SLIP OF THE 2004 SUMATRA-ANDAMAN EARTHQUAKE FROM JOINT INVERSION OF LONG-PERIOD GLOBAL SEISMIC WAVEFORMS AND GPS STATIC OFFSETS

Abstract

The 26 December 2004 Great Sumatra-Andaman earthquake opened a new era for seismologists to understand the complex source process of a great earthquake. This is the first event with moment magnitude greater than 9 since the deployment of high-dynamic-range broadband seismic and Global Positioning System (GPS) sensors around the globe. This study presents an analysis of the ruptured fault-plane geometry and slip distribution using long-period teleseismic data and GPS-measured static surface displacements near the fault plane. We employ a rupture geometry with six along-strike segments with and without a steeper down-dip extension. The fault segments are further subdivided into a total of 201 ~30 × 30 km fault patches. Sensitivity tests of fault-plane geometry and the variation in rupture velocity indicate that the dip and curvature of the fault plane are not well resolved from the given data set and the rupture velocity is constrained to be between 1.8 and 2.6 km/ sec. Error estimations of the slip distribution using a random selection of seismic and GPS station subsets (50% of all stations) illustrate that slip is well resolved along the whole rupture and the mean slip uncertainty is less than 1.5 m (about 11%). Although it is possible that near-field GPS data include contributions from additional postseismic transient deformation, our preferred model suggests that the Sumatra-Andaman earthquake had a magnitude of Mw 9.20 + 0.05/-0.06.