Combined Hazards

Lecture 9 Combined Hazards Seismic Hazard Maps of Indonesia and Geotechnical and Tsunami Hazard Assessment for Banda Aceh Masyhur Irsyam1, M. Asrurifak1, Hendriyawan1, Hamzah Latif2, Nazili Razali3, Anita Firmanti4 1) Faculty of Civil and Environmental Engineering, Institut Teknologi Bandung 2) Center for Marine and Coastal Development, Institut Teknologi Bandung 3) Research Group on Eco-Hydraulic, Engineering Center for Industry, Institut Teknologi Bandung 4) Research Institute for Human Settlements, Indonesia Ministry of Public Works ABSTRACT It is targetted that the renewal of Indonesian Earthquake Resistant Building Code, the SNI 03-1726-2002, will be finished by mid of 2010. Two major expected revisions are it’s Earthquake Hazard Map, to reflect the lattest advancement in Probabilistic Seismic Hazard Analysis (PSHA) technology accounting for potential larger disasters predicted in the near future, and changing the provision from UBC-97 concept to the lattest ASCE 7-05 and IBC2009 provisions. The main considerations are the much larger than maximum predicted the 2004 Aceh earthquake (Mw 9.0-9.3) and its following tsunami, and the fast destruction observed in the 2005 Nias Earthquake (Mw 8.7) and other similar earthquake disaster observed all over the country. This paper presents research works carried out in developing the latest spectral hazard maps. Improvement in seismic hazard analysis and careful inclusion of recent seismic records were augmented. Seismic sources were modeled by background, fault, and subduction zones considering truncated exponential model, pure characteristic model or both. Several well known attenuation functions were selected including the Next Generation Attenuation (NGA). Maps of Peak Ground Acceleration (PGA) and Spectral Response Acceleration (SRA) for 0.2 second (short periods) and 1-second period were developed using PSHA. To better reflect Indonesia overall potential disaster, additional geotechnical and tsunami hazard assessment researchs for Banda Aceh city, the capitol of Aceh Province, in particular were also submitted. The 2004 Aceh tsunami has caused at least 148,873 people killed, 142,123 people missing, 31,907 people injured (WHO) and staggering regional economic and social disasters. The earthquake and the following tsunami destroyed countless engineered and non-engineered buildings, infrastructure, and utilities. The case triggered the understanding for the need of better disaster mitigation strategy to anticipate future similar natural hazards. The research then focused on developing geotechnical and tsunami hazard maps for Banda Aceh. Dynamic soil properties and resistance of soil to liquefaction were obtained from field soil investigation consisted of deep borings and seismic downhole tests. Site specific response analysis was carried out using one dimensional shear wave propagation approach and evaluation of liquefaction potential was conducted using simplified empirical procedure. The results of site response analysis and liquefaction study at several points were utilized to generate contours of acceleration, amplification factor, design response spectra, and potential of liquefaction for Banda Aceh. The tsunami hazard study covered modeling of tsunami inundation, run-up, and developing tsunami zonation map. It was conducted using mathematical simulation and modeling leading to estimate the potential tsunami that may occur in the future. The output of geotechnical and tsunami hazard assessment were then overlayed ontop of the land use city planning in a Geographical Information Systems (GIS) database and used as criteria for tsunami warning system, an input in developing land use management for Banda Aceh, and enriching the basic regulation for new infrastructures and local building codes. 1. Background Research Indonesia has been well known as one of the most seismically active countries in the world. It is surrounded by three major active tectonic plates of the earth: Eurasian, Indo-Australian, and Philippine plates. Therefore, the construction design in Indonesia should consider these seismic activities. The most effective way to reduce disasters caused by earthquakes is to estimate the seismic hazard and to implement this information on a seismic code for use in building design and construction so that the structures possess adequate earthquake resistant capacity (Hu 1996 and Irsyam et al., 2008). The need to revise current Indonesian Seismic Hazard Map expressed as PGA contained in our latest Indonesian Earthquake Resistant Building Code SNI 03-1726-2002 (Figure 1) was driven among others by the desire to better reflect potential larger earthquake disasters faced by the nation predictably in the future (Irsyam et al., 2008). Several great earthquake occurrences in Indonesia in the last six years inquire revision of seismic hazard parameters. Some of the great earthquakes are the 2004 Aceh Earthquake (Mw9.0-9.3) which was followed by tsunami, the 2005 Nias Earthquake (Mw 8.7), the 2009 Tasik Earthquake (Mw 7.3), and the latest 2009 Padang Earthquake (Mw 7.6). The Aceh earthquake and the following tsunami destroyed countless engineered and non-engineered buildings, infrastructure, and utilities. Therefore, geotechnical and tsunami hazard assessment researchs for Banda Aceh city in particular were also required to better reflect Indonesia overall potential disaster. This paper presents the latest research works for developing seismic hazard maps of Indonesia. The maps are based on spectral acceleration rather than PGA, two maps of SRA are selected: for short period (0.2 s) and for long-period (1.0 s). The purpose of this research is to provide input for revision of the current seismic hazard map. The research also covers geotechnical microzonation study for Banda Aceh city for estimating the effects of local soil conditions to the surface ground motion and to the potential of liquefaction. Furthermore, tsunami hazard study is also performed to obtain tsunami inundation and run-up for Banda Aceh city. 2. Tectonic Setting of Indonesia Indonesia is located in a tectonically very active area at the point of convergence of three major plates and nine smaller plates as developed by Bird, P. (2003). The Eurasian, Pacific and Australian-Indian plates, along with some smaller plates (i.e. Philippine Sea plate), are all actively moving toward each other in the Southeast Asia region (Figure 2) creating a complex network of plate boundaries. Western Indonesia tectonically consists of the Sunda Shelf which includes the islands of Sumatra, Java, Bali, Borneo, and the southwestern part of Sulawesi (Hamilton, 1979). The active tectonics of western Indonesia is dominated by convergence of the Australia plate with Sumatra and Java. Along Sumatra the direction of convergence is highly oblique to the trench strike, and is partitioned into nearly arc-perpendicular thrusting at the trench and arcparallel, right lateral slip at the Sumatran fault (Bock et al., 2003). Figure 1. Indonesia seismic hazard map (SNI 03-1726-2002). Figure 2. Topographic and tectonic map of the Indonesian archipelago and surrounding region (Bock et al., 2003). For eastern Indonesia, the crustal composition south of Java and Bali and in the south Banda Sea are oceanic, and that the Arafura sea and Timor trough are underlain by continental crust. This, along with the geological evidence of exposed overthrust sheets of the Banda allochthon on the island of Timor, indicates collision of a continent with an island arc along the outer Banda arc, and suggests underthrusting of continental crust under Timor. Two plates subduct beneath the Banda Sea, one from the south at the Timor and Aru troughs, and the other from the north at the Seram trough, and that the two slabs are separated at the Tarera-Aiduna fault, which acts as a transform (Bock et al., 2003). Further East, the continental part of the Australian plate collides with the Banda arc, resulting in widespread deformation throughout the Banda island-arc. Further complicating the tectonics of East Indonesia, Australian continent also collides with the Pacific oceanic plate, resulting in uplift and extensive faulting on the island of New Guinea. Australia-Pacific convergence is highly oblique and appears to be partitioned into components perpendicular and parallel to the margin. The perpendicular component is taken up by crustal shortening in the Highlands thrust belt and very likely, subduction along the New Guinea and Manokwari trenches. The marginparallel component results in left-lateral shear zones along North New Guinea. The existence of subduction zones have created zones of earthquakes that contribute to the event earthquakes occurred in the Indonesian Region (Milsom et al., 1992). There are some potential active fault distributions around Indonesia islands (Figure 3). The Sumatran fault zone (SFZ) is a 1900-km-long structure that accommodates right lateral shear associated with the oblique convergence along the plate margin and widely accepted as highly segmented fault. The java Seismicity is significantly low compared to that along the SFZ (Lasitha et al., 2006). The strike-slip events may account for the presence of the Cimanderi, Lembang, Bumiayu, Semarang or Opak (Jogja) fault. The island of Sulawesi in eastern Indonesia has more active faults as Palu-Koro, Matano, Walanae, Poso, Batui thrust, Tolo thrust, Sulu thrust, Gorontalo and Lawanopo (Socquet et al., 2006). In the island of Papua, faults that have been identified include Yapen, Tarera-Aiduna, Sorong, Ransiki, Membrano thrust-belt, Manokwari trench, Lowland and Highland thrust-belt. 3. Development of Seismic Hazard Maps of Indonesia 3.1. Eartquake Catalog Historical earthquake events from 1900 to 2008 are compiled from many sources such as: a. Earthquake listings held by National Earthquake Information Service U.S. Geological Survey (NEIS-USGS) of the United State, which is a compilation of several catalogs from sources such as: The Bureau Central International de Séismologie (BCIS), the International Seismological Summaries (ISSN), the International Seismological Center (ISC), the Preliminary Determination of Epicenters (PDE), and The Advanced National Seismic System (ANSS) catalog. b. Indonesia earthquake listing prepared by the Bureau of Meteorology and Geophysics (BMG), Jakarta, Indonesia. c. Centennial Catalog which is compiled from Newcomb & McCann (1987) where several large event in Indonesia have been relocated and Pacheco & Sykes (1992) where the earthquakes were corrected for heterogeneity’s caused presumably by changes in instrumentation, reporting and/or detection capabilities. 3.2 Seismic Source Model The seismic source model for input to PSHA was defined using earthquake catalogs, tectonic boundaries, and fault information. The sources model was divided into subduction source, fault source, and background seismicity. This division followed previous studies by Irsyam et al. (a) (2009) and Irsyam et.al. (b) (2009). However, updating of seismic source parameters used in the previous studies have been performed based upon the most recent work in Indonesia. The updated parameters are already included in Figure 3. Subduction mechanism occurred when an oceanic plate is being subducted under an island arc or continent. Thrust fault mechanism may occur along the interface, while normal faulting may occur toward and along the outer arc high and in the trench. The subduction source models were limited to Megathrust zones (Figure 4). For earthquake events occurred in the Benioff zones below the Megathrust were accounted in deep background sources. Fault source represents individual fault for which data is sufficient to determine maximum earthquake magnitudes distributions and slip rate estimates. The major tectonic feature and sense of faulting, slip-rate, dip, width and maximum magnitude used in this work were estimated based on published data. At present, continuous updating of seismic source parameters is still conducted by the Indonesia Team for Revision of Seismisc Hazars Map. Figure 3. Major tectonic features of Indonesia and their parameters. Figure 4. Classification of main shocks based on type of seismic sources Background seismicity is used to account for region in which lack of seismogenic data but has seismic activities report from small to moderate earthquakes. Background source zones were modeled using gridded seismicity based on spatially smoothed earthquake rates (Frankel et al., 1995). This model consists of five depth intervals, i.e. shallow earthquakes (0-50 km), intermediate earthquakes (50–100 km and 100–150 km), and deep earthquakes (150–200 km and 200–300 km). 3.3 Attenuation Relations Selection of attenuation relations were conducted based on earthquake mechanism, which is generally categorized into background, fault and subduction source zones. This selection followed previous studies by Irsyam et al. (a) (2009) and Irsyam et.al. (b) (2009). Attenuation from Geomatrix subduction (Youngs et al., 1997), Atkinson-Boore BC rock and global Source (Atkinson and Boore, 2003) and Zhao et al., with variable Vs-30 (Zhao et al, 2006) were chosen for Megathrust zone (subduction interface). Attenuation from BooreAtkinson NGA (Boore, D.M., and Atkinson, G.M., 2008), Campbell-Bozorgnia NGA (Campbell, K.W., and Bozorgnia, Y., 2008) and Chiou-Young NGA (Chiou, B., and Youngs, R., 2008) were selected for faults and background sources. Attenuation from AtkinsonBoore intraslab (Atkinson and Boore, 2003), Geomatrix slab seismicity rock (Youngs et al, 1997) and Atkinson-Boore intraslab seismicity world data BC-rock condition (Atkinson and Boore, 1995) were used for Benioff (deep background sources). 3.4 Seismic Hazard Maps of Indonesia Hazard maps of PGA at bedrock with 10% and 2% probability of axceedance in 50 years (500 years and 2500 years return period of earhquake) are presented in Figure 5 and Figure 6 and hazard maps of spectral acceleration at 0.2, and 1.0 sec for 2% probability of exceedance in 50 years are shown in Figure 7 and Figure 8. Near the location of faults and subductions, results of this study are significantly higher than that of the SNI 03-1726-2002 for the same return period of earthquake (10% probability of axceedance in 50 years). 4. Geotechnical Microzonation Study for Banda Aceh This paper also presents the results of seismic microzonation study of Banda Aceh to be utilized for reconstrcution after the 2004 earthquake, enhanching land use management, and improving the accuracy of currently available and proposed new seismic resistant infrastructure and building codes. The map were obtained by performing ground response analysis utilizing one-dimensional (1-D) shear wave propagation method considering the nonlinearity of soil behavior and by evaluation the liquefaction resistance of soil deposits. The ground response analysis was performed using NERA software developed from SHAKE (Schnabel et. al., 1972) and EERA (Bardet et al., 1998) and based on material model developed by Iwan (1967) and Mroz (1967). The nonlinearity of the shear modulus and damping was accounted for by employing equivalent linear soil properties using an iterative procedure to obtain values for modulus and damping compatible with the effective strains in each layer. Figure 5. Map of Peak Ground Acceleration (PGA) of Indonesia for 10% probability of exceedance in 50 years. Figure 6. Map of Peak Ground Acceleration (PGA) of Indonesia for 2% probability of exceedance in 50 years. Figure 7. Map of 0.2 sec spectral acceleration of Indonesia for 2% probability of exceedance in 50 years. Figure 8.. Map of 1.0 sec spectral acceleration of Indonesia for 2% probability of exceedance in 50 years. Shear wave propagation analysis was performed for 30 data of soil borings including 10 seismic downhole (SDH) test to obtain peak acceleration, amplification factor, and response spectra at the ground surface. Shear wave velocity (VS) profiles were developed based on interpretation of Bored Log and SDH (Figure 9), around site location down to 30m below ground surface. Wave propagation analysis from bedrock to ground surface showed that peak acceleration on the ground surface ranged from 0.50g to 0.58g and from 0.42g to 0.68g for soil type SC and SD, respectively, for 10% probability of exceedance in 50 years. The results were then plotted to develope peak acceleration contour maps for Banda Aceh as shown in Figure 10. Evaluation of the liquefaction resistance of soil deposits was also conducted in this study. It was carried out using simplified empirical procedure proposed by Seed et al. (1971,1985) and was modified by NCEER (1997). The corrected SPT resistance of in-site soil stratum was compared with earthquake-induced cyclic shear stress. Proposed Iwasaki method (Iwasaki et al., 1982) was then adopted to estimate the severity of liquefaction degree at a given site based on the value of liquefaction potential index PL. The results of liqufaction study is presented in Figure 11, Banda Aceh can be divided into three zones, i.e. high, medium, and low liquefaction susceptibility. Figure 9. Field investigations for microzonation study of Banda Aceh (Gitamandalaksana, 2009). Figure 10. Microzonation map of peak surface acceleration of Banda Aceh for 10% probability of exceedance in 50 years (Gitamandalaksana, 2009). Figure 11. Microzonation map of liquefaction potential of Banda Aceh (Gitamandalaksana, 2009). 5. Tsunami Hazard Assessment for Banda Aceh Historically, subduction earthquakes and associated tsunamis have occurred repeatedly in Indonesia as reported by Ismail (1997) and by Puspito (2002) as shown in Figure 12. On December 26, 2004 a devastating megathrust earthquake occurred on the interface area of Indian plate that subducts beneath the overriding Burma plate with magnitude (Mw) of 9.1 to 9.3 and rupture length of 1600 km near Banda Aceh, Sumatra. The rupture propagated to the north along Andaman and Nicobar Islands (Lay, et al., 2005). The earthquake generated huge tsunami and attacked coastal area along the South and South East Asia and reached the Eastern part of Africa that caused 283.000 deaths (Figure 13 to Figure 15). Figure 12. Historical tsunami events in Indonesia (after Puspito, 2002) Figure 13. Tsunami height and effect generated by the 2004 Aceh earthquake (International Tsunami Survey Team-2005 and http://www.disasterscharter.org/disasters/CALLID_079a_e.html) Tsunami hazard criteria become essential as a basis for rehabilitation, reconstruction, and longterm development, as well as for developing tsunami warning system. Therefore, tsunami hazard assessment for Banda Aceh city were conducted. The 2004 Aceh tsunami source was designed based on the slip distribution given by Subarya, et. al. (2006), as shown on Figure 16. The fault of Aceh-Andaman earthquake was divided into 7 sub-faults as shown in Figure 17, the velocity rupture propagation of the earthquake from Sumatra to Andaman averagely was taken 1.7 km/s (Latief et. al., 2006). Calculation of tsunami inundation of Banda Aceh city was conducted by using several nested domains. The nested domains were used to calculate tsunami run-up and inundation area using more detail bathymetric and topography data. To obtain more accurate results, 5 nested domains were utilized, from domain-A up to domain-E, as presented in Figure 18. (b) Figure 14. Vast destruction at Lhoknga due to the 2004 Aceh earthquake. Figure 15. Tsunami damage to houses and buildings during the 2004 Aceh earthquake, Lam Paseh. There were 4 scenarios selected to simulate inundation model of Banda Aceh city. The first one was backcalculation of the 2004 Aceh tsunami with Moment Magnitude (Mw) of 9.2 used to calibrate input parameters. The other 3 cases were used to account for diffrenet magnitudes of earthquake; Mw=8.5, Mw=8.0, and Mw=7.5. Calculation of tsunami inundation was conducted by using TUNAMI-Code (Goto, C. and Ogawa, Y., 1992, and Imamura, F., 1995). The output of tsunami hazard assessment (Figure 19) was then overlayed ontop of the land use city planning in a Geographical Information Systems (GIS) database as shown in Figure 20. Figure 16. Coseismic slip distribution at Sunda mega thrust estimated from inversion geodetic data (Subarya et. al., 2006) Figure 17. Sub-fault position (Latief et. al., 2006) Figure 18. Grid size of nested domains (a) Inundation area for the 2004 Aceh tsunami (b) Inundation area for Mw=8.5 (c) Inundation area for M=8.0 (d) Inundation area for Mw=7.5 Figure 19. Tsunami inundation for Mw= 9.2, 8.5, 8.0, and 7.5 Figure 20. Tsunami inundation map of Banda Aceh city (Gitamandalaksana, 2009). 6. Conclusions A probabilistic hazard model for estimation of seismic hazard in Indonesia has been developed based upon updated available seismotectonic data, implementing new fault models, and incorporating new ground-motion prediction equations (NGA). Spectral hazard maps for 10% and 2% probability of exceedance in 50 years have been produced. Near the location of faults and subductions, results of this study are significantly higher than that of the SNI 03-1726-2002 for the same return period of earthquake. It indicates that the actual seismic hazard in Indonesia is higher than it was expected before. Geotechnical and tsunami hazard assessment for Banda Aceh have been conducted. 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