HR: 10:20h
AN: U22A-01    [Abstracts]
TI: Mapping The Ruptures of the Great Sumatra-Andaman Earthquakes
AU: Ammon, C J
EM: cja12@psu.edu
AF: Penn State, 440 Deike Building, University Park, PA 16802 United States
AU: Velasco, A A
EM: velasco@geo.utep.edu
AF: University of Texas at El Paso, Department of Geological Sciences, El Paso, TX 79902 United States
AU: * Lay, T
EM: tlay@es.ucsc.edu
AF: University of California, Santa Cruz, Earth Sciences Department & IGPP, Santa Cruz, CA 95064 United States
AB: We use Rayleigh waves with periods between 100 and 500 seconds to map seismic moment distribution during the 26 December, 2004 Great Sumatra-Andaman earthquake along the northern Indonesian subduction zone and the associated 28 March, 2005 event. To isolate source effects from the observed waveforms we deconvolved normal-mode synthetic seismograms computed using PREM and the Harvard CMT mechanism located at the USGS-NEIC epicenter. We obtain about 200 useable R1 source time functions with a good azimuthal distribution, admittedly more densely sampled to the north. Our results suggest that the major normal seismic moment release began about 40-60 seconds after the initial slip (identified by the USGS-NEIC origin time) and then continued for about 400-450 additional seconds. The 28 March event included two regions of large slip, one beneath Nias Island. For both events, the surface-wave time-function based models are consistent with seismogram modeling efforts using similarly parameterized faults. We also demonstrate the utility of R1 observations perpendicular to large, low-angle thrust events to construct simplified, smooth images of the rupture suitable for estimating tsunami potential of large earthquakes. An important lingering question regarding the 26 December rupture is the timing of the slow (postseismic) slip observed in GPS observations completed several weeks later, but not contained in seismogram-based models. We have examined the later, low-frequency signals of the surface-wave time functions during the hours immediately following the onset of slip. We find little coherent motion in the long-period seismic band during the first few hours after the event, which suggests very slow motion with a long time history, perhaps taking hours (or longer) to accumulate the meters of offset observed in the GPS observations on the northern third of the aftershock zone.
DE: 7215 Earthquake source observations (1240)
DE: 7240 Subduction zones (1207, 1219, 1240)
DE: 7255 Surface waves and free oscillations
SC: Union [U]
MN: Fall Meeting 2005