-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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