HR: 1340h
AN: S53A-1018    [Abstracts]
TI: An underwater landslide or slump on an active submarine fault Ea possible source of a devastating tsunami?
AU: * Matsumoto, T
EM: tak@sci.u-ryukyu.ac.jp
AF: University of the Ryukyus, 1 Senbaru Nishihara-cho, Okinawa, 903-0213, Japan
AB: A Mw 7.7 earthquake and subsequent large-scale tsunami occurred on 17th July 2006 off the southern coast of Java Island, Indonesia. A maximum of 7.7 m inundation height was recorded in Pangandaran on the southern coast of Java, according to the field survey just after the tsunami (Tsuji et al., 2006). However, since there are few residents who noticed the earthquake tremors, the earthquake may possibility be so-called "tsunami earthquake". The aftershocks from July to September occurred on the fore-arc area and their CMT solution suggests the predominant north-south tensile stress. R/V MIRAI passed the aftershock area in 2004 and 2005 with continuous multibeam bathymetric survey. The processed topographic map shows a lot of amphitheatres with the scale of 8-20 km along the fault scarps. Convexity landforms are located below the footwall of the amphitheatres, apparently the relics of an underwater landslide, and their relative elevation exceeds 1000 m in maximum. This area is characterised by the southeastern extension of Mentawai Fault and its associated minor faults ranging from the Sumatra fore-arc area. The observed amphitheatres and relics of underwater landslides are located along the active faults. Considering that the tsunami wave height distribution is concentrated on a specific narrow area compared with the scale of the mainshock, the mainshock possibly triggered underwater landslides on these amphitheatres and the slides generated a large-scale tsunami. The Ryukyu district was attacked by a M7-class earthquake followed by a devastating tsunami in 1771. 'The East Ishigaki Fault,' one of the across-arc active normal faults in this area was studied precisely by multibeam echo sounding. The study revealed a slump on the segmented active fault. The slump itself may generate a tsunami with the wave height of several meters and might cause the 1771 tsunami together with the faulting itself. Some recent geohazards also show that a landslide or a slump is associated with an identified active fault, for example, 17 Jul. 1998 Papua New Guinea Earthquake and Tsunami. This study is to compare the several similar cases mentioned above in order to discuss the liability to landlides on an active fault.
DE: 3000 MARINE GEOLOGY AND GEOPHYSICS
DE: 3045 Seafloor morphology, geology, and geophysics
DE: 3070 Submarine landslides
DE: 4564 Tsunamis and storm surges
DE: 7240 Subduction zones (1207, 1219, 1240)
SC: Seismology [S]
MN: 2007 Fall Meeting