Teleseimic Analysis of Repeating Events Beneath the Aceh Basin, Northern Sumatra

The 2004 Sumatra-Andaman Islands earthquake produced dramatic slip along more than 1,000 kilometers the megathrust plate boundary, and initiated adjustments in the regional stress distribution associated with an energetic aftershock sequence. We explore the seismic activity that has occurred beneath a small part of the 2004 rupture, focusing on the aftershock activity beneath the Aceh Basin. The Aceh Basin is the northernmost forearc basin in a series of forearc basins along the Sunda trench and is located more than 100 kilometers from the trench. Earlier work in the region included two OBS studies by Japanese and French researchers as well as teleseismic travel time locations of the larger events in the sequence. We estimated the depths of the larger aftershocks beneath the basin using depth-phase modeling and found results consistent with earlier studies that estimated depths of about 35-50 kilometers. Teleseismic P-waveforms that include sP bounces beneath the deep part of the basin produce a characteristic signature that we use to estimate first-order basin structure. We find that the basin is about 4.5 kilometers thick and underlain by a strong, sharp velocity contrast. Cross correlation of short-period teleseismic P-waveforms produced by the aftershocks reveals numerous repeating events that include multiplets and doublets. These repeating events correlate well (greater than 0.95 correlation coefficient) at several teleseismic stations. The largest cluster shows a systematic increase in time between events, and a systematic decrease in seismic moment, consistent with the events being driven by post-seismic activity decreasing with time following the mainshock. These events are located near the base of the seismogenic zone, which appears to agree well with the Aceh Basin edge, although the basin structures are 30-40 kilometers above the plate boundary. We present results of our analysis and efforts to relocate events using both body and surface waves, as well as a more detailed analysis of the repeating aftershocks.