Investigating the S Wave Velocity Structure in front Region of Subduction Zone by Seismogram Analysis of Sumatra's Earthquakes in UGM Station

The measured seismograms have been compared with synthetic ones at UGM observatory station, which the seismograms were triggered by several earthquakes, occurred in Nicobar, North Sumatra and Sunda straight. The wave paths from earthquakes hypocenter to UGM provide chance to investigate the S velocity structure along the front region of subduction zone. The synthetic seismogram was computed using GEMINI program, where the input consists of the earth model, and the CMT solution of the earthquake and location of the observatory station. The inverted response file of the station is imposed to the measured seismogram, so that the seismogram comparison is conducted in the same unit. Analysis of surface waveform shows that S wave velocity in front region of subduction zone has negative anomaly, but the waveform analysis of body wave shows that negative anomaly is also continued on earth mantle layers. The Love waveform is sensitive to the earth crust thickness, but the Rayleigh waveform is not. Velocity corrections on deeper earth mantle layers are required to obtain the fitting on S and ScS and multiple ScS body waves.Research's results show that the front region of subduction zone has negative S wave velocity anomaly in the upper mantle and deeper mantle layers. The assumed relation between the positive P wave model and S wave model anomaly should be not correct. The result is different with other seismological research, which based on travel time inversion or dispersion analysis.