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Source characteristics of small to moderate earthquakes in the Kanto region, Japan: application of a new definition of the S-wave time window length

Department of Geophysics Graduate School of Science Tohoku University, Aoba-ku, Sendai-shi 980-77, Japan
National Research Institute for Earth Science and Disaster Prevention, Tennodai 3-1, Tsukuba-shi, Ibaraki-ken 305, Japan

Abstract

The S wave from a local earthquake generally consists of several pulses, in contrast to a simple pulse of P wave. This results in a large uncertainty in the estimation of seismic moment and wave energy when the window length for S-wave analysis is not chosen properly. In this study, we propose a new method to define objectively an appropriate time window length of the S wave for estimation of source characteristics based on the S-wave to P-wave energy ratio for a point shear dislocation source. Analyzing waveform data of 97 local earthquakes in the Kanto region, Japan, whose magnitudes M range from 3.3 to 6.0, we obtained simple equations for predicting the time window length of the S-wave T_S as a function of earthquake magnitude or pulse width of P-wave T_P; we got T_S 1.8 T_P for the relation between T_P and T_S. Applying the S-wave window thus defined, we estimated seismic energy E, seismic moment M₀, and corner frequency f_c for both P and S waves. Regression analysis of those parameters revealed (1) our method to define T_S is confirmed by the fact that the seismic moment determined from P and S waves are consistent; (2) in the range of 10¹⁴ < M₀ < 10¹⁸ (N·m), M₀ is almost proportional to f^–3_c both for P and S waves; (3) the value of E/M₀ is 2.7 4.0 x 10^–5; and (4) breakdown of the scaling relation is seen at M 4.

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