Source processes of the 2005 West Off Fukuoka Prefecture earthquake and its largest aftershock inferred from strong motion and 1-Hz GPS data

Reiji Kobayashi,Shin’ichi Miyazaki,Kazuki Koketsu

doi:10.1186/bf03351914

Abstract

Abstract We simultaneously inverted strong motion and 1-Hz GPS data recorded during the 2005 west off Fukuoka prefecture earthquake (M JMA 7.0) for its source process. The data at a GPS station near the source region provided strong constraint on the fault geometry and asperity area. The resultant slip distribution suggests a single asperity close to the Genkai island, where many houses were severely damaged. The maximum slip is 1.4 m, and the total seismic moment is 1.0 × 1019 Nm (M W 6.6). We also inverted strong motion data recorded during the largest aftershock (M JMA 5.8). The resultant slip distribution shows two asperities on the fault plane. The maximum slip in the major asperity is 0.12 m and the total seismic moment is 2.0 × 1017 Nm (M W 5.5). The main shock and largest aftershock may cause stress change on the Kego fault, which is a major fault running through the city of Fukuoka.

Highlights

At 10:53 a.m. on March 20, 2005 (JST=UT+9 hours), a shallow earthquake with a magnitude (MJMA) of 7.0, which was estimated by the Japan Meteorological Agency, occurred in the west off Fukuoka prefecture, Japan (Fig. 1)
We simultaneously inverted strong motion and 1-Hz GPS data recorded during the 2005 west off Fukuoka prefecture earthquake (MJMA 7.0) for its source process
We inverted strong motion data recorded during the largest aftershock (MJMA 5.8)

Summary

Introduction

At 10:53 a.m. on March 20, 2005 (JST=UT+9 hours), a shallow earthquake with a magnitude (MJMA) of 7.0, which was estimated by the Japan Meteorological Agency, occurred in the west off Fukuoka prefecture, Japan (Fig. 1). Most the seismograms were well recovered (the variance reduction is 0.59), the synthetic vertical components at FKO006, FKOH03, FKOH10, and NGS022 were not in good agreement with the observed ones These stations are close to the P-wave radiation node of the strike slip fault. The peak is located the shallower subfaults as shown in the result from the GPS data inversion, but the synthetic strong motions are still in good agreement with the observed ones (the change of variance reduction from 0.59 to 0.56 was small). The observed seismograms were processed to those of the main shock except for the sampling rate of 5 Hz. For the fault geometry, we adopted a focal mechanism of (strike, dip, slip)=(133◦, 88◦, −33◦) determined by the JMA and the hypocenter determined by the Kyushu University (latitude 33.67◦N, longitude 130.29◦E, depth 13.0 km).

Discussion

Conclusion