-Dr. Rusnardi Rahamat Putra
Assistant Professor, Padang state University
Padang, Indonesia.

The Indonesian archipelago is located at the boundary of three major tectonic plates, the Indo-Australian, Pacific, and Eurasian plates, stretching from Sumatra in the west to Papua in the east. Indonesia is at the collision point of these three crustal plates. The high subduction-related seismicity in this region means that tsunami and other earthquake hazards are also high. Indonesia has approximately 17,504 islands, with a total land area of 1.92×106   km2 and a sea area of 3.26×106 km2. It has experienced a large number of earthquakes in the past. According to catalogued events, the number of earthquakes that have occurred in this region exceeds 45,778 with a magnitude greater than 4.0 from AD 1779 to 2010.
Most of the major historical earthquakes in Indonesia have caused significant damage to facilities (e.g., Utsu et al., 1992; Fauzi et al., 1999; EERI, 2010). Many large earthquakes have occurred in the shallow seas of the area that can produce massive tsunami like the 2004 Banda Aceh event. This earthquake off the coast of Sumatra resulted in hundreds of thousands of deaths and a million people homeless (Ghobarah. A et al., 2006). Tectonic and plate boundaries, Large arrows indicate the direction of plate motion.

The most recent one is the Mentawai tsunami that occurred on October 25, 2010 (Hi. When Indonesian region clustered into two regions, the west cover Sumatera, Kalimantan and Java, on the other hand the east covers Bali, Flores, Timor, Ambon, Sulawesi and Papua. In western Indonesia (i,e. Java and Sumatra), Sumatra subduction zone is formed by the subduction of the Indian-Australian plate beneath the Eurasian plate at a rate of about 50 to 70 mm per year and this is the main source of subduction-related seismicity (Prawirodirjo et al., 2000). Based on our catalog, several giant earthquakes occurred in this region: 1779 (Mw8.4), 1833 (Mw9.2), 1861 (Mw8.3), 2004 (Mw9.2), 2007 (Mw7.9 and 8.4) and 2009 (Mw7.6). Although the source of the 2009 Padang earthquake on September 30 located in the ocean slab of the Indian-Australian plate at (-0.81S, 99.65E) and its depth of 80km. It produced a large shaking and severe damage to houses and building in Padang and Padang Pariaman, because its epicenter was about 60km offshore from Padang. As the Padang earthquake was an intra-slab earthquake at intermediate depth with comparable magnitude, the event did not generate a tsunami of significance (EERI. 2010). Due to this earthquake, about 1,117 people were killed, 1,214 severely injured, 1,688 slightly injured, and 3 missing. Damage to houses is about 114,797 heavily, 67,198 moderate and 67,837 slightly. About 4,000 buildings and 93 schools in Padang city were sustained damaged (Report, BNPB. 2009).

This event occurred just a few minutes after office and school hour, but if it struck earlier time, the higher number of causalities would definitely occur resulted by buildings collapse. From comparison response spectra between actual Padang earthquake and existing Indonesian code (SNI2002), the peak acceleration required for designing building is larger than Indonesian code at small period. The oblique convergence also results in lateral displacement along the Sumatera fault (Peterson et al., 2004). This fault also generates large destructive earthquakes such as in 1892 (Mw7.1), 1943 (Mw7.6) and 2007 (Mw6.4). These faults are capable of generating future strong ground motion that would affect to vulnerable structures. According to our catalogs, the Sumatera fault produces a very high annual rate of earthquakes and many of the major earthquakes occurred in shallow region under the Sumatra Island.
In eastern Indonesia that covers Bali, Flores, Timor, Ambon, Sulawesi and Papua, three major tectonic plates (Indian-Australian, Pacific, and Eurasian plates) interact each other. The Pacific plate subducts beneath the Eurasian plate with a rate of about 100-110mm per year at eastern Papua island (Hall et al., 2000).
Subduction of Indian lithosphere beneath the Banda sea and the northern Australia plate boundary is a complex- and active-deforming region which has one of the fastest relative plate motions on the Earth (Hall et al., 2000). Based primarily on the tsunami records from 1608 to 2010 (Major et al. 2008), the eastern Indonesia experienced over 30 significant earthquake events such as 1938 (Mw8.5), 1976 (Mw7.1), 1992 (Mw7.8), 1996 (Mw8.1), 2006 (Mw7.1), 2008 (Mw7.5), 2009 (Mw7.6) and 35 tsunamis.
Probabilistic Seismic Hazard Analysis (PSHA) aims to quantify the uncertainties and produces an explicit description of the distribution of future shaking that may occur at a site (Baker, 2008). We consider all possible earthquake events and estimate ground motion along with their associated probabilities of occurrence in order to assess design ground motion for structure. The annual probability of exceedance is determined for some level of earthquake shaking at site. In this study, we consider the earthquakes of which magnitudes are larger than 4.0 in moment magnitude scale, and adopt an areal model to determine source because earthquake events may occur anywhere in the region . For accelerometer records observed, we compared several existing attenuation equations and selected a suitable one for Indonesia. In addition, we calculated the seismic hazard for peak ground acceleration (PGA) with 10% probabilities of exceedence in 50 years and proposed design spectra at several sites.

In conclusion by comparison attenuation, we adopted Fukushima attenuation as appropriate equation and applied it to seismic hazard analysis. From the comparison of existing hazard map and proposed one, the result of this study shows that the ground peak acceleration of 475 years at every site is higher about 30% through 90% compared with existing seismic hazard of Indonesia. The differences of the values might be mainly caused by the number of data collected, in which many earthquakes with magnitude greater than 6 occurred after 2002 are included. The other reasons are difference on attenuation equation and the definition for bedrock. The definition of engineering bedrock in our study is average shear wave velocity of upper 30m soil profile (Vs30) > 400m/sec and Vs > 750m/sec for existing hazard map (SNI. 2002).
From the comparison of total hazard curve for Banda Aceh city and Padang city, Banda Aceh has higher value of the peak ground acceleration at 10% probability of exceedance in 50 years is 0.73g and 0.7g for Padang city. The proposed response spectra is larger than existing response spectra about 50% through 121%, it is due to deference of attenuation equation and bedrock level. The next prospective is to estimate the effective peak velocity in Indonesia region for determining the exactly value of To and Tc. Because based on Indonesian code, it does not take into account the peak velocity for determining the value of To and Tc. 

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