2005 年福岡県西方沖地震の破壊初期段階の特徴

抄録

A P-phase with smaller amplitude (P1)preceding another P-phase with larger amplitude (P2)has often been observed for major seismic events, which indicates a small-scale rupture, occurring around the rupture initiation point, precedes large-scale rupture. These two phases as well as the corresponding two S-phases (S1 and S2)were observed on seismograms of the 2005 West off Fukuoka Prefecture Earthquake (M_JMA=7.0) that occurred on March 20, 2005. In order to investigate the initial rupture process, we made the following analyses. We checked whether these double phases were generated from different but adjacent ruptures along the fault. We first picked the four phases (P1, P2, S1 and S2) using AIC for AR model fitting from the seismograms recorded by Digital Strong-Motion Seismograph Network and High Sensitivity Seismograph Network that deployed by NIED. We then relocated the corresponding hypocenters by means of the master event hypocenter relocation technique. The distance and time interval between the initial and main ruptures were estimated to be about 3.44 km and 3.38 s, respectively. The relationship between the duration of the initial rupture and the magnitude is consistent with the empirical scaling relation in preceding studies. The estimated apparent “average rupture propagation speed” of this initial rupture was 1.02 km/s, which is lower than the average rupture velocity in usual crustal events. The moment magnitude of the initial rupture portion was estimated to be around 4.7. We estimated the source time functions for the initial rupture of the main shock and two aftershocks of similar size using empirical Green’s functions. The moment of the initial rupture of the main shock gradually increased and had long duration, which is quite different from the pulse-shaped source time functions with shorter duration of the two aftershocks. This result, combined with the empirical scaling relation mentioned above, implies that the initial rupture is different from those of earthquakes of the same size and related to the final rupture size of the main shock.