Using Earthquake Rupture Variations along the Sumatra-Andaman Subduction System to Examine Fault Zone Variations

The 26 December 2004 great Sumatran-Andaman earthquake in the Indian Ocean caused extensive damage and loss of life from intense shaking and the resulting tsunami. Several studies of this earthquake suggest that portions of the fault ruptured at variable speeds, with both fast and slow rupture velocities observed along the 1200 km long rupture length. Variations in rupture velocity during the earthquake may indicate along-strike variations in megathrust frictional conditions that may influence other earthquakes along the zone. Previous work on global subduction zone systems suggests depth-dependent frictional conditions arising from heterogeneous conditions along the fault. In many of the circum-Pacific subduction zones, shallow earthquakes along the subduction megathrust have longer scaled source durations than deeper earthquakes, possibly resulting from variations in frictional conditions with depth. This study focuses on thrust mechanism earthquakes on the Sumatra- Andaman megathrust, examining aftershocks with Mw 6.0 of the 2004 earthquake, as well as earthquakes that occurred in the region between 1992 and 2004. Source duration, depth, and slip distribution are determined for this set of earthquakes to explore the possibility of both along-strike and depth-dependent variations in source parameters and frictional conditions. There is evidence of depth-dependent source parameters for these events, with longer scaled durations for the shallower earth- quakes, consistent with previous global studies. No temporal change is apparent in this relationship, as source parameters for previous large earthquakes are similar to those after the earthquake. Along-strike patterns suggest long-duration character for several earthquakes in the southern portion of the rupture zone but no strong evidence of slow character in the northern portion of the rupture zone. It appears unlikely that any long-term variations in the fault-zone character influenced possible slow rupture of the 2004 earthquake.