, 2000).

The historical record shows that the segments of the Sumatra

fault have caused numerous major earthquakes, but their

magnitudes are limited to about 7.5 to 7.8, with the rupture

lengths not greater than 100 km. On the other hand, the

subduction zone has generated several large earthquakes

with Mw ranging from 8.5 to 9.0. Three regions have thus

been assigned, as shown in Figure 1 and 4. Region 1, from

250 to 500 km, covers the closest segments of the Sumatra

fault and has a maximum magnitude of 7.8. Region 2, from

500 to 600 km, covers the deeper region of the subductionzone and has a maximum magnitude of 8.5. Region 3, from

600 to 700 km, covers the shallow part of the subduction

zone, where the 1833 event occurred, and has a maximum

magnitude of 9.0. Earthquakes occurring farther than 700

km are not considered because their contributions to FI(i) in

equation (1) are negligible, as will be shown later.

Table 1 summarizes the Mmin, Mmax, M, and k of each

region. The f(m) for each region is plotted in Figure 5, to-

gether with the number of earthquakes of a certain magni-

tudem recorded in the 37-yr period.

f(m) for Kuala Lumpur

Figure 6 shows the moment magnitude and distance dis-tribution of earthquakes occurring within 600 km of Kuala

Lumpur, from January 1963 to December 1999. The solid


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Figure 5. Probability density function for anearthquake of magnitudemto occur,f(m), of Regions1, 2, and 3, with respect to Singapore, together withthe number of earthquakes of magnitude m recordedfrom January 1963 to December 1999.

Figure6. Moment magnitude and distance distri-bution of earthquakes occurring within 600 km ofKuala Lumpur.

Figure 7. Probability density function for anearthquake of magnitudemto occur,f(m), of Regions1 and 2, with respect to Kuala Lumpur, together withthe number of earthquakes of magnitude m recordedfrom January 1963 to December 1999.

circles denote the 612 events with Mw4.5 recorded for the

period. To determine the maximum magnitude, the older

earthquake catalogs are used. There were nine large events

that occurred between 1900 and 1962, plotted as triangles in

Figure 6. Two regions are assigned, as shown in Figures 1

and 6. Region 1 covers only the Sumatra fault, and Region

2 covers the subduction zone where the historical Mw 8.5

earthquake occurred in 1861. The maximum magnitude for

Regions 1 and 2 are 7.8 and 8.5, respectively.

Table 1 summarizes the Mmin, Mmax, M, and k of each

region. The f(m) for each region is plotted in Figure 7, to-

gether with the number of earthquakes of a certain magni-tudem recorded in the 37-yr period.

Probability Density Function h(r)

The probability density function h(r) of earthquake oc-

currence with respect to distance is determined using the

earthquake distributions from January 1963 to December

1999, assuming that future earthquakes with Mw 4.5 will

have the same spatial distribution as those that have occurred

in the past 37 yr.

h(r) for Singapore

The probability density function h(r) for Singapore is

determined using the earthquake distribution shown in Fig-

ure 4. The area is divided into concentric rings of 50-km

width. Theh(r) is then calculated by dividing the number of

earthquakes within each of the 50-km-width rings by the

total number of earthquakes and further divided by the in-

terval width of 50 km. Theh(r) function is shown in Figure


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Figure 8. Probability density function of earth-quake occurrence as a function of distance,h(r), withrespect to Singapore.

Figure 9. Probability density function of earth-quake occurrence as a function of distance,h(r), withrespect to Kuala Lumpur.

8, together with the number of earthquakes within a distancer. The total area of is equal to 1.0.700 h(r)dr250

h(r) for Kuala Lumpur

The probability density function h(r) for Kuala Lumpur

is determined using the earthquake distribution shown in

Figure 6. Similar to what was done for Singapore, the area

is divided into concentric rings of 50-km width. The h(r)

function is shown in Figure 9, together with the number

of earthquakes within a distance r. The total area of

is equal to 1.0.600 h(r)dr200

Attenuation Relationship for Long-DistanceSumatra Earthquakes

Usually an attenuation relationship for a specific region

is empirically developed from statistical regression analyses

of many earthquake ground-motion records in the region.

However, for distant Sumatra earthquakes, where systematic

recording was set up in Singapore only in 1996, it is not

possible to do so because the accumulated data have not been

adequate. To overcome this limitation, several existing at-

tenuation relationships derived for other regions are exam-

ined to find a suitable attenuation relationship for Sumatra.

Most of the existing attenuation functions were derived for

short to medium distance earthquakes, and they are extrap-olated to long distance (5001400 km) to examine the fitting

to the recorded PGAs in Singapore.

The first attenuation relationship considered is by Fu-

kushima and Tanaka (1992), which was a slight revision of

Fukushima and Tanaka (1990). The model was derived

mainly from earthquakes in Japan, with supplementary data

from earthquakes in the United States and other countries.

A two-step stratified regression procedure was applied to

avoid interaction between magnitude and distance of differ-

ent earthquakes. This model was intended to be used in Ja-

pan for earthquakes with epicentral distance less than 300

km. The equation is given as

log(PGA) 0.42M log(R 0.025w (7)0.42Mw 10 ) 0.0033R 1.22,

where PGA is the peak horizontal acceleration in cm/sec2,

Mwis the moment magnitude, and R is the shortest distance

from the fault plane to the site in km. A multiplier of 0.6

should be applied to the calculated PGA of equation (7), to

obtain the peak horizontal acceleration for a rock site.

The second attenuation relationship examined is themodel derived by Youngset al.(1997) for subduction zone

interface earthquakes. The data used in this model were from

worldwide subduction earthquakes of moment magnitude

5.0 and greater and for distances of 10500 km. The rela-

tionship for rock sites is given as

ln(PGA) 7.1304 1.414M 2.552ln(Rw rup0.554Mw 1.7818e ) 0.00607 H, (8)

where Rrup is the closest distance to the rupture surface in

km,His the focal depth in km; His taken to be 33.0 km inthe present study because the majority of the Sumatra earth-

quakes have shallow depth.

The third equation investigated is the attenuation rela-

tionship derived by Booreet al.(1997) from Western North

American earthquakes. The data set was restricted to shallow

earthquakes with moment magnitude greater than 5.0 and

with fault rupture mainly above a depth of 20 km. The equa-

tion was intended to estimate the PGAs of earthquakes with

distance smaller than 80 km in a seismically active region.

The equation is given as


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Figure10. Recorded PGAs in Singapore due torecent Sumatra earthquakes, together with four atten-uation relationships considered in this study.

ln(PGA) 6.647 0.527(M 6)w

Vs2 0.778 ln R 31.025 0.371ln , (9) jb

1396

where Rjb is the closest horizontal distance to the vertical

projection of the rupture in km, and Vsis the average shear-

wave velocity to 30-m depth in m/sec, which is taken to be

750 m/sec for rock sites. The coefficients in the equation

were determined using a weighted, two-stage regression pro-

cedure.

The fourth equation examined is the attenuation derived

by Atkinson and Boore (1995) for eastern North America.

Due to low seismicity in this stable continental region, the

attenuation relationship was based on stochastically simu-

lated ground motions instead of recorded ones. The equation

was derived for estimating ground motions from eastern

North American earthquakes ofMw between 4.0 and 7.25,

at distances of 10 to 500 km. The equation is given as

log(PGA) 3.79 0.298(M 6)w2

0.0536(M 6) logR 0.00135R , (10)w hypo hypo

whereRhypois the hypocentral distance in km.

In accordance with the public awareness of seismic haz-

ard, the Meteorological Service Singapore installed a net-

work of seven seismic stations in Singapore in 1996. The

main station in Bukit Timah, which is a member of the

Global Seismic Network, is equipped with a comprehensive

set of sensors to record ground tremors continuously. The

station is located on a rock outcrop site. It has recorded 52

seismic tremors, including the mainshock and two major af-

tershocks of the recent Bengkulu earthquake. The earth-

quakes had moment magnitudes ranging from 4.7 to 7.7, at

distances of 4701400 km. The majority of the earthquakes

had focal depths shallower than 60 km. The recorded PGAs

are shown in Figure 10, together with the four attenuation

relationships (equations 810). Different source-to-site dis-

tance measures used in the previous equations are treated as

epicentral distance, so that they can be drawn in the same

figure. This is an acceptable treatment for long-distance

earthquakes. In the long-distance range, the Boore et al.

(1997) relationship predicts the largest PGA among the four

models, much larger than the observed PGAs. This may be

attributed to the fact that their equation was derived to es-

timate PGA of near-field earthquakes only. Although theAtkinson and Boore (1995) attenuation relationship was to

cover long-distance earthquakes, it also overpredicts the ob-

served PGAs. Eastern North America, which is a stable con-

tinental region, might have a lower attenuation rate than the

seismically active Sumatra region. Fukushima and Tanaka

(1992) and Youngset al.(1997) give similar predictions for

magnitude less than 6.0 and distance less than 200 km, but

the former predicts much lower PGA as the distance in-

creases. Among the four models, that of Fukushima and Ta-

naka (1992) gives the highest attenuation rate for distance

larger than 200 km, and it fits the observed data very well.

Fukushima and Tanaka (1992) applied distance selection cri-

teria to obtain consistent attenuation rates within and beyond

the distance range of the data used in deriving the relation-

ship (Fukushima and Tanaka, 1990; Fukushima, 1997).

Table 2 shows the observed PGAs that have values


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Table 2Fourteen Earthquakes with Observed PGA 0.01 gals, with log (PGA)

for Observed and Predicted Values*

Magnitude log(PGA)

No. DateUTC m b Ms Mw

Distance

(km)

Depth

(km)

PGA

( gal ) O bs er ved P re di ct ed De vi at io n

1 19961010152104 5.7 6.1 6.3 689 33 0.0426 1.3706 1.4747 0.1041

2 19970317080548 5.8 6.2 6.4 909 33 0.0112 1.9508 2.2779 0.3271

3 19970422055559 5.6 5.5 5.9 557 107 0.0259 1.5867 1.1136 0.4731

4 19970518221418 5.0 5.1 5.4 576 33 0.0116 1.9355 1.3986 0.5370

5 19970707112437 5.4 5.7 5.9 689 28 0.0213 1.6716 1.6405 0.0311

6 19970820071515 5.9 6.0 6.0 875 33 0.0365 1.4377 2.3154 0.8777

7 19971218054657 5.3 5.5 5.7 592 33 0.0113 1.9469 1.3383 0.6086

8 19980401175623 6.2 6.9 7.0 545 55 0.1080 0.9666 0.6138 0.3528

9 19990918125235 5.1 5.2 5.6 602 33 0.0141 1.8508 1.4260 0.4248

10 19990922072257 4.7 4.4 4.9 484 33 0.0105 1.9788 1.2206 0.7582

11 19991221141457 6.1 6.2 6.6 934 55 0.0269 1.5702 2.2892 0.7190

12 20000604162825 8.0 7.7 705 33 0.3820 0.4179 0.9677 0.5498

13 20000604163945 6.6 6.9 7.0 691 33 0.4560 0.3410 1.2097 0.8687

14 20000607234526 6.2 6.7 6.8 694 33 0.0900 1.0458 1.2741 0.2283

*Predicted values according to the Fukushima and Tanaka (1992) attenuation relationship.

Figure 11. Probability of ground motion I ex-ceeding a value ofi if an earthquake occurs in a dis-tance between 250 and 700 km from Singapore, FI(i).

greater than 0.01 gal. There are 14 such events. The values

predicted by Fukushima and Tanaka (1992) and the devia-

tion of the log(PGA) between the observed and predicted

values are also listed. The average deviation is calculated to

be 0.49, while the average deviation for the whole set of 52

events is 0.52, which is just slightly higher. We therefore

adopted as the attenuation relationship the Fukushima and

Tanaka (1992) model, with a standard deviation of 0.49. The

probability of PGA exceeding a given value i due to an earth-

quake with magnitudem at distancer,P[Ii|m,r] in equa-

tion (1), can then be calculated, assuming a normal distri-

bution of log(PGA). Therefore, the standard deviation of the

attenuation relationship affects the exceedance probability,

in the way that a smaller standard deviation results in a lower

exceedance probability.

Estimation of Peak Ground Acceleration

All elements f(m), h(r), and P[I i|m, r] that are re-

quired by equation (1) for the probabilistic seismic hazard

assessment of Singapore and Kuala Lumpur have been de-

fined. The probability of a given ground acceleration being

exceeded by any earthquake in the region, FI(i), and the

PGAs for various levels of probability of being exceeded in

50 yr are calculated. The magnitude interval, dm, and dis-tance interval, dr, in equation (1) are taken to be 0.1 and

10.0 km, respectively.

Estimated PGA for Rock Sites in Singapore

Figure 11 shows the probability of ground motion Iex-

ceeding a value of i if an earthquake occurs at a distance

between 250 and 700 km from Singapore, FI(i). Two atten-

uation equations, Fukushima and Tanaka (1992) and Youngs

et al. (1997), are considered. Two values of standard devi-

ation are used for Fukushima and Tanaka (1992) r 0.21

andr 0.49. The former value is the standard deviation of

Fukushima and Tanaka (1990) derived from earthquake data

up to a distance of 300 km, and the latter is the value derivedin the present study for longer distance. The standard devi-

ation of Youngs et al. (1997) is taken to be 0.35, which is

the value suggested for magnitude 6.5 in the study. The fig-

ure shows that for a certain i, the exceedance probability

using the Youngs et al. (1997) attenuation relationship is

larger than that using Fukushima and Tanaka (1992), which

can also be seen in Figure 10, where the former predicts a

larger PGA than the latter does. For the same attenuation

relationship, the larger the standard deviation, the higher the

exceedance probability.


10/13


Figure 12. Probability of ground acceleration ibeing exceeded in 50 yr, for Singapore.

Table 3Ground Acceleration (gal) That Has 10% Probability of Being

Exceeded in 50 Years, for Singapore and Kuala Lumpur

Fukushima and Tanaka

(1992) Youngset al.(1997)

City r 0.21 r 0.49 r 0.35

Singapore 3.3 12.7 44.3

Kuala Lumpur 7.4 29.5 44.7

Figure 12 shows the probability of ground acceleration

i being exceeded in 50 yr. In the region considered, 922

earthquakes have occurred between January 1963 and De-

cember 1999. Thus, the average occurrence rate is 24.9

earthquakes per year. For 10% probability of i being ex-

ceeded,PE 0.1, which is equivalent to FI(i) 8.447

105, the values ofi are summarized in Table 3. Note that

the PGA with 10% probability of being exceeded in 50 yris often used to define the design-basis earthquake. There-

fore, PGA 12.7 gal is the value associated with the

design-basis earthquake. Considering Fukushima and Ta-

naka (1992) with r 0.49, the PGAs of 50% and 2% prob-

ability of being exceeded in 50 yr are 5.6 and 24.3 gal, re-

spectively. These probability values are generally referred

to as the probable earthquake and the maximum credible

earthquake, respectively.

To understand which earthquakes create the anticipated

ground motions, Figure 13 shows the FI(i) ofi 12.7 gal

as functions of magnitude m and distance r, using the Fu-

kushima and Tanaka (1992) model with r 0.49. The total

volume under the surface is equal to the probability of 12.7gal being exceeded by an earthquake occurring in the region,

which is equal to 8.477 105. It can be seen that the main

contributions, about 81% of the total volume, come from

earthquakes within 250 to 500 km, although the probability

of earthquake occurrence in this distance range is only

17.8%. Contributions from earthquakes within the range

from 600 to 700 km are about 3%, although the maximum

magnitude considered is 9.0. Looking at the trend, the con-

tribution from earthquakes located more than 700 km away

would be negligible.

Estimated PGA for Rock Site in Kuala Lumpur

Figure 14 shows the probability of ground motion Iex-

ceeding a value of i if an earthquake occurs in the region

between 200 and 600 km from Kuala Lumpur, FI(i). It also

shows that for a given i, the exceedance probability using

the Youngs et al. (1997) attenuation relationship is larger

than that of Fukushima and Tanaka (1992).

Figure 15 shows the probability of ground acceleration

i being exceeded in 50 yr. Between January 1963 and De-

cember 1999, 612 earthquakes occurred in the region con-

sidered. Thus, the average occurrence rate is 16.5 earth-

quakes per year. For 10% probability of i being exceeded,

PE 0.1, which is equivalent to FI(i) 1.274 104,

the values ofi are summarized in Table 3. The PGA is 29.5gal for the design-basis earthquake. Considering Fukushima

and Tanaka (1992) with r 0.49, the probable and the

maximum credible PGAs are 13.3 and 55.1 gal, respectively.

Figure 16 shows the FI(i) ofi 29.5 gal as functions

of magnitudemand distancer, using Fukushima and Tanaka

(1992) with r 0.49. The total volume under the surface

is equal to 1.274 104. It can be seen that the main con-

tributions, about 91% of the total volume, come from earth-

quake within a distance of 200400 km, although the prob-

ability of earthquake occurrence in this distance range is

only 27.3%. Contributions from earthquake farther than 500

km are less than 1%, so that excluding earthquakes with

distance greater than 600 km is justifiable.

Discussion

The choice of an attenuation relationship is a critical

step in seismic hazard assessment analysis. Based on the

PGAs recorded in Singapore of 52 recent Sumatra earth-

quakes, the Fukushima and Tanaka (1992) relationship ap-

pears to fit much better to the observed data than the other

attenuation relations do, as shown in Figure 10. However,

Figure 10 also shows that for Mw larger than 6.0, the Fu-

kushima and Tanaka (1992) relationship seems to underpre-dict the observed data. The fact that the long-distance data

have been observed over a short 4-yr period at a single sta-

tion might suggest unsteadiness in the fitting. The derived

standard deviation of 0.49, which is larger than the 0.21

suggested by Fukushima and Tanaka (1990), was therefore

adopted in the present study to accommodate the unsteadi-

ness. The Youngs et al. (1997) relationship is the second

best in fitting the observed data, especially for large distance

and magnitude. By using the Youngs et al. (1997) attenua-

tion relationship, the PGAs estimated for 10% exceedance


11/13


Figure13. Probability of PGA exceeding 12.7 gal for Singapore as a function ofearthquake magnitude and distance, using Fukushima and Tanaka (1992) as the atten-

uation relationship withr

0.49.

Figure 14. Probability of ground motion I ex-ceeding a value ofi if an earthquake occurs at a dis-tance between 200 and 600 km from Kuala Lumpur,FI(i).

Figure 15. Probability of ground acceleration ibeing exceeded in 50 yr, for Kuala Lumpur.

probability in 50 yr in Singapore and Kuala Lumpur are 3.5

and 1.5 times, respectively, of those estimated based on the

Fukushima and Tanaka (1992) relationship. The estimated

values for Kuala Lumpur do not differ much between the

two attenuation relationships, because the contribution from

earthquakes with epicentral distance between 200 and 350

km (see Fig. 16) is predominant, and the PGAs predicted by

the two attenuation equations in this distance range are con-

sistent.

The probability of ground motions in the two cities con-

tributed by other earthquake sources has not been included

here. First, the pattern of the spatial distribution of earth-

quakes is based on earthquake data from 1963 to 1999. If

the reference time span had been longer, earthquakes occur-

ring closer to the cities might have had to be taken into

account. Although these are rare events, their contributions

to the total probability of a given PGA being exceeded can

be significant. This trend has been shown in Figures 13 and

16 that earthquakes occurring closer to the respective cities


12/13


Figure16. Probability of PGA exceeding 29.5 gal in Kuala Lumpur as a functionof earthquake magnitude and distance, using Fukushima and Tanaka (1992) as the

attenuation relationship withr

0.49.

contribute significantly to the exceedance probability of a

given PGA value, although the probability of occurrence of

the short-distance earthquakes is small. Second, no allow-

ance has been made for local earthquakes, and only Sumatra

earthquakes have been taken into consideration. The Malay

Peninsula is considered a low-seismicity region. Several

known inactive faults have not slipped in the modern history

of the Peninsula. These faults might slip at some time in the

future and could generate small- to moderate-size earth-

quakes, which would affect the area much more greatly than

huge earthquakes at a far distance.

Although Singapore and Kuala Lumpur are located in

the same low-seismicity region, Kuala Lumpur has a 500-yr

return-period PGA more than two times that of Singapore.

This is because Kuala Lumpur is closer to the northern Su-

matra seismic zone, as shown in Figure 1. There are more

earthquakes within a radius of 350 km from Kuala Lumpur

than those within the same radius from Singapore.

Conclusions

Based on the PGAs recorded recently in Singapore of

distant Sumatran earthquakes, it has been found that the re-gion has a high attenuation rate, which might be attributed

to the seismic and volcanic activities along the western coast

of Sumatra Island. Given that the tectonic setting of Sumatra

and the Malay Peninsula is similar to that of Japan, the Q

value of the Sumatra region is likely to be comparable to

that of Japan. The attenuation relationship of Fukushima and

Tanaka (1992) has been shown to fit well to the observed

long-distance Sumatran seismic data.

The design-basis PGAs on rock outcrop sites in Singa-

pore and Kuala Lumpur have been computed based on prob-

abilistic analysis of the earthquake data of Sumatra in the

past 37 yr and adopting the Fukushima and Tanaka (1990)

attenuation relationship. The PGA values are 12.7 and 29.5

gal for Singapore and Kuala Lumpur, respectively. The de-

sign PGA value is higher for Kuala Lumpur because of its

proximity to the northern Sumatra seismic zone.

Current building design codes in Singapore and Malay-

sia have been formulated largely based on the BS8110 Code

(British Standards Institution, 1985), which does not have

any provision for seismic loading. It does, however, require

that all buildings be capable of resisting a notional ultimate

horizontal design load applied at each floor level simulta-

neously, for structural robustness. The notional horizontal

load is equal to 1.5% of the characteristic dead weight of a

structure, and the design wind load should not be taken less

than this value. Given the moderate design wind speed of

30 m/sec in Singapore, the notional horizontal load is gen-

erally greater than the wind loading for most buildings.

Thus, the notional horizontal load is usually the governing

lateral load. The base shear force resulting from the calcu-

lated design-basis PGAs of 1.3% g and 3.0% g for rock

outcrop site are therefore comparable to the capacity re-

quired by the notional horizontal load. The maximum cred-ible PGAs of 2.4% g in Singapore and 5.5% g in Kuala

Lumpur would certainly produce a base shear force higher

than that stipulated in the current building codes.

The design-basis PGAs for soft-soil sites or reclaimed

land could be estimated based on the PGA values for rock

outcrop sites using the site response analysis. Knowing that

soft-soil deposits are exposed extensively in the two cities

and that areas of reclaimed land are increasing, the seismic

hazard, especially to medium- and high-rise buildings in

these sites, should be investigated further.


13/13


Acknowledgments

The authors would like to acknowledge the contribution of Research

Fellow Dr. Jichun Sun in the initial stage of this study. The Sumatra earth-

quake data used in the analysis were compiled by Research Scholar Mr.

Chin Long Lee.

ReferencesAbe, K. (1981). Magnitudes of large shallow earthquakes from 1904 to

1980,Phys. Earth Planet. Interiors27, 7292.

Abe, K. (1982). Magnitude, seismic moment and apparent stress for major

deep earthquakes,J. Phys. Earth 30, 321330.

Abe, K. (1984). Complements to magnitudes of large shallow earthquakes

from 1904 to 1980,Phys. Earth Planet. Interiors 34, 1723.

Abe, K., and H. Kanamori (1979). Temporal variation of the activity of

intermediate and deep focus earthquakes, J. Geophys. Res. 84, 3589

3595.

Abe, K., and S. Noguchi (1983a). Determination of magnitude for large

shallow earthquakes 18981917, Phys. Earth Planet. Interiors 32,

4559.

Abe, K., and S. Noguchi (1983b). Revision of magnitudes of large shallow

earthquakes 18971912,Phys. Earth Planet. Interiors33, 111.

Abrahamson, N. A., and K. M. Shedlock (1997). Overview, Seism. Res.

Lett.68, 923.

Atkinson, G. M., and D. M. Boore (1995). Ground-motion relations for

Eastern North America,Bull. Seism. Soc. Am. 85, 1730.

Boore, D. M., W. B. Joyner, and T. E. Fumal (1997). Equations for esti-

mating horizontal response spectra and peak acceleration from West-

ern North American earthquakes: a summary of recent work, Seism.

Res. Lett.68, 128153.

British Standards Institution (1985). Structural Use of Concrete, Part 1.

Code of Practice for Design and Construction, BS 8110: Part 1: 1985,

British Standards Institution, London.

Demets, C., R. G. Gordon, D. F. Argus, and S. Stein (1990). Current plate

motions, Geophys. J. Int. 101, 425478.

Dong, W. M., A. B. Bao, and H. C. Shah (1984). Use of maximum entropy

principle in earthquake recurrence relationships,Bull. Seism. Soc. Am.

74, 725737.

Dunbar, P. K., P. A. Lockridge, and L. S. Whiteside (1992). Catalog of

significant earthquakes, including quantitative casualties and damage,

National Oceanic and Atmospheric Administration, National Envi-

ronment Satellite Data and Information Service, NationalGeophysical

Data Center, Report SE-49, 320 pp.

Fauzi, R. McCaffrey, D. Wark, Sunaryo, and P. Y. P. Harydi (1996). Lateral

variation in slab orientation beneath Toba Caldera, northern Sumatra,

Geophys. Res. Lett. 23, 443446.

Fitch, T. J. (1972). Plate convergence, transcurrent faults, and internal de-

formation adjacent to Southeast Asia and the western Pacific, J. Geo-

phys. Res. 77, 44324460.

Fukushima, Y. (1997). Comment on Ground motion attenuation relations

for subduction zones, Seism. Res. Lett. 68, 947949.

Fukushima, Y., and T. Tanaka (1990). A new attenuation relation for peak

horizontal acceleration of strong earthquake ground motion in Japan,Bull. Seism. Soc. Am. 80, 757783.

Fukushima, Y., and T. Tanaka (1992). Revised attenuation relation of peak

horizontal acceleration by using a new data base, Programme Ab-

stracts Seism. Soc. Jpn., 2, 116 (in Japanese).

Gutenberg, B., and C. F. Richter (1949). Seismicity of the Earth, Princeton

Univ. Press, Princeton, New Jersey.

Heaton, T., F. Tajimi, and A. W. Mori (1986). Estimating ground motions

using recorded accelerograms, Surv. Geophys. 8, 2583.

International Conference of Building Officials (1991). Uniform Building

Code, Whittier, California.

Katili, J. A., and F. Hehuwat (1967). On the occurrence of large transcurrent

faults in Sumatra, Indonesia,J. Geosci. Osaka City Univ. 10, 517.

Lomnitz, C. (1974). Global Tectonics and Earthquake Risk, Elsevier, Am-

sterdam.

McCaffrey, R. (1991). Slip vectors and stretching of the Sumatran fore arc,

Geology19, 881884.

National Earthquake Information Center (1994). Earthquake Data Base

System Documentation, United States Geological Survey, Denver,

Colorado.

Newcomb, K. R., and W. R. McCann (1987). Seismic history and seis-

motectonics of the Sunda Arc,J. Geophys. Res. 92, 421439.

Pacheco, J. F., and L. R. Sykes (1992). Seismic moment catalog of large

shallow earthquakes, 1900 to 1989, Bull. Seism. Soc. Am. 82, 1306

1349.

Pan, T.-C. (1995). When the doorbell ringsa case of building response

to a long distance earthquake,Earthquake Eng. Struct. Dyn.24, 1343

1353.Pan, T.-C., and J. Sun (1996). Historical earthquakes felt in Singapore,Bull.

Seism. Soc. Am. 86, 11731178.

Pan, T.-C., K. Megawati, J. M. W. Brownjohn, and C. L. Lee (2001). The

Bengkulu, Southern Sumatra, Earthquake of 4 June 2000 (Mw 7.7):

another warning to remote metropolitan areas, Seism. Res. Lett. 72,

171185.

Seed, H. B., M. P. Romo, J. P. Sun, A. Jaime, and J. Lysmer (1987).

Relationships between soil conditions and ea rthquake ground motions

in Mexico City in the earthquake of September 19, 1985,UCB/EERC-

87/15, Univ. of California, Berkeley, California.

Sieh, K., and D. Natawidjaja (2000). Neotectonics of the Sumatran fault,

Indonesia,J. Geophys. Res. 105, 28,29528,326.

Tregoning, P., F. K. Brunner, Y. Bock, S. S. O. Puntodewo, R. McCaffrey,

J. F. Genrich, E. Calais, J. Rais, and C. Subarya (1994). First geodetic

measurement of convergence across the Java Trench, Geophys. Res.Lett.21, 21352138.

Youngs, R. R., S.-J. Chiou, W. J. Silva, and J. R. Humphrey (1997). Strong

ground motion attenuation relationships for subduction zone earth-

quakes,Seism. Res. Lett. 68, 5873.

Zachariasen, J., K. Sieh, F. W. Taylor, R. L. Edwards, and W. S. Hantoro

(1999). Submergence and uplift associated with the giant 1833 Su-

matran subduction earthquake: evidence from coral microatolls,Geo-

phys. Res. Lett.104, 895919.

Protective Technology Research Centre

School of Civil and Structural Engineering

Nanyang Technological University

Block N1, Nanyang Avenue

Singapore 639798

Republic of Singapore

Manuscript received 27 February 2001.

earthquake design

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