Effect of The Heterogeneous Structure Near The Source on The Teleseismic Body Waveforms From The Sunda Trench Earthquakes Analyzed by a 2.5D Finite-Difference Method

Teleseismic body waveform analysis is one of the major methods in the source process study of large earthquakes. It is also possible to relocate small earthquakes by using teleseismic waveforms to provide improved distribution of earthquakes (e.g., Okamoto 1994). For shallow subduction zone earthquakes, it is quite important to consider the effects of the laterally heterogeneous structure near the source on the teleseismic body waveforms: the thick ocean (water) layer with dipping ocean bottom and the thick sediments with low seismic wave velocity often cause very large effects which are not reproduced by flat-layered model structure (e.g., Wiens 1989; Yoshida 1992; Okamoto 1993). In this paper, we study how the lateral heterogeneity in the Sunda trench affect the teleseismic waveforms and discuss the effect on the study of the source process of the recent large earthquakes on 12 September 2007. We preliminary compared the observed waveforms and 1D synthetics computed for a flat-layered model structure from a shallow event (2007/09/14 06:01 Mw6.2) that is one of the aftershocks closest to the trench axis. Relatively large later phases were observed at stations located close to one of the nodal lines (i.e., stations at about 180 to 280 degrees in azimuth): these phases were not reproduced by a 1D model structure near the source assumed based on the laterally varying crustal model obtained by a detailed seismic experiment (Kopp et al. 2001). This suggests that the lateral heterogeneity near the source is the likely cause of the later phases. The structural effect appears to be minor on the waveforms at stations far from the nodal lines. We will present analysis using teleseismic waveform modeling with a 2.5D finite-difference method (Okamoto 1994; Takenaka and Kennett 1996) which incorporates the lateral heterogeneity in the Sunda trench.