S WAVE VELOCITY STRUCTURE IN NONTECTONIC SE ASIA BY SEISMOGRAM ANALYSIS OF THE EARTHQUAKES IN SUMATRA–JAVA AT TATO STATION, TAIWAN

Bagus Jaya Santosa

doi:10.1142/s1793431110000911

Abstract

The S wave velocity structure beneath South-East Asia and South China Sea due to earthquakes in Sumatra–Java subduction zone has been investigated through seismogram analysis in time domain and three components simultaneously, using data recorded in TATO, Taiwan seismological station. The synthetic seismogram was calculated using the GEMINI method, which consists of the earth model and the CMT solution of the earthquake. A low-pass filter with corner frequency of 20 mHz is imposed to the seismograms. Response file inversion subjected on the measured seismogram will compare the measured and the synthetic seismogram in the same unit. The seismogram comparison indicated that the synthetic seismogram constructed from PREMAN earth model deviates greatly from the measured one. The deviation occurred on the arrival time of surface wave of Rayleigh and Love as well as S body waves.The S, Love, and Rayleigh waveform deviations on arrival time or oscillation number are solved by changing the gradient of βhinto positive in the upper mantle layers, and corrections for zero-order coefficients of β speed polynomial in every earth mantle layers.The interpretation results of seismogram analysis using waveform comparison indicate that the nontectonic South-East Asia area in front of subduction zone has strong negative correction of βvin the upper mantle and with smaller factor also at earth layers below. This result shows stronger vertical anisotropy than that indicated by the PREMAN earth model.

Highlights

The Earth’s solid mantle is about 2900-km thick and divided into upper and lower mantle (Dziewonski and Anderson, 1981; Kennett, 1991)
The S wave velocity of the region ahead of the subduction field and non-tectonic areas beneath South-East Asia have been investigated using seismogram analysis of the earthquakes that occurred on the subduction plane of Java – Sumatra that was recorded in TATO station, Taiwan, in the time domain and the three Cartesian components simultaneously
The data set used in this research contains all information in seismograms, unlike the travel time data set, which is just a small part of the information contained in the seismogram

Summary

Introduction

The Earth’s solid mantle is about 2900-km thick and divided into upper and lower mantle (Dziewonski and Anderson, 1981; Kennett, 1991). The lower mantle comprises the region located between the seismically defined discontinuity at 670-km depth and the core–mantle boundary (CMB) at 2900-km depth. It represents 70% of the volume of the mantle and is still being a controversy in Solid Earth Geophysics, whether the convection is the driving force for the plate tectonics movement. Due to the slacking strain, Replumaz et al (2004), Engdahl et al (1998) and Grand et al (1997) have interpreted that the earth structures below the area of South China Sea and South East Asia have a negative anomaly of the P wave velocity. Due to the slacking strain, Replumaz et al (2004), Engdahl et al (1998) and Grand et al (1997) have interpreted that the earth structures below the area of South China Sea and South East Asia have a negative anomaly of the P wave velocity. Romanowicz (2003) has interpreted the S wave velocity beneath South East Asia with negative S anomaly in the upper 410 km earth mantle and without anomaly in the mantle layers below, refer to fig. 1

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Conclusion