PROCEEDINGS PIT IAGI YOGYAKARTA 2012 The 41 st IAGI Annual Convention and Exhibition

Transcription

1 EG-25 COMPARING QUALITATIVE AND QUANTITATIVE METHOD TO DETERMINE EARTHQUAKE SUSCEPTIBILITY LEVEL AT KULON PROGO, YOGYAKARTA by: Deasy Rimanda Cahyaningtyas (1), Prof. Dr. Kirbani Sri Brotopuspito (2) Salahuddin Husein, Ph.D. (3) (1) Graduate student of Gadjah Mada University (2) Geophysics Sub-Department, Gadjah Mada University (3) Geological Engineering Department, Gadjah Mada University *Corresponding author: ABSTRACT New development planning for international airport in Kulon Progo Regency becomes strategic issues, especially for Yogyakarta area. International airport relocation requires earthquake susceptibility ilustration as part of earthquake mitigation. Three sub-districts are selected as study area: Temon, Wates, and Panjatan to determine earthquake susceptibility level with various method, qualitative and quantitative. The qualitative parameters are collected using geophysical methods, where as the quantitative parameters are collected using geological method. The quantitative parameters are frequency, and shear wave velocity. Whereas, the qualitative parameters are lithology, Quaternary sediment thickness, hydrogeology, and fault. These parameters are collected and compared, to examine the correlation among the parameters. Based on various parameters, three earthquake susceptibility map are made: (1) Qualitative earthquake microzonation map, (2) Peak ground acceleration map using Kanai attenuation function, and (3) Peak ground acceleration map using Boore-Atkinson attenuation function. These three maps are compared with building damage distribution of 2006 earthquake to observe their relevance with building damage distribution. The three earthquake susceptibility maps indicate Panjatan sub-district as the most susceptible area. They present good correlation with damage house distribution at sub-district level, but not present significant correlation with damage house distribution at village level especially for PGA map using Kanai attenuation function. Key words: earthquake susceptibility, peak ground acceleration, site amplification, earthquake microzonation, Kulon Progo, Indonesia. INTRODUCTION Kulon Progo Regency is part of Yogyakarta Province which becomes the most developing regency in Yogyakarta, especially for Temon, Wates, and Panjatan sub-district (Figure 1). These three subdistricts become strategic development area, where new strategic infrastructure will build (i.e. Tanjung Adikarto harbor and new international airport). New international airport as substitution of Adisucipto Airport currently becomes strategic development issue. Due to earthquake that stroke Yogyakarta Province on May 27, 2006 many important structure in Yogyakarta, including Adisucipto Airport are damage. This fact encourages the existence of the earthquake susceptibility illustration as parts of mitigation. The susceptibility illustrations of Temon, Wates, and Panjatan sub-districts in the paper are made based on several methods. In general, the methods divided into two categories: (1) Qualitative method based on geological and hydrological condition; (2) Quantitative method using several geophysical data using probabilistic seismic hazard analysis (PSHA) method. This paper focused to discuss comparation between two kinds of methods, qualitative and quantitative. This two methods also compared with damage houses distribution to find the most appropriate method to represent the earthquake damage houses distribution. GEOLOGICAL CONDITION Study area located at the southern part of Java Island, part of Kulon Progo Regency, Yogyakarta Province. The study area are part of Southern Mountain Range. The morphology at the southern categorize as alluvial plain and coastal plain; whereas the hills present at the northern part of the study area as part of West Progo Mountain (Van Bemmelen, 1949). The area divided into five lithology units: (1) Andesite breccias unit, part of Old Andesite Formation, (2) Limestone unit, part of Sentolo Formation, (3) Clay sand unit (alluvial origin), (4) Clay sand unit (qolluvial origin), and (5) Iron sand unit (eolian origin), sometimes form sand dunes. Unit (3), (4), and (5) are part of alluvial plain and coastal 12

2 plain (Van Bemmelen, 1949), whereas Sir MacDonald & Partners (1984) categorize these unit as part of Wates Formation. The normal faults and folds occur within the area (Figure 2). METHODOLOGY TO DETERMINE EARTHQUAKE SUSCEPTIBILITY LEVEL Earthquake susceptibility level of the study area determine based on several method. The scenario of May 27, 2006 earthquake is used in order to make the earthquake susceptibility maps which causative fault located 5 10 km east part of Opak River fault (Abidin, et al, 2009). (1) Qualitative seismic microzonation (Noack & Fäh, 2001). This method in this paper classified as qualitative method due to all parameters based on qualitative data (surface mapping, borehole data, groundwater table). It based on seven parameters that qualitatively characterized the local soil conditions and the expected influence in amplification during the earthquake (see Table 1 and Figure 3). The rating value ranges from 2 to 14. (2) Probabilistic seismic hazard analysis (PSHA), two different attenuation functions are used to make two different peak ground acceleration maps. a. PGA map using Boore-Atkinson attenuation function (2008). This PGA map is made based on measured Vs 30 value derived from MASW measurement (Figure 5). PGA values range from 0.17 to 0.34 g. b. PGA map using Kanai attenuation function (Douglas, 2004). This PGA map is made based on dominant period value derived from microseismic measurement (Figure 6). PGA values range from 0.05 to 0.69 g. PSHA method in this paper classified as quantitative method due to geophysical measurement using microseismic and MASW method. DISCUSSION The result between qualitative and quantitative method is different. It occurs as the result of different parameter that use in calculation/scoring. Each method has its own advantage and disadvantage. Qualitative seismic microzonation shows the susceptibility level depend on the type and thickness of sediment in the area. The higher rating value, the higher susceptibility level. Whereas in PGA map; whether using Kanai attenuation function or Boore- Atkinson attenuation function; the susceptibility level present in quantitative data based on measurement. Kanai PGA map using dominant period value; whereas Boore-Atkinson using shear velocity (Vs 30 ). The higher PGA value, the higher susceptibility level. In PGA maps, distance from active fault is calculated, but in qualitative seismic microzonation map, distance from active fault has homogen scoring in different place within the study area. PGA map using Kanai attenuation function show high susceptibility level in high topography area, whereas PGA map using Boore-Atkinson attenuation function and qualitative seismic microzonation show low susceptibility level in high topography area. Quarternary sediment thickening exist around flat area in Panjatan sub-district. Especially, at Tayuban, Depok, and Bojong village (Table 1, Parameter No. 3 and 4), this part show the gradient due to lateral variation of sedimentary thickness. These two parameters, in qualitative seismic microzonation map, give high susceptibility value, which means higher damage level than surrounding area. To observe the relevance of earthquake susceptibility maps with the damage houses, each map compared with damage house distribution. But the data is limited only in sub-district level for the whole area. Only Panjatan sub-district has damage houses data at village level (Figure 6). All susceptibility maps show Panjatan sub-district as the most susceptible area among the other area. But, PGA map using Kanai attenuation function (Figure 5) show the low susceptibility level at Depok, Tayuban, and Bojong village; which has higher damage houses than the other village. It is very different with the qualitative seismic microzonation map and PGA map using Boore-Atkinson attenuation function result. These two map shows high susceptibility, appropriate with the damage houses distribution map. Based on these maps, it conclude that qualitative seismic microzonation map and PGA map using Boore- Atkinson attenuation function have better correlation with damage house distribution. Better correlation with damage houses distribution that made by qualitative seismic microzonation map and PGA map using Boore-Atkinson attenuation function imply Quarternary sediment characteristic (i.e. consolidation, type, thickness and lateral variation) is important factor to calculate in determine earthquake susceptibility level; considering Quarternery sediment as material that spread the earthquake wave. PGA map using Boore- Atkinson attenuation function is another alternative 13

3 to determine earthquake susceptibility level better than PGA map using Kanai attenuation function. Measured Vs 30 represent the Quarternary sediment characteristic better than measured dominant period. PGA map using Boore-Atkinson attenuation function is an alternative to obtain preliminary earthquake susceptibility illustration in the area without borehole data. Sir MacDonald&Partners, 1984, Greater Yogyakarta Ground Water Study, Ground Water Development Project (P2AT) PU, Yogyakarta. Van Bemmelen, 1949, The Geology of Indonesia, vol.1a : General Geology, Martinus Nijhof, The Haque, 684p. REFERENCES Abidin, H. Z., Andreas, H., Kato, T., Ito, T., Meilano, I., Kimata, F., Natawidjaya, D. H., Harjono, H., 2009, Crustal Deformation Studies in Java (Indonesia) Using GPS, Journal of Earthquake and Tsunami, Vol. 3 No. 2, p , World Scientific Publishing Company. Boore, D.M., and Atkinson, G. M., 2008, Ground Motion Prediction Equations for the Average Horizontal Component of PGA, PGV, and 5%- Damped PSA at Spectral Periods between 0.01 s and 10.0 s, Earthquake Spectra Vol. 24 No. 1, p Douglas, John, 2004, Ground Motion Estimation Equations , Department of Civil & Environmental Engineering Soil Mechanics, Imperial Collage, London. Noack, T. and Fäh D., 2001, Earthquake Microzonation : site effect and local geology. A case study for the Kanton of Basel-Stadt. downloaded from oack.pdf Fig 1. Location of the study area; Temon, Wates, and Panjatan sub-district, Kulon Progo Regency Table 1. Schematic representation of qualitative rating scheme. The zonation map is the sum of all the different contributions at each grid cell (Noack & Fäh, 2001). The local contribution of each characteristic parameter are mapped on 25 x 25 m grid. The parameters are modified appriopriate with Temon, Wates, and Panjatan sub-district condition. 14

4 PARAMETER WEIGHT REMARKS MAP 1. Consolidation of the Quaternary sediments (as a function of age) Map is compiled from geologic maps, well data, and outcrops. Pleistocene alluvium (highly consolidated) 0 The study area are part of Sentolo, Holocene alluvium (medium consolidated) 2 Wates Formation, and sand dunes Pleistocene and Holocene slopewash and 3 (Sir MacDonald & Partners, 1984). Pleistocene loess (low consolidation) Artificial fill (very low consolidation) 4 2. Type of Quaternary sediments (grain size cementation) Sand dominance - sand dune/eolian origin 1 (d) Sand interbedded with clayey sand and/or 2 sandy clay; clayey sand; sandy clay (dc) Sandy clay interbedded with clayey sand; or 3 clay interbedded with sandy sand and/or clayey sand (cd) Clay and marl; clay dominance (c) 4 3. Thickness of Quaternary sediments (depend on the type of the sediment) d 45 m; dc < 45 m; cd < 45 m 1 c < 45 m; cd > 45 m; dc > 45 m 2 c > 45 m 3 4. Lateral variations of the thickness of the Quaternary sediments Gradient < Gradient Map is compiled from generalized lithologic unit of borehole log correlation. Weights are dependent on the type of Quaternary sediment. Map is calculated based on map of Parameter 2. Map is calculated from the map of the thickness of Quaternary sediments. Map is compiled from generalized lithologic unit based on well data and outcrops. 5. Depth of ground water table Map is calculated from the dry > 20 m 1 season ground water table of m 2 Sentolo Formation and Wates 3 10 m 3 Formation 1 3 m 4 Old Andesite Formation classified as poor aquifer with weight = Lithologic variations in the lithified Pre Quaternary sediments Andesite breccias 0 Clastic limestone (calcarenite) 1 Map is compiled from geologic map of the study area. 7. Lateral influence of active master fault In this study, the scenario refers to Outside the area of influence (> 1000 m) 0 May 27, Active fault located Within the area of influence (1000 m) km east part of Opak River fault (Abidin, et al, 2009). 15

5 Fig 2. Geological map of study area Fig 3. Qualitative seismic microzonation map of study area 16

6 Fig 4. PGA map using Boore-Atkinson attenuation function Fig 5. PGA map using Kanai attenuation function 17

7 Fig 6. Village damage houses distribution of 27 May 2006 earthquake at Panjatan village 18