earthquake microzonation of yogyakarta

Qualitative Analysis on Seismic Microzonation of Yogyakarta City Deasy R. Cahyaningtyas*) and Salahuddin Husein **) *) Undergraduate student at Dept. of Geological Engineering, Gadjah Mada University *) Lecturer at Dept. of Geological Engineering, Gadjah Mada University; corresponding email: [email protected]

Abstract On May 27, 2006, many people died and infrastructure damaged due to a 5.8 Richter scale shallow earthquake. The distribution of damage buildings and houses is different in each sub district and has uneven pattern, particularly due to different geological, geotechnical, and seismic properties. One of methods to study those factors is using quantitative microzonation that give an illustration of susceptibility level of an area in response to the earthquake. This method relies on existing subsurface information, could be claimed as a desk study or preliminary seismic microzonation. However, a quantitative microzonation conducted in Yogyakarta City reveals that this method is suitable to shows appropriate representation with damage infrastructure.

Introduction On May 27, 2006, a 5.8 Richter scale earthquake had stroke Yogyakarta Province and surrounding area. It was thought that the quake resulted from movement of the Opak Fault, that lies in the eastern part of Bantul Regency, which caused near 6000 fatalities and thousands of building and houses in Yogyakarta area damaged or collapsed. In Yogyakarta City, like elsewhere in adjacent areas, distribution of damage buildings and houses was different in each sub-district and has uneven pattern. This uneven pattern occurred because different geological and geotechnical properties, as well as seismic properties of the rocks (amplification, period and frequency) in each area. Therefore, an earthquake microzonation map that could describe those properties is in need to improve mitigation works for future hazard. Earthquake microzonation map is defined in this study as a semi detail scale map (in 1:25000 or greater scale) which illustrates zonation of distribution and susceptibility levels. There are many methods used to give an illustration susceptibility of an area in response to the earthquake. In this study, there are three methods that would be discussed to show the susceptibility levels of Yogyakarta and surrounding area in response to the earthquake, i.e. ground amplification, ground period, and quantitative microzonation. These three methods would be compared with damage buildings and houses data of May 27, 2006 earthquake in order to know the most relevant method which shows susceptibility illustration and the factors that affect damage building and houses as the result of earthquake.

Geological Setting of Yogyakarta The study area is part of distal zone of Merapi Volcano, which consists of Quaternary fluvio-vulcanic deposits. MacDonald & Partners (1984), categorize the Quaternary deposits into several formations, which are the Old Merapi, Sleman, and Yogyakarta. The Old Merapi Formation consists of strongly fractured basalt and andesite lavas, with indurated breccias, outcropped around the upper cone of Merapi and were deposited during Upper Pleistocene. The Sleman Formation has designated as the lower part of a major volcaniclastic unit which was formerly included in the Younger Merapi Volcanics formation. In Simposium Geologi Yogyakarta - 23 Maret 2010

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the north, on the Merapi Upper Slopes, it consists of sands and gravels with interspersed boulders, all derived from volcanic ejecta. From Yogyakarta to the south, the formation is overlain by the Yogyakarta Formation so that full thickness of the former formation can only be identified in boreholes. The Sleman Formation is thought to be Upper Pleistocene to Holocene in age. The Yogyakarta Formation forms the surface outcrop throughout most of the lowland area of the Yogyakarta Basin from the Merapi Middle Slopes to the coast. It constutes the upper part of the former Younger Merapi Volcanics. It consists of an interbedded sequence of sands, gravels, silts, and clays. The amount of clay increases southwards. The study area is part of Yogyakarta Formation (MacDonald & Partners, 1984) (Figure 1). Based on boreholes data Sleman Formation undernearth the Yogyakarta Formation, and undernearth Sleman Formation, there is Sentolo Formation, which predominantly consists of limestone, also consists of marls, tuffs, and conglomerates.

Quantitative Microzonation The quantitative microzonation is based on seven parameters that qualitatively characterized the local soil conditions and the expected influence in amplification during the earthquake. Detailed description of the scheme is given in Noack & Fäh (2001). Those 7 parameters are: 1. The consolidation of the Quaternary deposits expressed by their age. The lower a sediment is consolidated, the lower its shear wave velocity and the higher is the expected amplification during earthquakes. The study area is consist of Quaternary sediment (Yogyakarta Formation) which thought to Holocene in age (class 2) (Figure 2). It is shown by 32 borehole data (Figure 3) and geological map of study area. 2. The type (grain size and lithification) of the Quaternary sediments. As the qualitative rule it can be assumed that, the smaller the grain size of a sediment, the lower the shear wave velocity. The study area divided into 4 classes depend on the lithology (Table 1) (Figure 4). 3. The thickness of the Quaternary sediments weighted by their type. The thickness of the sediments define the frequency band at which amplification effects occur. The thicker the soils, the lower the fundamental frequency of resonance. Assuming the simple model of one layer over a half space the expected amplification is maximal at the fundamental frequency of resonance and significant for the frequency range above. In study area, the thickness of Quaternary sediments is homogen. The thickness of all type of Quaternary sediments is more than 45 m (Class 3) (Figure 5). 4. Lateral variation in the thickness of the Quaternary sediments to account for the excitation of local surface waves and resonances. It is often observed during earthquakes, that lateral heterogeneities can excite local surface wave, which, due to their strong attenation only propagate a limited distance, but within this range, can significantly increase the duration and amplitude of ground motion. At least for the areas covered by Quaternary gravels, the parameter for lateral variation also rates some effects of subsurface and surface topography. Basin type structures and topographic features can be subject to resonance phenomena. Based on borehole data, gradient of study area is less than 0.01, that means there is no lateral variation in the thickness of the Quaternary sediment in study area (Class 0) (Figure 6). 5. The fifth parameter considers the potential of liquefaction. This factor is limited to watersaturated, cohesionsless, granular sediments at depths less than 10 m. Liquifaction potential in the rating scheme is expressed by the depth to the water table. The study area is divided into 3 classes depend on the depth of water table (Table 1) (Figure 7). Simposium Geologi Yogyakarta - 23 Maret 2010

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6. The sixth parameter characterizes the differences in shear wave velocity given by the lithology of the PreQuaternary sediments underneath the Quaternary gravels. Geologically, they are bedrock, but seismically, these sediments belong partly to the soft sediments. This can be expected from the interpretation of ambient noise measurements, that clearly show low fundamental frequencies of resonance between 0.4 – 1 Hz that can only be explained by low shear wave velocities of some of the Tertiary sediments (Fäh, et al., 1997 vide Noack & Fäh, 2001) in the order of 500-900 m/s. There is no outcrop of Tertiary sediments in study area, so we can conclude no lithologic variation in the lithified preQuaternary sediments (Class 0) (Figure 8). 7. The seventh parameter rates the influence of the lateral variation of the fault. Resonance effects may be expected at very low frequency of the order of 0.4 – 0.5 Hz, due to basin structure of the area within the nearby the fault. The extent of the influence has been fixed to 1000 m inside. In this study, Opak Fault is the master fault that influence the area. The distance between Opak Fault and study area is more than 1000 m, outside the area of influence (Class 0) (Figure 9). Each of the seven parameter contributes to the microzonation (Table 1). The contribution of the effects of each parameter are classified to values between 0 and 4 units on a qualitative scale (Table 2). This value is assigned to a 25  25 m grid. The value 0 is equivalent to no contribution to local amplification and 4 means a high contribution. Included in this figure is also the percentage of the area covered by each class (0 – 4). Each map depicts geographically an independent distribution of the different classes. The quantitative microzonation map of Yogyakarta and surrounding area are divided into 5 units area, with score from 9 to 13 (Figure 10). The higher score in the area shows the higher probability of the building and houses damage in the area (Figure 11). Gamping, Kasihan, Sewon, Umbulharjo, Kotagede, and Banguntapan subdistricts are highly damage area with score 11 to 13. The area with score from 9 to 10, such as, Godongtengen, Danurejan, Gondokusuman, Wirobrajan, Ngampilan, Gondomanan, Pakualaman, Kraton and Mergangsan are subdistricts with low damage infrastructure.

Conclusion  

Quantitative microzonation is a suitable method that shows appropriate representation with damage infrastructure. Gamping, Kasihan, Sewon, Umbulharjo, Kotagede, and Banguntapan subdistricts are highly damage area with score 11 to 13 in quantitative microzonation of Yogyakarta and surrounding area.

References Jogja Media Centre (2007) Damage Distribution of Yogyakarta Earthquake 2006, http://www.freewebs.com/gempadiy_mediacenter.htm, downloaded on December 22, 2009. MacDonald, Sir M. & Partners (1984) Greater Yogyakarta Groundwater Resource Study, Volume 3, Groundwater Development Project, Direct General of Water Resources Development, Ministry of Publicworks, Government of Indonesia.

Simposium Geologi Yogyakarta - 23 Maret 2010

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Mahendra, R.O. (2008) Pengaruh Kondisi Hidrogeologi dan Tataguna Lahan terhadap Tingkat Kandungan Nintrat pada Airtanah di Kota Yogyakarta dan Sekitarnya. Tugas Akhir tipe Skripsi, Geological Engineering Gadjah Mada University, Yogyakarta (not published). Noack, T. and D. Fäh (2001) Earthquake Microzonation : site effect and local geology. A case study for the Kanton of Basel-Stadt.


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Table 1. Schematic representation of the application of the qualitative rating scheme. The local contribution of each characteristic parameter are mapped on 25  25 grid. The zonation map is the sum of all the different contributions at each grid cell (modified from Noack & Fäh, 2001). The parameters are modified appropriate with Yogyakarta and surrounding area condition

1.

PARAMETER Consolidation of the Q uaternary sediments (as a function of age)

WEIGHT

 Pleistocene alluvium (highly consolidated)

0

 Holocene alluvium (medium consolidated)

2

 Pleistocene and Holocene slopewash and Pleistocene

3

loess (low consolidation) 2.

4



Sand with gravel dominant and little clay

2



Sand with clay dominant and little gravel

3

3.

4.

5.

Sand with clay Thi ck ne ss of Q uate r nar y se di me nts (de pe nd on the type of the sedi ment ) No variation of thickness of Quaternary sediments The thickness of the Quaternary sediment is more than 45 m Late ral var i ati ons of the thick ne ss of the Q uater nar y se di me nts  The gradien value at study area is homogen less than 0.01  Quaternary sediment thickness is homogen Depth to gr ound water table  10 - 20 m 

3 - 10 m  1-3m 6.

3

A buiding site

 Map is compiled from lithologic

descriptions of boreholes

 Weights are dependant on the type of the

Quaternary deposit

0

 Map is calculated from the map of the thickness of the Quaternary sediments

2

 Map is calculated from the map of the mean

3 4

groundwater table  Map is compiled from lithologic

0

. Lateral influence of Opak master fault  Outside the area of influence (> 1000 m)  Within the area of influence (1000 m)

 Map is compiled from geologic maps, well

4

Prequaternary sediments

7.

lithified sedimentary rocks

1

Li thol ogi c var i ati on s i n the l i thi fie d No outcrop of tertier sediments

 No contribution from Prequaternary

data, map of hazardous waste and outcrop

 Artificial fill (very low consolidation) Type of Q uate r nar y se di me nts (gr ai n si z e cementati on)  Sand with gravel



REMARKS

0 1

descriptions of boreholes and outcrop  No outcrop of tertiary sediment in study area  In this study Opak Fault movement is believed as the cause of Bantul earthquake.


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Table 2. Buildings and houses damage of Mei 27, 2006 earthquake from Jogja Media Center (2007) No

Subdistrict

Collapse

Buildings and houses damage (units) Heavy Damage Light Damage

Total

1

Sewon

8281

8496

6004

22781

2

Banguntapan

5557

8232

7452

21241

3

Kasihan

1790

4657

12103

18550

4

Umbulharjo

1738

2249

0

3987

5

Kotagede

238

864

490

1592

6

Gamping

160

1922

1551

3633

7

Mlati

99

487

2157

2743

8

Depok

85

656

3148

3889

9

Jetis

73

594

1207

1874

10

Gondokusuman

50

310

33

393

11

Kraton

38

0

114

152

12

Tegalrejo

18

38

0

56

13

Ngaglik

12

132

528

672

14

Pakualaman

9

92

190

291

15

Mantrijeron

0

225

0

225

16

Mergangsan

0

83

0

83

17

Danurejan

0

61

365

426

18

Gondomanan

0

26

11

37

19

Wirobrajan

0

25

175

200

20

Ngampilan

0

10

15

25

21

Gedongtengen

0

0

17

17


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Figure 1. Regional geological map of Yogyakarta Basin (modified from MacDonald & Partners, 1984)


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Figure 2. State of Quaternary sediment consolidation, based on boreholes data.

Figure 3. Fence diagram of Quaternary sediments in Yogyakarta City, based on boreholes data.


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Figure 4. Types of Quaternary sediments, based on boreholes data.

Figure 5. Thickness of Quaternary sediments in Yogyakarta City, based on boreholes data.


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Figure 6. Lateral variation of Quaternary sediments in Yogyakarta City, based on boreholes data.

Figure 7. Depth to ground water table in Yogyakarta City (Mahendra , 2008)


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Figure 8. Surficial pre-Quaternary lithologies in Yogyakarta City, based on geological map.

Figure 9. Lateral influence of the master fault, i.e. the Opak Fault, based on geological map.


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Figure 10. Quantitative seismic microzonation of Yogyakarta City.

Figure 11. Distribution of damages in Yogyakarta City.


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