Preliminary Report of the Urgent Survey Cruise off Sumatra Island by the Research Vessel NATSUSHIMA: Leg 1 (Feb. 18 - Mar. 4, 2005)
Off Sumatra Earthquake and Tsunami Survey Group
IFREE/DSR: Wonn Soh, Masa Kinoshita
Tel +81+46-867-9323 fax +81-46-867-9315
Public Relations: Kenichi Takahashi
Tel +81+46-867-9066 fax +81-46-867-9055
Collaborating with the Agency for the Assessment and Application of Technology of the Republic of Indonesia (BPPT, Head : Dr. Kusayanto Kadiman), the Japan Agency for Marine-Earth Science and Technology (JAMSTEC, President: Mr. Yasuhiro Kato) conducted a survey around the epicenter of theSumatran Earthquake of Dec. 26, 2004, by JAMSTECfs Research Vessel NATSUSHIMA (1,739 GT) with Remotely-Operated Vehicle HYPER DOLPHIN, from Feb. 18 to Mar. 4, 2005 as the first leg out of two. The onboard scientific party conducted a survey for precise seafloor bathymetry data by the multi-narrow beam echo sounder onboard the NATSUSHIMA, and visually observed the seafloor for detection of possible events of the earthquake, and monitored aftershocks using array of Ocean-Bottom Seismometers (OBSs).
This research has been supported by the Special Coordination Fund of the Ministry of Education, Culture, Sports, Science and Technology (MEXT) of the Government of Japan, entitled gImmediate Study of the Great Sumatran Earthquake and Tsunami Disasters.h
2. Summary of Survey During Leg 1 (Feb. 18-Mar.4, 2005)
1) 17 OBSs, owned by JAMSTEC and the Earthquake Research Institute, Univ. of Tokyo (ERI), were deployed for three week long observation of nearby seismicity, with 2 long-term OBSs of ERI for four month long observation. 17 OBSs will be recovered during Leg 2, before March 22, whereas the rest of 2 long-term OBSs are planned to be recovered afterwards during a cruise of another vessel(see Appendix 1).
2) Taking advantage of a super high-resolution TV camera installed on the HYPER DOLPHIN, some video images were obtained near the epicenter area for the first time since the last earthquake. Among these images were open fissures due to the slope failure, collapsed cliffs and muddy layer on the downstream side, which were probably caused by the last earthquake (see Appendix 2).
3) A precise bathymetric map covering approximately 3,000 km2 area, around the focal area which should have caused maximum seafloor deformation was drafted and at the same time, several lines of Single-Channel Seismic (SCS) survey(*) were carried out.
4) Data including video / photo images were obtained for the first time since the earthquake, enabling scientists to understand how geological and environmental changes such as the extinction of living benthos observed near the focal area might have occurred on the seafloor due to the earthquake.
* SCS survey:
A seismic survey method to examine the crustal structure below sea floor by emitting large energy sound waves (or seismic waves) from an air-gun towed from the survey vessel, and by receiving reflected waves from the sea floor and formation boundaries such as sand/mud layer interface with a towed hydrophone (microphone in the water) to process to create a cross-sectional image of the sediment structure.
3. Plan for Leg 2 (March 10-22, 2005)
@In the latter half of the survey cruise, the onboard scientific party will continue similar investigations such as bathymetry and SCS surveys, dives and aftershock monitoring, by the NATSUSHIMA and HYPER DOLPHIN. Every effort will be made based upon integrated results from Leg1 to identify evidence for the crustal deformation such as faulting. Main target area will be the outer rise zone near the trench and main seismic fault.
@Preliminary daily results will be available at the JAMSTEC website (www.jamstec.go.jp).
4. Post-cruise analyses
@Post-cruise analyses will be carried out at JAMSTEC based upon data obtained throughout this cruise:
1) It is expected that through analyses of OBS data, a seismicity distribution map including aftershocks below magnitude three (M>3.0) will be created. It is further expected that study of this micro-seismicity map will enable scientists to obtain the 3D image of the fault plane that caused the last earthquake, and will provide insights into the possible mechanism of how the slip along the fault propagates for the earthquakes over magnitude nine (M>9.0).
2) It is considered that the currently obtained high-precision bathymetry data would highly contribute to the hazard map on tsunami propagation and wave heights for great earthquakes likely to occur in the Sumatran region in the future . Also, there is the expectation that comparison of simulated seafloor modulation from the tsunami inversion with this bathymetry data will provide more reliable model of tsunami propagation.
Appendix 1: Survey area of the survey cruise
(1) Fresh edge of collapse at 2105 m water depth
(2) Open fissures that develop on the upper slope of steep cliff.