Abstract
Spatial and temporal variations in coseismic slip distribution are often obtained by rupture process analyses using teleseismic body waves. Many analyses using teleseismic body waves were based on the ray theory because of the very efficiently computable direct P-, S-, and major reflected waves near the source. The 2004 Sumatra-Andaman earthquake was one of the largest earthquakes recorded in history, and the data that are required for the entire rupture process analysis include later phases such as PP waves and a very long period phase called a W phase. However, calculating these later phases using the conventional ray theoretical method is difficult. Here we investigate the rupture process of the 2004 Sumatra-Andaman earthquake using complete Green’s functions, including all later phases such as PP waves and W phase. We use the direct solution method, which computes complete synthetic seismograms up to 2 Hz for transversely isotropic spherically symmetric media, to calculate the Green’s functions. The obtained synthetic seismograms generated fit the observed seismograms quite well from a short period to a long period. The estimated slip distribution consists of four large slip areas: the largest slip occurred in the shallow part off the Sumatra west coast with a maximum slip of approximately 29 m, the second and third largest slips occurred in the shallow and deep parts of the Nicobar region with maximum slips of approximately 8 m and 7 m, respectively, and the fourth slip occurred in the middle Andaman region with a maximum slip of approximately 6 m. The estimated average rupture velocity is 2.8 km/s, but the rupture may have slowed between the Sumatra and the shallow Nicobar slip areas, and between the Nicobar and the middle Andaman slip areas. The delayed initiation of the shallow slips in the Nicobar region may possibly have been triggered by the deeper slip in the Nicobar region. There were no distinct depth-varying properties for the shallow and deep slips in the Nicobar region, as were reported for the 2011 Tohoku-Oki and 2010 Chile earthquake.
Highlights
The 26 December 2004 Sumatra-Andaman earthquake was one of the largest earthquakes ever to be instrumentally recorded; it caused the Indian Ocean tsunami, which killed over 250,000 people
The estimated slip distribution consists of four large slip areas: the largest slip area is located in the shallow part of the Sumatra region with a maximum slip of approximately 29 m, the second and third areas of strong slip are located in the shallow and deep Nicobar region with maximum slips of approximately 8 and 7 m, respectively, and the fourth slip area is located in the middle depths of the Andaman region with a maximum slip of approximately 6 m
After 60 s, the rupture of area A started to propagate to the northwest with an average rupture velocity of 3.0 km/s and lasted for 140 s (200 s after origin time). (Here, the average rupture velocity of each large slip area is defined to track the onset when the slip value reaches 2 m, which is approximately 30% of the maximum slip value for slip areas B, C, and D.) the rupture of area C started to propagate to the north with an average rupture velocity of 3.0 km/s at 200 s
Summary
The 26 December 2004 Sumatra-Andaman earthquake was one of the largest earthquakes ever to be instrumentally recorded; it caused the Indian Ocean tsunami, which killed over 250,000 people. Some problems arise when these methods analyze that the total duration of a rupture process is longer than the time window between the later phase PP waves and the P waves, such as the 2004 Sumatra-Andaman earthquake (Ji 2005; Yagi 2005; Yamanaka 2005). These methods cannot accurately calculate later phases such as PP waves because such waves are complicated by the upper mantle structure. These conventional methods cannot explain the long-period (up to 1,000 s) W phase, which has been observed in the displacement records of huge earthquakes and can be explained as the superposition of the fundamental mode and several overtones of spheroidal modes or Rayleigh waves (Kanamori 1993; Kanamori and Rivera 2008)
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