The rupture process of the MJ=7.2 1995 Hyogo-ken Nanbu (Kobe) earthquake deduced from S-wave polarization analysis

Abstract

The use of S-wave polarization analysis for constraining high-frequency (> 1 Hz) source parameters of large earthquake has been previously illustrated by Bouin & Bernard (1994) and Guatteri & Cocco (1996). In this paper, we show the efficiency of such a methodology applied to a data set in the presence of strong non-linear and liquefaction effects for the particular case of the 1995 Hyogo-ken Nanbu (Kobe) earthquake. The 11 strong-motion recordings from the closest stations to the fault plane have been analysed in the 0.6-2.0 Hz frequency band. We first correct the recorded ground motion data for the effect of shallow crustal anisotropy in the Kobe area, which allows us to remove part of the complexity of the observed S-wave polarigrams. Our results in terms of anisotropy parameters are in good agreement with those of previous studies performed in the same area. They show evidence, far from the faults, of a fast S-wave polarization oriented nearly parallel to the regional compressive stress direction, and, close to the faults, of a fast S-wave polarization oriented parallel to the faults' strikes. The corrected S-wave polarigrams are then interpreted in terms of rupture propagation along an extended fault. We consider three different faulting mechanisms: a pure right-lateral mechanism and two mechanisms with 30 and 50 per cent reverse components. The polarization data and the triggering times at the selected stations allow the identification of a set of subsources, demonstrating the need for a reverse slip component during the rupture of the two high-frequency (2.0 Hz) subevents on the Kobe fault section. The resulting locations of the main subevents in space and time provided an interval of possible values of rupture velocity ranging between 2.65 km s-1 (that is, 80 per cent of the shear wave velocity) and 3.46 km s-1 (that is, the value of the shear wave velocity). This study emphasizes that S-wave polarization can be a useful tool to restrict the number of reliable starting models for iterative waveform inversions.