A PALEOSEISMIC RECORD OF REPEATED GREAT EARTHQUAKES ON THE SUNDA SUBDUCTION MEGATHRUST, NORTHERN SUMATRA

The great Aceh-Andaman earthquake of December 2004 presented the scientific community with an unprecedented opportunity to learn more about great subduction zone earthquakes while at the same time provided a sobering reminder of the social and economic cost of such earthquakes. Over 230,000 fatalities and billions of dollars of damage repair and replacement income amply demonstrate the socio-economic importance of better understanding such earthquakes. The 2004 Aceh-Andaman earthquake on the Sunda megathrust has already provided a wealth of data on coseismic and postseismic land-level displacements associated with the Sunda megathrust rupture. During the 2004 Aceh-Andaman earthquake, a ~1500-km stretch of the Sunda megathrust ruptured along the Australian and Indian plates producing a magnitude 9.3 earthquake. Slip along the megathrust exceeded 20 m and produced 0.5 to >2 m of coseismic subsidence along the northwest coast of Sumatra, with over 30% additional fault slip in following two months. The 2004 earthquake is a rare event and provides an unprecedented opportunity to understand the vertical land-level changes caused by such an event in northern Sumatra, and to inquire whether earthquakes of comparable magnitude have occurred in the past and how these paleoseismic events compare to the 2004 earthquake.

We will show preliminary results from our exploration of the paleoseismic history of the southern portion the Sunda megathrust that ruptured in the 2004 Aceh-Andaman earthquake. We will investigate the recurrence interval of megathrust paleoearthquakes, the width of megathrust rupture during paleoearthquakes and document whether some of the slip on the megathrust occurred as postseismic slip after paleoearthquakes. Our work will be focused north of the offshore islands in an area that is landward of the region directly above the 2004 rupture and in an area that typically subsides as a result of megathrust earthquakes.

We will show the application of subsidence stratigraphy as a paleoseismic tool in Sumatra. Subsidence stratigraphy is a valuable tool for: (1) determining recurrence intervals of megathrust earthquakes; (2) assessing the amount of coseismic subsidence during paleo-subduction zone earthquakes; and (3) constraining dimensions of paleo rupture extent at the down dip end of rupture.