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THE DETERMINATION OF PEAK GROUND ACCELERATION 12-1

The Determination of Peak Ground Acceleration at Bantul

Regency, Yogyakarta Province, Indonesia

Teuku Faisal Fathani

Civil and Environmental Engineering Department, Faculty of Engineering, Gadjah Mada

University, Yogyakarta, Indonesia, 55281, email: [email protected] Darmawan Adi

Civil and Environmental Engineering Department, Faculty of Engineering, Gadjah Mada

University, Yogyakarta, Indonesia, 55281, email: [email protected]

Subagyo Pramumijoyo

Geological Engineering Department, Faculty of Engineering, Gadjah Mada University,

Yogyakarta, Indonesia, 55281, email: [email protected]

Dwikorita Karnawati

Geological Engineering Department, Faculty of Engineering, Gadjah Mada University,

Yogyakarta, Indonesia, 55281, email: [email protected], [email protected]

ABSTRACTThe horizontal peak ground acceleration at Bantul area, Yogyakarta

Province is calculated based on the Indonesian code of SNI-1726-2002

coupled with the local soil conditions determined from Standard

Penetration Test (SPT) results and based on empirical prediction by using

attenuation relationships. The average value of SPT until a depth of 30 m at

the 10 sites surrounding Bantul area was investigated. The SPT values

varied from 18.60 to 36.85. Accordingly, by referring to SNI-1726-2002, the

soil at 10 study sites was classified as medium soil with peak ground

acceleration from 0.225g to 0.288g. The empirical prediction of peak

ground acceleration at Bantul area is determined based on the M w 6.3

Yogyakarta earthquake of May 27, 2006, considering two Scenarios ofepicenter coordinate and hypocenter depth based on the Indonesia

Meteorological and Geophysical Agency (BMG) and United States

Geological Survey (USGS). According to the attenuation relationships, the

peak ground accelerations at the study sites vary from 0.209g to 0.322g. An

attenuation relationship based on the dominant period at the observed sites,

earthquake magnitude and hypocenter distance has been used to estimate

the peak ground acceleration. The dominant period of the ground is

assumed as the ground period produced by the micro-tremor survey

conducted on the study area. As the result, the peak ground accelerations

vary from 0.140g to 0.534g. In the south and east part of the study area, the

peak ground accelerations produced by the Indonesian code of SNI-1726-2002 are considerably lower than those produced by the attenuation

relationships based on the Yogyakarta earthquake of May 27, 2006 and the

results of micro-tremor survey. Therefore, it is prudent to reconsider and

revise the Indonesian building code based on the local soil condition and

ground amplification, for use in regional planning and development

projects.


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THE YOGYAKARTA E ARTHQUAKE OF M AY 27, 2006 12-2

INTRODUCTION

Peak Ground Acceleration is one of the

most difficult parameters to determine. It

represents an acceleration that will be

induced sometime in the future by an

earthquake on a particular area. Since it is

not possible to predict earthquakes, the

value of peak ground acceleration must be

based on prior earthquakes and faults

studies. The peak ground acceleration is

determined based on source, seismicity, and

attenuation relationships. Some of the more

commonly used methods to determine the

peak ground acceleration at a site are

historical earthquake, maximum credible

earthquake, maximum probable earthquake,

code or other regulatory requirement, and

earthquake maps (Day, 2002).In response to the Mw 6.3 Yogyakarta

earthquake of May 27, 2006 an

investigation to determine horizontal peak

ground acceleration in the area affected

earthquake was conducted. The

investigation was done by core drilling,

Standard Penetration Test (SPT) and micro-

tremor survey in the area surrounding

Bantul Regency, Yogyakarta Province. This

research deals with the analysis of peak

ground acceleration based on (1) the

Indonesian code of SNI-1726-2002 coupledwith local soil conditions determined from

Standard Penetration Test (SPT) results; (2)

the empirical prediction by using

attenuation relationships, which relate the

peak ground acceleration to the earthquake

magnitude and the distance between the site

and the seismic source; and (3) the

attenuation relationship based on the

dominant period at the observed sites

produced by the micro-tremor survey.

INDONESIAN CODE OF SNI-1726-2002

In each country, there is a local building

code that specifies design values of peak

ground acceleration. In Indonesia, the peak

ground acceleration can be obtained by

using the building code of SNI-1726-2002.

This code divided Indonesia into 6 seismic

zones considering the probability of

exceedance of buildings with 50 years life

time is 10 percent and seismic design load

of 500 years return period. The

determination of national standard for peak

ground acceleration in this code was based

on the historical earthquakes, maximum probable earthquakes, and local soil

conditions.

The maximum probable earthquake is

the largest predicted earthquake that a fault

is capable of generating within a specified

time period based on a study of nearby

active faults. Maximum probable

earthquake are most likely to occur within

the design life of the project, therefore they

have been commonly used in assessing

seismic risk. The peak ground acceleration

is determined as the value that has a certain probability of exceedance in a specific

number of years. A commonly used

definition of a maximum probable

earthquake is an earthquake that will

produce a peak ground acceleration with a

50 percent probability of exceedance in 50

years (USCOLD, 1985). The design basis

ground motion can often be determined by

a site-specific hazard analysis, or from a

hazard map. Various maps showing peak

ground acceleration with a 2, 5, or 10

percent probability of exceedance in 50

years provide the choice of the appropriate

level of hazard or risk. Such an approach is

termed the probabilistic method, with the

choice of peak ground acceleration based

on the concept of acceptable risk.

Earthquake Return Period

Earthquake return period for lifelines

facilities should be based on the following

criteria (Ferritto, 1992).

1) Ordinary category of construction onaverage seismicity sites

Ordinary facilities can be designed

based on earthquake with an

approximate 10 percent probability of

exceedance in 50 years.

2) High seismicity or essential category ofconstruction

Facilities that are deemed important


3/15


and essential shall use a two-earthquake

procedure. Level 1 earthquake has a 50


years. Level 2 earthquake has a 10


years.

3) Facilities containing polluting orhazardous material

Facilities containing polluting or

hazardous material should be designed

based on Level 3 earthquake having a 10


years.

In any case, a design load less than

suggested by a code is not permitted. The

seismic load in Indonesian building code of

SNI-1726-2002 is based on three factors: a

probability of exceedance in a certain period, ductility factor, and structural over

strength factor. In the explanation above,

the relation between probability of

exceedance, facility life time and the

earthquake return period are as follows.

%1001 ⎟⎟

⎠

⎞⎜⎜

⎝

⎛ −= −

T

L

e p …………..…. (1)

or

( )( ) p

e LT

−

−=

1log

log …………….... (2)

Seismic Zones

According to plate tectonic theory,

earthquakes commonly occur at the

locations around the plate boundaries. The

plate boundaries in Indonesia can be seen in

Figure 1.

Figure 1 Earthquake epicenter and plate boundaries in and adjacent to Indonesia (USGS).

According to SNI-1726-2002, Indonesia

is divided into 6 seismic zones and its

response spectra is shown in Figure 2,

where seismic zone 1 has the lowest

earthquake hazard and seismic zone 6 hasthe highest earthquake hazard. These

seismic zones are divided based on the peak

ground acceleration of earthquake with 500

years return period, where the average

values of peak bedrock acceleration and

peak ground acceleration for each seismic

zone can be seen in Figure 2 and Table 1.

From Figure 2, it can be seen that the study

site is located in Bantul Regency

Yogyakarta, which is in seismic zone 3. Ifthe peak ground acceleration cannot be

analyzed using the wave propagation

theory, the peak ground acceleration in each

zone for each type of soil can be taken from

Table 1.

Earthquakes Plate Boundaries


4/15


16o

14o

12o

10o

8o

6o

4o

2o

0o

2o

4o

6o

8o

10o

16o

14o

12o

10o

8o

6o

4o

2o

0o

2o

4o

6o

8o

10o

94o

96o

98o

100o

102o

104o

106o

108o

110o

112o

114o

116o

118o

120o

122o

124o

126o

128o

130o

132o

134o

136o

138o

140o

94o

96o

98o

100o

102o

104o

106o

108o

110o

112o

114o

116o

118o

120o

122o

124o

126o

128o

130o

132o

134o

136o

138o

140o

Banda Aceh

Padang

Bengkulu

Jambi

Palangkaraya

Samarinda

BanjarmasinPalembang

Bandarlampung

Jakarta

Sukabumi

Bandung

Garut Semarang

Tasikmalaya Solo

Blitar Malang

BanyuwangiDenpasar Mataram

Kupang

Surabaya

Jogjakarta

Cilacap

Makasar

Kendari

Palu

Tual

Sorong

Ambon

Manokwari

Merauke

Biak

Jayapura

Ternate

Manado

Pekanbaru

: 0,03 g

: 0,10 g

: 0,15 g

: 0,20 g

: 0,25 g

: 0,30 g

Wilayah

Wilayah

Wilayah

Wilayah

Wilayah

Wilayah

1

1

1

2

2

3

3

4

4

56

5

1

1

1

1

1

1

2

2

2

22

2

3

3

3

33

3

4

4

4

44

4

5

5

5

55

5

6

6

6

4

2

5

3

6

0 80

Kilometer

200 400

Zone

Zone

Zone

Zone

Zone

Zone

Figure 2. Seismic zones in Indonesia (SNI-1726-2002).

Table 1. Bedrock acceleration and peak ground acceleration for each seismic zone and type of soil in

Indonesia

Peak ground acceleration amax (g)

Seismic ZonesBedrock

acceleration (g) Hard Soil Medium

Soil

Soft Soil Special Soil

1

2

3

4

56

0,03

0,10

0,15

0,20

0,250,30

0,04

0,12

0,18

0,24

0,280,33

0,05

0,15

0,23

0,28

0,320,36

0,08

0,20

0,30

0,34

0,360,38

Required

special

evaluation in

each zone

Soil Classification

Soil condition, where a structure is built

on it, has a very significant influence to the

level of seismic load to be considered.

According to SNI-1726-2002, soil

conditions can be classified into three

types: soft soil, medium soil, and hard soil.

These three soil classifications can be

determined if the top 30 m of soil thicknesssatisfies one of the requirements listed on

Table 2.

In Table 2, sv , N and uS are the

average values of each soil layer that can be

determined by using a weighting method

with the following equations:

si

m

i

i

m

i

i

vt

t

sv/

1

1

∑

∑

=

== ……………..……. (3)

i

m

i

i

m

i

i

N t

t

N /

1

1

∑

∑

=

== ………………...….. (4)

ui

m

i

i

m

ii

S t

t

uS /

1

1

∑∑

=

== …………………..... (5)

where t i is the soil thickness of the ith

soil

layer, vsi is the shear wave velocity through

the ith

soil layer, N i is the value of SPT of

the ith

soil layer, S ui is the undrained shear

strength of the ith

soil layer and m is the


5/15


number of soil layer above the bedrock. In

Table 2, PI is the Plasticity Index of the

clay, wn is natural moisture content and S u

is undrained shear strength of the soil. The

special soil mentioned in Table 2 is a soil

type that does not satisfy any requirement

in the table. Moreover, the special soil canalso be classified as a soil that has a high

potential for liquefaction, or may be

sensitive clay, or a deteriorated cemented

sand, or a soil with a high content of

organic matter where the thickness is more

than 3 m, or is a soft clay with a PI of more

than 75% and the thickness is more than 10

m, clay layer with 25 kPa < S u < 50 kPa and

the thickness is more than 30 m. The peak

ground acceleration of special soil must bedetermined from a wave propagation

analysis.

Table 2. Soil classification based on SNI-1726-2002

Type of soil Average shear wave

velocity, sv (m/s)

Average Standard

Penetration Test (SPT),

N

Average undrained

shear strength, uS

(kPa)

Hard soilsv > 350 N > 50 uS > 100

Medium Soil 175 < sv < 350 15 < N < 50 50 < uS < 100

sv < 175 N < 15 uS < 50Soft Soil

or, any soft soil profile where the total thickness is more than 3 m withPI > 20%, wn > 40 % and S u < 25 kPa

Special Soil Required special examination on every site

EMPIRICAL GROUND MOTION

PREDICTION

Empirical prediction of peak ground

acceleration invariably incorporates a

dependence on the earthquake size anddistance to the hypocenter or active faulting.

Based on the past recordings of strong

earthquake motion, the peak ground

acceleration can be described by a ground

motion prediction equation as a function of

the earthquake magnitude, distance from

the hypocenter or the fault to the site, and

general site conditions parameter. Figure 3

shows one of such relationships, developed

by Abrahamson and Silva (1997) for crustal

earthquakes in tectonically active areas.

Since peak ground acceleration is themost commonly used ground motion

parameter, many peak ground acceleration

relationships have been developed. The

most broadly based relationship between

local magnitude (Richter scale), hypocenter

distance and peak ground acceleration was

provided by Donovan (1973) as shown in

Equation 6. The equation expresses the

mean of 678 acceleration values of Western

U.S., Japan and Papua New Guinea, and

represents a conservative estimation of

mean peak ground acceleration on sites

with 6 m or more of soil overlying the rock.

Figure 3. Peak ground acceleration as afunction of magnitude and distance from the

hypocenter or the fault (Abrahamson and

Silva, 1997).

( ) 32.15.0

max25

1080

+

⋅=

R

ea

M

…………………. (6)

where amax is peak ground acceleration

(cm/sec2), R is distance from the hypocenter


6/15


(km), and M is earthquake local magnitude

of Richter scale.

Another attenuation relationship based

on statistically evaluated data is that of

Esteva (1974), who gives the expression for

peak ground acceleration based on

California data and is valid for focaldistances in excess of 15 km, as shown in

Equation 7. Matuschka (1980) also

provided the similar attenuation

relationship in Equation 8.

( )28.0

max40

5600

+

⋅=

R

ea

M

………………. (7)

( ) 15.181.0

max25

119

+

×=

R

ea

M

……..………… (8)

It should be noted that attenuation

equations are generally inappropriate forthe epicentral area, i.e. within a distance of

about 15-20 km from the epicenter. This

area needs special consideration, and the

understanding of it is still very limited.

Moreover, Campbell (1981) used

worldwide data to develop an attenuation

relationship for the mean peak ground

acceleration for sites within 50 km of the

fault rupture in magnitude 5.0 to 7.7

earthquakes:

) M

R M a7.0

max 606.0ln09.1868.0141.4ln +−+−= ……………………………………. (9)

where M is the local magnitude or surface

wave magnitude for magnitudes less than or

greater than 6, respectively, and R is the

closest distance to the hypocenter or the

fault rupture in kilometers.

An attenuation equation for peak

horizontal acceleration

applicable to the

near source region in Japan was developed

by Fukushima and Tanaka (1990). The data

base consists of 1372 horizontalcomponents of peak

ground acceleration

from 28 earthquakes in Japan and 15

earthquakes in the United States and other

countries. Coefficients describing

the

decrease in acceleration with increasing

distance found by most previous studies of

Japanese data are significantly smaller than

those found by analyzing individual

earthquakes. The resulting relation in Japan

is

log amax = -0.41 M – log( R+0.032.100.41 M

) –

0.0034 R + 1.30 …………..(10)

where amax is the mean of peak acceleration

from two horizontal

components at each site(cm/sec

2), R is the shortest distance

between the site and hypocenter or fault

rupture (km), and M is the surface-wave

magnitude. Effects of four different ground

conditions (rock, hard-, medium- and soft-

soils) on the attenuation relation were also

examined. Average peak horizontal

accelerations for the rock and the soft-soil

sites are 60 percent and 140 percent,

respectively of the value predicted from the

equation.

Kanai (1966) proposed an attenuationrelationship for peak ground acceleration

based on the dominant period at the

observed site, earthquake magnitude of

Richter scale and the hypocenter distance,

as shown in Equation 11.

R R

R M

gT

a

83,1167.0log

6.366.161.0

max 105 −+⎟ ⎠

⎞⎜⎝

⎛ +−

=

…………………………….…… (11)

where amax is the peak ground accelerationat the site (cm/sec2), T g is the dominant

period or the fundamental period of the

ground (s), R is the shortest distance

between site and the hypocenter or the fault

rupture (km), and M the earthquake

magnitude of Richter scale.

THE CALCULATION RESULTS AND

DISCUSSION

In order to provide a peak ground

acceleration distribution map which can

estimate the zonation of earthquakevulnerability and susceptibility at various

levels of risk, it is necessary to define a

scoring system representing each value of

peak ground acceleration appropriate with

the local building strength conditions in

Bantul Regency, Yogyakarta. Table 3

shows five levels of scoring system for

peak ground acceleration used in this study.


7/15


Table 3. Scoring system used in developing peak

ground acceleration distribution map

No Peak Ground

Acceleration (g)Level

ScoreLevel of

Risk

1 amax < 0.10 1 Very

low risk

2 0.10 ≤ amax < 0.20 2 Lowrisk

3 0.20 ≤ amax < 0.30 3 Mediumrisk

4 0.30 ≤ amax < 0.40 4 Highrisk

5 amax ≥ 0.40g 5 Veryhigh

risk

Peak ground acceleration based on the

code of SNI-1726-2002By considering the seismic zones shown

in Figure 2, the study area at Bantul

Regency, Yogyakarta is located in the

seismic zone 3. The peak ground

acceleration in each location for each type

of soil can be taken from Table 1.

Meanwhile, the soil classification is

decided by using the results of site

investigations of standard penetration

testing at 10 bore holes. From SPT results,

the values of N until a depth of 30 m are

shown in Figure 4. The average values of N

are used for determining the soil

classification on each site. By using

Equation 4, the average values of N until a

depth of 30 m at the sites surrounding

Bantul area, Yogyakarta vary from 18.60 to

36.85. Therefore, the soil can be classified

as medium soil. Table 4 shows thecalculation results of peak ground

acceleration based on SNI-1726-2002.

The soil classification system in SNI-

1726-2002 uses a very rough interval. From

Table 2, the average N-SPT value in the

interval of 15 to 50 at the same seismic

zone produces a similar value of peak

ground acceleration. Likewise, the average

N-SPT values at Watu, Tempuran, Pranti,

BPKP-1, BPKP-2, Karangsemut,

Segoroyoso, Bambanglipuro, Wijirejo and

Krajan area differ from 18.60 to 36.85. Thevalues of peak ground acceleration at these

10 study sites are determined by

interpolating the value in Table 2 based on

the average value of N-SPT. Accordingly,

by referring to SNI-1726-2002, the peak

ground accelerations at 10 study sites are

0.225g to 0.288g. On the basis of the

calculation results shown in Table 4, the 10

study sites are classified of having a

medium level of risk (level 3).

Table 4. The calculation results of peak ground acceleration based on SNI-1726-2002 in seismic zone 3

Boring

No.

Location Average

SPT

( N )

Soil

Classification

Bedrock

Acceleration

(g)

Peak Ground

Acceleration

(g)

Level

of

Risk

BH1 Watu 27.53 Medium soil 0.15 0.257 3

BH2 Tempuran 36.85 Medium soil 0.15 0.225 3

BH3 Pranti 18.60 Medium soil 0.15 0.288 3

BH4 BPKP-1 24.00 Medium soil 0.15 0.269 3

BH5 BPKP-2 25.90 Medium soil 0.15 0.263 3BH6 Karangsemut 30.50 Medium soil 0.15 0.247 3

BH10 Segoroyoso 33.20 Medium soil 0.15 0.238 3

BH11 Bambanglipuro 26.47 Medium soil 0.15 0.261 3

BH12 Wijirejo 31.13 Medium soil 0.15 0.245 3

BH13 Krajan 26.93 Medium soil 0.15 0.259 3


8/15


Location: BH-2 Tempuran

N = 36.85

0

5

10

15

20

25

30

35

40

0 20 40 60

Standard Penetration Test (N )

D e p t h ( m )

Location: BH-3 Pranti

N = 18.60

0

5

10

15

20

25

30

35

40

0 20 40 60


D e p t h ( m )

Loca tion : BH-4 BPKP-1

N = 24.00

0

5

10

15

20

25

30

35

40

0 20 40 60


D e p t h ( m )

Locat ion: BH-5 BPKP-2

N = 25.90

0

5

10

15

20

25

30

35

40

0 20 40 60


D e p

t h ( m )

Location: BH-6 Karangsemut

N = 30.50

0

5

10

15

20

25

30

35

40

0 20 40 60


D e p

t h ( m )

Location: BH-10 Segoroyoso

N = 33.20

0

5

10

15

20

25

30

35

40

0 20 40 60


D e p t h ( m )

Location: BH-11 Bb.lipuro

N = 26.47

0

5

10

15

2025

30

35

40

0 20 40 60


D e p t h ( m )

Location: BH-12 Wijir ejo

N = 31.13

0

5

10

15

20

25

30

35

40

0 20 40 60


D e p t h

( m )

Location: BH-13 Krajan

N = 26.93

0

5

10

15

20

25

30

35

40

0 20 40 60


D e p t h

( m )

Location: BH-1 Watu

N = 27.53

0

5

10

15

20

25

30

35

40

0 20 40 60


D e p t h ( m )

Figure 4. Results of Standard Penetration Test.


9/15


Peak ground acceleration based on the

attenuation relationships

Earthquake Magnitude Scales

The previously described earthquake

magnitude scales are empirical quantities

based on various instrumentalmeasurements of ground shaking

characteristics. As the total amount of

energy released during an earthquake

increases, however, the ground-shaking

characteristics do not necessarily increase at

the same rate. For strong earthquakes, the

measured ground-shaking characteristics

become less sensitive to the size of the

earthquake than for smaller earthquakes.

This phenomenon is referred to as

saturation; the body wave (mb) and Richter

local magnitudes ( M L) saturate atmagnitudes of 6 to 7 and the surface wave

magnitude saturates at about M s = 8. To

describe the size of very large earthquakes,

the only magnitude of scale that does not

depend on ground-shaking levels, and

consequently does not subject to saturation

is the moment magnitude M w (Kanamori,

1977; Hanks and Kanamori, 1979), since it

is based on the seismic moment ( M o),

which is a direct measure of the factors that

produce rupture along the fault. Figure 5

shows the approximate relationships between several earthquake magnitude

scales. Saturation of the instrumental scales

is indicated by their flattening at higher

magnitude values.

The lines drawn in Figure 5 should only

be considered as approximate relationships,

representing a possible wide range in values.

Considering the limitations of Figure 5, it

could be concluded that the local magnitude

( M L) and moment magnitude scale ( M w) are

reasonably close to one another below a

value of about 7 (Day, 2002). Meanwhile,

the surface wave magnitude ( M s) slightly

deviates from the local magnitude ( M L) and

moment magnitude scale ( M w) below a

value of about 6. At high magnitude values,

the moment magnitude ( M w) tends to

significantly deviate from these other two

magnitude scales. The local magnitude

scales become saturated at an M L of about

7.3.

Figure 5. Approximate relationships between

moment magnitude scale ( M w) and other

magnitude scales: Richter local magnitude

( M L), surface wave magnitude ( M s), short-period body wave magnitude ( m b), and

Japanese Meteorological Agency magnitude

( M JMA) (After Idris, 1985).

Empirical Prediction of Peak Ground

Acceleration

The peak ground acceleration at 10 study

sites near the hypocenter of the Yogyakarta

earthquake can be determined roughly

based on Equations 6 ~ 10. By inputting the

hypocenter distance and the local

magnitude or surface wave magnitude of

the earthquake, the peak ground

acceleration (amax) can be calculated. The

empirical prediction of peak ground

acceleration in the study area is determined

based on the M w 6.3 Yogyakarta earthquake

of May 27, 2006. The calculation was done

by considering two Scenarios of earthquake

magnitude, epicenter coordinate and

hypocenter depth based on the Indonesia

Meteorological and Geophysical Agency

(BMG) and United States GeologicalSurvey (USGS) reports, as described

below:

a. Scenario 1 (BMG): the epicentercoordinate was located at 423960.78 E,

9115638.42 N, with the hypocenter

depth of 11.8 km. The short period body

wave magnitude (mb) is 5.9. Based on


10/15


Figure 5, the value of mb = 5.9 is

approximately equal to M w = 6.3.

b. Scenario 2 (USGS): the epicentercoordinate was located at 440265.66 E,

9119863.97 N, with the hypocenter

depth of 10 km and the moment

magnitude M w is 6.3.

The attenuation relationships between

peak ground acceleration, hypocenter

distance and earthquake magnitude in

Equations 6 ~ 10 are used to calculate the

peak ground acceleration surrounding

Bantul area. Table 5 shows the calculation

results of peak ground accelerations based

on Scenario 1 (BMG). The calculation

results of peak ground accelerations based

on Scenario 2 (USGS) are shown in Table 6.

The peak ground acceleration distributionmaps on each site based on two Scenarios

mentioned above are shown in Figure 6 and

Figure 7 for Scenario 1 (BMG) and

Scenario 2 (USGS), respectively.

Table 5. The calculation results of peak ground acceleration based on Scenario 1 (BMG)

Peak Ground Acceleration (amax) Boring

No.

Location

Donovan

(1973)

(g)

Esteva

(1974)

(g)

Matuschka

(1980)

(g)

Campbell

(1981)

(g)

Fukushima

& Tanaka

(1990)

(g)

Highest

amax

(g)

Level

of

Risk

BH1 Watu 0.216 0.322 0.311 0.241 0.287 0.322 4

BH2 Tempuran 0.204 0.302 0.295 0.210 0.265 0.302 4

BH3 Pranti 0.202 0.300 0.293 0.207 0.263 0.300 4

BH4 BPKP-1 0.161 0.233 0.241 0.132 0.195 0.241 3

BH5 BPKP-2 0.161 0.233 0.241 0.132 0.194 0.241 3

BH6 Karang semut 0.179 0.261 0.263 0.160 0.223 0.263 3

BH10 Segoroyoso 0.164 0.238 0.245 0.136 0.199 0.245 3

BH11 Bambang-

lipuro0.213 0.316 0.306 0.232 0.281 0.316 4

BH12 Wijirejo 0.191 0.281 0.279 0.182 0.243 0.281 3

BH13 Krajan 0.196 0.289 0.285 0.192 0.252 0.289 3

From Table 5 and 6, the highest peak

ground accelerations at 10 study sites which

are underlain by the medium soil vary from

0.241g to 0.322g and from 0.209g to 0.302g

for Scenario 1 and Scenario 2, respectively.

The highest values of peak ground

acceleration are provided by Esteva (1974),

Matuschka (1980) as well as the Indonesian

code of SNI-1726-2002. The Donovan

(1973) equation yields lower values of peak

ground acceleration since this method

represents a conservative estimation of

mean peak ground acceleration on sites

with 6 m or more of soil overlying the rock.

Meanwhile, Fukushima and Tanaka (1990)

equation produces higher values of peak

ground acceleration compared with

Donovan (1973) and Campbell (1981)

equations. Average peak horizontal

accelerations for the rock and the soft-soil

sites are 60% and 140% respectively of the

value predicted from the Fukushima and

Tanaka (1990) method. Hence, the mean

peak ground acceleration for medium soil at

10 study sites is assumed 100% of the value

predicted from this equation.

From Figures 6 and 7, the peak ground

accelerations at 10 study sites in the Bantul

area are dominated by the value higher than

0.25g. Therefore, the study sites are

classified from high to very high levels of

risk. The epicenter of BMG version is

located at the southern part of the study area,

hence, the very high risk area located at the

south-east part of the study area. On the

other hand, the epicenter of USGS version

is located at the east part, subsequently the

east and south-east part of the study area is


11/15


classified as a very high risk area. The

results reveal that the distance between the

site and the seismic source greatly affects

the peak ground acceleration determined by

the attenuation relationship.

Table 6. The calculation results of peak ground acceleration based on Scenario 2 (USGS)

Peak Ground Acceleration (amax

) Boring

No.

Location

Donovan

(1973)

(g)

Esteva

(1974)

(g)

Matuschka

(1980)

(g)

Campbell

(1981)

(g)

Fukushima

& Tanaka

(1990)

(g)

Highest

amax

(g)

Level

of

Risk

BH1 Watu 0.184 0.270 0.270 0.170 0.232 0.270 3

BH2 Tempuran 0.201 0.300 0.291 0.203 0.260 0.300 4

BH3 Pranti 0.201 0.300 0.292 0.204 0.261 0.300 4

BH4 BPKP-1 0.172 0.249 0.254 0.147 0.211 0.254 3

BH5 BPKP-2 0.171 0.249 0.254 0.147 0.210 0.254 3

BH6 Karang semut 0.202 0.301 0.292 0.205 0.262 0.301 4

BH10 Segoroyoso 0.202 0.302 0.293 0.206 0.262 0.302 4

BH11 Bambang-lipuro

0.173 0.251 0.256 0.149 0.213 0.256 3

BH12 Wijirejo 0.163 0.235 0.243 0.134 0.197 0.243 3

BH13 Krajan 0.137 0.193 0.209 0.101 0.157 0.209 3

Figure 6. Peak ground acceleration distribution map based on Scenario 1 (BMG).

Peak Ground Acceleration

BH-5

amax < 0.10g

0.10g ≤ amax < 0.20g0.20g ≤ amax < 0.30g0.30g ≤ amax < 0.40gamax ≥ 0.40g

BH-4

BH-12

BH-13

BH-11

BH-6

BH-10

BH-1

BH-2

BH-3

Epicenter(BMG Version)


12/15


Figure 7. Peak ground acceleration distribution map based on Scenario 2 (USGS).

Based on the calculation results of peak

ground acceleration by using SNI-1726-

2002 (Table 4) and attenuation relationships

(Table 5 and 6), it is prudent to provide an

earthquake microzonation and hazard map

based on the local condition in order torevise the Indonesian code of SNI-1726-

2002. By using this earthquake

microzonation and hazard map, the

zonation of earthquake vulnerability and

susceptibility at various levels of risk can

be estimated.

Attenuation Relationship based on the

dominant period at the observed sites

An attenuation relationship based on the

dominant period at the observed site,earthquake magnitude of Richter scale and

the hypocenter distance proposed by Kanai

(1966) is used to estimate the peak ground

acceleration. The resonant frequency

calculated from the micro-tremor data

theoretically has a close value with the

frequency calculated from the standard

analysis of direct measurement of high

magnitude earthquakes. In Equation 11, the

dominant period of the ground (T g) is

assumed as the ground period produced by

a micro-tremor survey. In this study, micro-

tremor survey was conducted at 243 sites

by Ratdomopurbo (2006) from the VolcanicSurvey of Indonesia (VSI), Yogyakarta.

By using the attenuation relationship

based on the dominant period at the

observed sites, the peak ground

accelerations at 10 study sites in the Bantul

area vary from 0.140g to 0.480g and from

0.146g to 0.534g for Scenario 1 (BMG) and

Scenario 2 (USGS), respectively. By

considering Table 3, the distribution map of

peak ground acceleration based on the data

from micro-tremor survey Scenario 1

(BMG) and the Scenario 2 (USGS) areshown in Figure 8 and 9.

From Figure 8 and 9, the study sites

consist of the low risk to very high level of

risk, depending on the dominant period at

the observed site based on the micro-tremor

survey and the distance between the site

and the seismic source.


amax < 0.10g


BH-4

BH-5

BH-12

BH-10

BH-6

BH-3

BH-2

BH-1

BH-11

BH-13

Epicenter(USGS Version)


13/15


Figure 8. Peak ground acceleration distribution map based on the data from micro-tremor

survey for Scenario 1 (BMG).

Figure 9. Peak ground acceleration distribution map based on the data from micro-tremor

survey for Scenario 2 (USGS).


amax < 0.10g

0.10g ≤ amax < 0.20g0.20g ≤ amax < 0.30g

0.30g ≤ amax < 0.40gamax ≥ 0.40g

Epicenter

(USGS Version)


amax < 0.10g


Epicenter(BMG Version)


14/15


According to the attenuation relationship

based on the dominant period in the

observed site for Scenario 1 (BMG), the

south-west and the center part of the study

site has a very high level of risk, whilst the

south and south-east part have a high levelof risk and the north part of the study area

has a low to medium level of risk.

Meanwhile, Scenario 2 (USGS), the south

and south-east of the study area have a high

to very high level of risk, while the north

part has a low to medium level of risk. The

results show that Scenario 2 (USGS) yields

a better distribution of the peak ground

acceleration, since the high to very high

level of risk reflects the presence of

structures related to the Opak-Oya fault

system. The peak ground accelerations

based on the data from micro-tremor survey

produced considerably higher values than

those produced by the Indonesia code of

SNI-1726-2002 and the other attenuation

equations.

CONCLUSIONS

The peak ground acceleration at Bantul

area, Yogyakarta was determined based on

the Indonesian code of SNI-1726-2002

coupled with the local soil conditionsdetermined from SPT results and based on

empirical prediction by using attenuation

relationships. The SNI-1726-2002 divided

Indonesia into 6 seismic zones based on the

peak bedrock acceleration of earthquake

considering the probability of exceedance

of buildings with 50 years life time is 10%

and seismic design load of 500 years return

period. The average values of N until a

depth of 30 m at the 10 study sites vary

from 18.60 to 36.85. According to SNI-

1726-2002, the soil at the study sites can beclassified into medium soil, the peak

ground accelerations are 0.225g to 0.288g,

and having a medium level of risk.

The empirical prediction of peak ground

acceleration in the study area was

determined based on the M w 6.3 Yogyakarta

earthquake of May 27, 2006. The

calculation was done by considering two

Scenarios of earthquake magnitude,

epicenter coordinate and hypocenter depth

based on the BMG and USGS version. The

attenuation relationships between peak

ground acceleration, hypocenter distance

and earthquake magnitude are used tocalculate the peak ground acceleration

surrounding Bantul area. The highest values

of peak ground acceleration are provided by

Esteva (1974), Matuschka (1980) and the

Indonesian code of SNI-1726-2002. As the

results of calculation, the peak ground

accelerations at 10 sites vary from 0.241g

to 0.322g and from 0.209g to 0.302g for

Scenario 1 and Scenario 2, respectively.

Moreover, the peak ground accelerations at

10 study sites are dominated by the value

higher than 0.25g and therefore could be

classified as medium risk to high level of

risk. An attenuation relationship based on

the dominant period at the observed site,

proposed by Kanai (1966) was used to

estimate the peak ground acceleration. The

results indicate that the peak ground

acceleration vary from 0.140g to 0.480g

and from 0.146g to 0.534g for Scenario 1

and Scenario 2, respectively. Based on this

attenuation relationship, the study sites are

classified as the low risk to very high levelof risk. It is prudent to provide an

earthquake microzonation and hazard map

at the study sites in order to revise the

Indonesian code of SNI-1726-2002, which

can estimate the zonation of earthquake

susceptibility at various levels of risk.

REFERENCES

Abrahamson, N.A. and Silva, W.J., 1997, Empirical

Response Spectral Attenuation Relations for

Shallow Crustal Earthquake, Seismol. Res. Lett.

Campbell, K.W., 1981, Near Source Attenuation ofPeak horizontal Acceleration, Bulletin of theSeismology Society of America 71.

Day, R.W., 2002, Geotechnical EarthquakeEngineering Handbook, Mc Graw-Hill, New

York.

Donovan, N.C., 1973, A Statistical Evaluation of

Strong Motion Data Including the February 9,1971 San Fernando Earthquake, Proc. 5

th

World Conference on Earthquake Engineering,

Rome.


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Esteva, L., 1974, Geology and Predictability in theAssessment of Seismic Risk, Proc. 2

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Report UG-0025, Naval Civil Eng. Laboratory,

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Hanks, T.C. and Kanamori, H., 1979, A Moment

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Ratdomopurbo, A., 2006, Micro-tremor Survey: theMeasurement of Ground Amplification at

Bantul Regency Yogyakarta, Reports, Volcanic

Survey of Indonesia, Yogyakarta (inIndonesian).

USCOLD, 1985, Guidelines for Selecting Seismic

Parameters for Dams Projects, Report of

Committee on Earthquakes, U.S. Committee on

Large Dams.

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