Abstract
We investigate the fault parameters of the 2005 Miyagi-Oki (Japan) earthquake using duration variations of teleseismic P-waves. The results show that the earthquake has a thrust-type mechanism and a seismic moment of 4.46 × 10 19 Nm. Rupture directivity analysis suggests that the earthquake occurred as a result of a bilateral faulting on the fault plane with a strike of 247°, a dip of 17° and a slip of 125°. The optimal rupture azimuth, measured counterclockwise from the strike on the fault plane, is 170° (or 350°). The rupture length and average source duration are estimated to be 73.4 km and 14.5 sec, respectively. Thus the rupture velocity is 2.53 km sec -1 (~0.57 times the value of S-wave velocity), which is lower than the value for other similarly sized earthquakes. This implies that the 2005 Miyagi-Oki earthquake was probably a slow event. Consequently, there may have been less release of high-frequency seismic energy, leading to lower radiated seismic energy and radiation efficiency (~0.32 - 0.48). In other words, relatively larger fracture energy occurred during earthquake faulting in addition to the heat due to friction. The ratio of the static stress drop to the apparent stress (> 2.0) also suggests that the earthquake can be modeled as a frictional overshoot in a stress model, which implies the transformation of a lower percentage of strain energy into seismic-wave energy during the process of earthquake rupturing.
Highlights
Fault parameters and rupture directivity for earthquakes provide important information for understanding the physical behavior of earthquake ruptures (Ben-Menahem 1961; Kanamori and Heaton 2000)
We find the minimum misfit for a combination of bilateral faulting and fault plane (247°, 17°, 125°) when searching a series of Θ [rupture azimuths as indicated in Eqs. (4) and (5)]
Large earthquakes frequently occur in northeastern Japan where their rupture areas overlap and cover each other (Yamanaka and Kikuchi 2004)
Summary
Fault parameters and rupture directivity for earthquakes provide important information for understanding the physical behavior of earthquake ruptures (Ben-Menahem 1961; Kanamori and Heaton 2000). Okada et al (2005) analyzed the rupture process and suggested that the 2005 Miyagi-Oki earthquake possibly caused rupturing in the source area of the 1978 Miyagi-Oki earthquake They found that the coseismic slip distribution more or less followed the dip-direction of the fault. The aftershocks of the 2005 Miyagi-Oki earthquake (see Fig. 1) occurred mainly along the dip-direction of the fault They mostly surrounded the main area of the rupturing as derived by Yagi (2005). We attempt to infer the source mechanism and parameters of the Miyagi-Oki earthquake by utilizing the far-field P-waveform inversion procedure proposed by Lin et al (2006) With this inversion process, we can simultaneously derive the fault plane solution, the seismic moment, and the rupture directivity of the earthquake. The estimated fault parameters can be used to calculate the macroscopic source parameters, which give us a better understanding about the physical behavior of rupture during the 2005 Miyagi-Oki earthquake
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