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40th Foundation Year Lecture (6): Source of the 2004 Indian Ocean Tsunami from Inverse Modelling of Observed Data by Dr Kenji Satake
Tsunamis are generated by submarine geological processes such as earthquakes, volcanic eruptions or landslides. Once the tsunami source, or generation process, is estimated from seismological analysis, the tsunami propagation process can be simulated by numerically solving shallow-water (long-wave) equations on actual bathymetry. Computed tsunami arrival times and amplitudes from such forward modeling can be used for tsunami warning system. The tsunami source, however, is usually not well known and need to be estimated by inverse modeling of observed tsunami data. Tsunami is recorded in various ways: run-up heights measured by survey teams, waveforms recorded on tide gauges, and sea surface heights captured by satellite altimeters. The December 26, 2004, Sumatra-Andaman earthquake generated tsunamis that propagated across the Indian Ocean and caused the worst tsunami disaster in history. The tsunami source, particularly its northern end, was not well resolved. While the aftershocks and crustal deformation were extended from off northwestern Sumatra Island through Nicobar Islands to Andaman Islands, seismic wave analyses indicated shorter source length, several hundred km. We used tsunami waveforms recorded at 12 tide gauge stations around the source and the sea surface heights measured by three satellites: Jason-1, TOPEX and Envisat. Joint inversion of tide gauge records and sea surface heights indicated that the tsunami source was about 900 km long. The largest slip on fault, about 13 to 25 m, was located off Sumatra Island and the second largest slip, up to 7 m, near Nicobar Islands. The slip distribution is similar for different rupture velocities or rise time, while slow velocity of 1 km/s and rise time of 3 min yield the largest variance reduction.
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