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Coseismic Slip Distributions of the 26 December 2004 Sumatra–Andaman and 28 March 2005 Nias Earthquakes from GPS Static Offsets

¹ Wadia Institute of Himalayan Geology
Dehra Dun, India 248001
 (P.B.)
² U.S. Geological Survey
Menlo Park, California 94025
 (F.P.)
³ Geodetic and Research Branch
Survey of India
Dehra Dun, India 248001
 (B.N.)
⁴ Department of Earth and Planetary Science
University of California
Berkeley, California 94720
 (R.B.)

Static offsets produced by the 26 December 2004 M 9 Sumatra– Andaman earthquake as measured by Global Positioning System (GPS) reveal a large amount of slip along the entire 1300 km-long rupture. Most seismic slip inversions place little slip on the Andaman segment, whereas both near-field and far-field GPS offsets demand large slip on the Andaman segment. We compile available datasets of the static offset to render a more detailed picture of the static-slip distribution. We construct geodetic offsets such that postearthquake positions of continuous GPS sites are reckoned to a time 1 day after the earthquake and campaign GPS sites are similarly corrected for postseismic motions. The newly revised slip distribution (M_w 9.22) reveals substantial segmentation of slip along the Andaman Islands, with the southern quarter slipping 15 m in unison with the adjacent Nicobar and northern Sumatran segments of length 700 km. We infer a small excess of geodetic moment relative to the seismic moment. A similar compilation of GPS offsets from the 28 March 2005 Nias earthquake is well explained with dip slip averaging several meters (M_w = 8.66) distributed primarily at depths greater than 20 km.

Online material: Offset estimation procedure, comparison of results to other studies and simpler solutions, and tables of coseismic offsets.

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