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Rupture Properties of the Giant Sumatra Earthquake Imaged by Empirical Green’s Function Analysis

¹ Géosciences Azur, IRD
250 avenue Albert Einstein
06560 Valbonne, France

Empirical green’s function (EGF) analysis has remained little used to image the rupture properties of the giant 26 December 2004 Sumatra earthquake. The 2 November 2002 foreshock (M_w 7.2), close to the mainshock epicenter, gives us the opportunity to use its waveforms to empirically simulate the Rayleigh-wave propagation of the Sumatra earthquake. We first show that the exceptional size of the Sumatra earthquake does not prevent use of the EGF technique. Four aftershocks (M_w 5.9–6.1), distributed along the Sumatra–Andaman trench, are shown to have consistent Rayleigh waves for periods between 100 and 200 sec. At a lower frequency, we present two large earthquakes of the Mexican subduction zone (M_w 7.2– 7.3, close to the selected EGF magnitude) for which long-period Green’s functions (100–2000 sec) remain very similar, even if event epicenters are separated by about 650 km. This justifies the possibility of using the 2002 foreshock as an EGF for the whole rupture process of the Sumatra earthquake and shows more generally the very broad range of application of EGF technique. Then, a specific analysis reveals that seismic moment magnitude is close to 9.1 (seismic moment equal to 5.6 x 10²² N m). Moment release analysis along the Sumatra–Andaman trench shows two main slip episodes: one next to the northern extremity of Sumatra (20-m slip) and the other one along the Nicobar Islands (10-m slip), with a global extent of 1150–1200 km. Rupture velocity varies between values around 2.5 km/sec in the first half of the rupture and values closer to 2 km/sec in the second half. Total duration imaged by Rayleigh waves is 580 sec (±20 sec) and no activity of the fault is found in the time scale between 600 and 2000 sec. In the hypothesis of even longer timescale slip, this phenomenon would be of the order of 10%–20% of the global moment and likely restricted to the Andaman Islands.

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