Three-dimensional seismic velocity structure as determined by double-difference tomography in and around the focal area of the 2005 West off Fukuoka Prefecture earthquake

Abstract

On March 20, 2005 the West off Fukuoka Prefecture earthquake (magnitude of 7.0 on the JMA scale) occurred in southeastern Japan. The earthquake fault was a left-lateral strike-slip having a nearly vertical fault plane and a strike in the WNW-ESE direction. The largest aftershock with a magnitude of 5.8 (JMA) followed 1 month later. To gain more detailed aftershock data, several teams from different Japanese universities jointly installed a number of temporary seismic stations and positioned Ocean Bottom Seismometers (OBSs) immediately above the focal area. Double-difference tomography was used to estimate the three-dimensional (3D) (Zhang and Thurber, 2003) velocity structures in and around the focal area based on the travel time data collected during seismic observations. The high-velocity regions estimated by the inversion are located on the edge of the aftershock area and on the shallow part of asperity, as inferred from the slip distribution. Conversely, the V p/V s ratio is not always as high as that found at the location of the asperity. This finding suggests that the construction of the medium is not uniformly elastic but complex, with different relations between elastic constants and strength.

Hypocenter distribution and heterogeneous seismic velocity structure in and around the focal area of the 2008 Iwate-Miyagi Nairiku Earthquake, NE Japan—Possible seismological evidence for a fluid driven compressional inversion earthquake

“…Large coseismic slip area (shown by circle) has been estimated in this part, and it seems to be in the complex and relatively high velocity area. This correspondence between high-velocity body and asperity has observed also in the 2003 northern Miyagi earthquake (M 6.4) in NE Japan (Okada et al, 2007a), the 2004 Mid Niigata earthquake in central Japan (Okada et al, 2005, the 1995 southern Hyogo (Kobe) earthquake (M 7.2) (Okada et al, 2007b), the 2005 West off Fukuoka Prefecture earthquake (M 7.3) (Hori et al, 2006) and Park eld, California (Michael and Eberhart-Phillips, 1991;Eberhart-Phillips and Michael, 1993). Such high-velocity area might act as "asperity" which can store large strain and occur large slip (e.g.…”

Spatial heterogeneities in tectonic stress in Kyushu, Japan and their relation to a major shear zone

“…The hypocenters in this area, with the exception of the Fukuoka earthquake aftershocks, can be determined by using three-dimensional velocity structures obtained through the tomographic inversion of travel-time data (Saiga et al 2010). The hypocenter for the Fukuoka area was determined by the velocity structure of Hori et al (2006) because those data provided a finer structure after high-aftershock activity in the area as compared to the data of Saiga et al (2010). As many as 40,981 hypocenters have been located over approximately 20 years.…”

Inelastic strain rate in the seismogenic layer of Kyushu Island, Japan

“…The data cover the period from January 2000 to July 2013, during which the detectability would be expected to be constant. Saiga et al (2010) and Hori et al (2006) determined earthquake hypocenters in this area using three-dimensional velocity structures. The focal mechanisms were estimated from P-wave polarity data observed at eight or more stations using the HASH algorithm developed by Hardebeck and Shearer (2002).…”