site response characteristics of h/v spectrum by...

J. SE Asian Appl. Geol., Jan–Jun 2013, Vol. 5(1), pp. 1–9

SITE RESPONSE CHARACTERISTICS OF H/VSPECTRUM BY MICROTREMOR SINGLESTATION OBSERVATIONS AT PALU CITY,INDONESIA

Pyi Soe Thein∗1, Subagyo Pramumijoyo1, Kirbani Sri Brotopuspito2 , Wahyu Wilopo1, JunjiKiyono3, and Agung Setianto1

1Geological Engineering Department, Faculty of Engineering, Universitas Gadjah Mada, Yogyakarta, Indonesia2Physics Department, Universitas Gadjah Mada, Yogyakarta, Indonesia3Graduate School of Global Environmental Studies, Kyoto University, Japan

Abstract

In this study, we estimated predominant period ofan H/V spectrum in Palu City, Indonesia, by usingmicrotremor single station observations. SulawesiIsland, eastern Indonesia, is located at the junctionbetween the converging Pacific-Philippine, Indo-Australian Plates and the Eurasian Plate. One ofthe major structures in Central Sulawesi is the Palu-Koro Fault system, which extends NNW-SSE direc-tion and cross-cuts Sulawesi along more than 300km from the North Sulawesi trench passing south-ward through Palu Bay then turn to the southeast,connecting to the Matano and Lawanopo Faults andfurther eastward both faults join to Tolo trench. Sev-eral earthquakes have been known along Palu-KoroFault system such as Gimpu earthquake (1905), Ku-lawi earthquake (1907), Kantewu earthquake (1934),and offshore Donggala earthquake (1968) whichcaused tsunami that destroyed 800 houses and killed200 people at Donggala district. Palu City, locatedat the northern tip of Palu depression, is a capitalof the Central Sulawesi Province. It is located inthe active seismic zone of the Palu-Koro fault. Spec-tral ratios for horizontal and vertical motion (H/V)from single-station microtremor records were usedto identify the predominant periods of the ground vi-

∗Corresponding author: P. S. THEIN, Department ofGeological Engineering, Faculty of Engineering, GadjahMada University, Jl. Grafika 2 Yogyakarta, 55281, Indone-sia. E-mail: [email protected]

brations. Understanding the parameters of predom-inant period[s] and seismichazard is important formitigation and environmental planning of the Paluregion.Keywords: H/V spectrum, predominant period[s],microtremor single station observation

1 Introduction

One of the major active structures in Sulawesiis the left lateral Palu-Koro Fault, which ex-tends NNW-SSE direction and cross cuts Cen-tral Sulawesi. This fault connects to the Northto the North Sulawesi trench and to the South,then turns to South East and connects with bothMatano and Lawanopo sinistral Faults (Hamil-ton, 1979). Further eastward, both faults joininto Tolo trench (Figure 1). Based on earthquakecatalog from USGS and Indonesia Institute ofMeteorology and Geophysics, during period of1968 to 2012, a hundred earthquakes with mag-nitude of 3–7 Richter were recorded in Sulawesiarea (Figure 2). These earthquakes clustered atnorthern arm of Sulawesi, where the active tec-tonic plates meet. In the Central Sulawesi, a lotof earthquakes occured in the zone associatedwith the Palu-Koro and Matano faults. Sev-eral better-known earthquakes along the Palu-Koro fault occured at Gimpu–1905, Kulawi–1907, Kantewu–1934 and offshore Donggala–

1

THEIN, et al.

Figure 1: Tectonic map of Sulawesi (Priadi,1993). Red circle: Palu depression area.

1968. The last earthquake with the focal depthof 23 km caused tsunami wave up to 10 m high.It has destroyed 800 houses and killed 200 peo-ple at Donggala district.

City of Palu, located at northern tip of Paludepression, is a capital of the Central SulawesiProvince. As a capital of province, Palu willbe a center of development where popula-tion growth by urbanisation usually increasesrapidly. Considering the seismic activity alongthe Palu-Koro fault zone, understanding theparameters of predominant periods and seis-michazard is important for mitigation andenvironmental planning.

In recent years, microtremor observationshave become popular for the purpose of inves-tigating soil structure because they do not re-quire much manpower and cost. Microtremoris a small ground vibration excited by artificialsources or natural phenomena such as factories,traffic, wind, waves, etc. As the vibration prop-agates through the ground surface, the surfacewaves such as the Rayleigh and Love waves aredominant.

In this study, Palu, where a large earthquakeis expected in the near future, is considered to

be a target area. We carried out higher den-sity single-point observations. Based on the ob-served data, we calculated the distribution ofpredominant Rayleigh wave.

2 Geometry of Palu depression

Geometry of the northern part of Palu-KoroFault system shows an asymmetric grabencalled Palu depression which is up to 7 kmwide bordered by N-S orientation, up to 60 mhigh steep triangular facets and truncated al-luvial fans at West side, and gently step faultsat East side along approximately 20 km lengthfrom Palu bay to the north and Gumbasa valleyto the south.

The topography of the Palu region in detailbecome firstly important to observe and to ana-lyze. Topographic map of 1:50000 scale and dig-ital elevation model (DEM) from SRTM with 30m resolution can be combined in order to deter-mine structural lineaments (Figure 3).

3 Microtremor single station observa-tions

A microtremor is a very small ground motionthat can be recorded on the ground surface. Itcan be produced by a variety of excitations (eg.,wind, traffic, breaking sea waves), A full mi-crotremor record can be described by one ver-tical and two horizontal components. Our anal-ysis was conducted by using the recorded mi-crotremor. First, the horizontal and verticalspectrum ratios (HVSR) were computed for allof the observation sites. The peak period of theHVSR is known to correspond to the resonantperiod of the site.

A three-component accelerometer with GPL-6A3P data logger (produced by Akashi Co.Ltd.) was used. The number of single-pointobservations was 151 (Figure 4). The samplingfrequencies were 100 Hz or 500 Hz and the ob-servation times were 10 to 15 minutes.

4 Predominant period of H/V spectra

The spectral ratio of horizontal and vertical mo-tion obtained by microtremor observations is

2 c© 2013 Department of Geological Engineering, Gadjah Mada University

SITE RESPONSE CHARACTERISTICS OF H/V SPECTRUM BY MICROTREMOR SINGLE STATION

OBSERVATIONS AT PALU CITY, INDONESIA

Figure 2: Epicentral distribution of some important earthquakes around Palu province.

c© 2013 Department of Geological Engineering, Gadjah Mada University 3

THEIN, et al.

Figure 3: DEM of Palu area.




Figure 4: Location of the microtremor observation sites.


THEIN, et al.

called the H/V spectrum. The predominant pe-riod of an H/V spectrum is thought to be equiv-alent to the predominant period of the grounddirectly beneath the site. H/V spectra at eachsite in the target area were calculated. We classi-fied the H/V spectra calculated into three typesaccording to the shape of the spectra (Figure 5).

Type A: those with short period peak (Figure5(a))

Type B: those with long period peak (Figure5(b))

Type C: those without clear peaks (Figure 5(c))

Distinct peaks express layers characteristicsin which the shear wave velocity is quite differ-ent. According to this interpretation, the lowerand higher periods in Type A represent the ef-fects from deep and shallow soil layers, respec-tively. In Type B, the difference between the twolayers is not marked and the effect of one layeris absorbed into that of the other layer. Type Cis an observation site that has hard soil or didnot work well. Thus, we established the datafor both long and short predominant periods.Although the predominant period does not al-ways indicate the characteristics of an individ-ual layer because typically the actual shakingmode of the ground is complex, we assumedthat the long and short periods reflected infor-mation from each layer.

Although there are 151 observation points,the points are not adequate to cover all the tar-get area if each value of the predominant periodobtained is considered to be a realization of astochastic random field. Space interpolation isconducted by ordinary Kriging technique. Theresults are shown in Figure 6. The predominantperiods of 1.5–2.0 seconds were on the alluvialfan area. The spatial correlations between pre-dominant periods on the Westside Mountain(which slope is steep) are shorter than those inthe eastside mountain and change more rapidly.

5 Conclusions

Our observations and analysis provide usefuland practical data for earthquake disaster mit-igation in Palu. The procedure employed and

conclusions obtained in this study area are asfollows.

1. Microtremor observations were carriedout for constructing a subsurface groundmodel in Palu. Single-point observationswere conducted at 151 sites which coveredalmost the whole city area.

2. H/V spectra were calculated at all thesingle-observation sites and a distributionof predominant periods was obtained.

3. The Kriging method can be used for the in-terpolation of subsurface information suchas predominant period and shear wave ve-locity.

4. The predominant periods of 1.5–2.0 sec-onds were on the alluvial fan area.

Acknowledgements

This study was supported by the Grant-in-Aidof JICA/SEED-Net. We gratefully acknowledgeDr. Noguchi and Dr. Ono in Tottori Univer-sity, and Dr. Rusnardi Rahmat Putra in Ky-oto University for their cooperation with themicrotremor observations. We sincerely thankTadulako University for their help in undertak-ing the observations in Palu, Indonesia.

References

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Noguchi, T., Horio, T., Kubo, M., Ono, Y., Kiyono, J.,Ikeda, T. and Rusnardi, P. R. (2009) Estimation ofSubsurface Structure in Padang, Indonesia by Us-ing Microtremor Observation, Report on Earth-quake Disaster Prevention Field, Tono Research




Figure 5: Example of the H/V spectrum ratio (mean value and 1 deviation).


THEIN, et al.

Figure 6: Spatial distributions of longer and shorter predominant periods, Ts.




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