HR: 14:40h
AN: U53A-04    [Abstracts]
TI: The 2007 Sumatra seismic sequence revealed by a regional seismic network in Indonesia
AU: * Nakano, M
EM: mnakano@bosai.go.jp
AF: NIED, 3-1 Tennodai, Tsukuba, 305-0006, Japan
AU: Inoue, H
AF: NIED, 3-1 Tennodai, Tsukuba, 305-0006, Japan
AU: Kumagai, H
AF: NIED, 3-1 Tennodai, Tsukuba, 305-0006, Japan
AU: Yamashina, T
AF: NIED, 3-1 Tennodai, Tsukuba, 305-0006, Japan
AU: Sunarjo
AF: Meteorological and Geophysical Agency, Kemayoran, Jakarta, 0000, Indonesia
AU: Fauzi
AF: Meteorological and Geophysical Agency, Kemayoran, Jakarta, 0000, Indonesia
AU: Suhardjono
AF: Meteorological and Geophysical Agency, Kemayoran, Jakarta, 0000, Indonesia
AB: On September 12, 2007, a great earthquake with Mw 8.4 occurred at 11:10 (UTC) off Bengkulu, Sumatra, Indonesia. This event was followed by a large earthquake with Mw 8.0, which occurred at 23:49 (UTC) on the same day in the northwest of the Mw 8.4 earthquake. Another earthquake with Mw 6.8 occurred off Padang, in the northwest of the second earthquake, at 03:35 (UTC) on the next day. These earthquakes caused dozens of casualties and damage to the buildings in Bengkulu and Padang areas. Tsunami with a hight of 1m was observed in Padang, but no severe damage by tsunami was reported. We have been developing an automated system for rapid source parameter determinations of earthquakes in Indonesia, using data obtained from a broadband seismic network in this country (JISNET). This network is operated by NIED and Indonesia Meteorological and Geophysical Agency (BMG). In our method, the moment function is estimated simultaneously with a centroid moment tensor (CMT) solution based on the waveform inversion carried out in the frequency domain. The source parameters for an earthquake greater than Mw 5 can be determined automatically by our system within 15 minutes after the occurrence of an earthquake. The automatic inversion result is then checked manually. The source parameters of earthquakes that are not determined by the automatic system are also determined manually. The source parameters of the 2007 Sumatra earthquakes were succesfully determined by our system. The estimated focal mechanisms show similar thrust-type faultings, suggesting that these earthquakes occurred in association with the subduction of the Indo-Australian Plate beneath Sumatra Island. Both the Mw 8.4 and 8.0 events occurred at a depth of about 20 km. The rupture durations estimated from the moment functions are 140 s and 108 s for the Mw 8.4 and Mw 8.0 events, respectively. These rupture durations are slightly longer than typical values for earthquakes of these magnitudes. The aftershocks are distributed in a region extending about 300 km along the subduction zone of the Indo-Australian Plate. The depths of aftershocks range from 15 to 60 km. In the off-Bengkulu region, the earthquakes with M8 or above occurred in 1381, 1608 and 1833, approximately every 230 years. Now 174 years have been passed since the last event in 1833. We did not experience M8 class earthquakes during the last 210 years in the off-Padang region, which is to the north of off-Bengkulu. The off-Padang region corresponds to a seismic gap between the source regions of the 2007 Sumatra seismic sequence and M8.7 Nias earthquake on March 2005. The sequence of the M8.4, M8.0, and M6.8 earthquakes moving towards the northwest from off-Bengkulu in the 2007 seismic activity suggests the imminency of a large earthquake off Padang. We have been deploying broadband seismograph networks in Indonesia, including JISNET, by an international cooperation among Indonesia, Germany, China, and Japan, aiming at improving the capabilities to monitor seismic activity and tsunami generation in Indonesia. The seismic networks are now in operation, and totally 150 seismic stations will be installed by the end of 2008. Seismic monitoring based on these regional networks would contribute to early notification of a large earthquake anticipated to occur in the off-Padang region.
DE: 7209 Earthquake dynamics (1242)
DE: 7215 Earthquake source observations (1240)
DE: 7230 Seismicity and tectonics (1207, 1217, 1240, 1242)
DE: 7240 Subduction zones (1207, 1219, 1240)
DE: 7294 Seismic instruments and networks (0935, 3025)
SC: Union [U]
MN: 2007 Fall Meeting