Aftershock distribution and 3D seismic velocity structure in and around the focal area of the 2004 mid Niigata prefecture earthquake obtained by applying double-difference tomography to dense temporary seismic network data

Abstract

A destructive large earthquake (the 2004 mid Niigata prefecture earthquake) sequence occurred in the central part (Chuetsu district) of Niigata prefecture, central Japan on October 23, 2004. We have deployed a temporary seismic network composed of 54 stations for aftershock observation just above and around the focal area of the earthquake for about a month. Using travel time data from the temporary seismic network and surrounding routine stations, we obtained precise aftershock distribution and 3D seismic velocity structure in and around the fault planes of the earthquake and four major (M ≥ 6) aftershocks by double-difference tomography. The results clearly show three major aftershock alignments. Two of them are almost parallel and dipping toward the WNW. The shallow and deep aftershock alignments correspond to the fault plane of the mainshock and that of the largest aftershock (M6.4), respectively. The third alignment is almost perpendicular to the WNW-ward dipping planes and perhaps corresponds to the fault plane of the M6 aftershock on October 27. General feature of the obtained velocity structure is that the hanging wall (western part of the focal area) has lower velocity and the footwall (eastern part of the focal area) has higher velocity. Major velocity boundary seems to shift westward in comparison to in northern and southern parts at a location near the central part of the focal area, where the main shock rupture started. Some parts of the fault planes were imaged as low velocity zones. This complex crustal structure would be one of possible causes of the multi-fault rupture of the 2004 mid Niigata prefecture earthquake sequence.

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

“…This is the same as the previous compressional earthquakes as the 2003 northern Miyagi earthquake and others recently occurred in NE Japan (e.g. Okada et al, 2005Okada et al, , 2007a.…”

“…This is the same as for the conjugate M 6 aftershock of the 2004 Mid Niigata (Chuetsu) earthquake (e.g. Okada et al, 2005). This might suggest eastward-dipping alignment may means relatively newly developed fault or the fault which is less developed in Miocene in comparison with the major westward-dipping fault plane.…”

“…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.…”

High-resolution aftershock observations in the source region of the 2004 mid-Niigata Prefecture Earthquake

“…Consequently, several research groups in Japan deployed temporary seismic stations following the mid-Niigata Prefecture Earthquake. Intensive researches such as hypocenter relocations, focal mechanism, seismic tomography and imaging of reflection planes or scatterings in the crust, have been conducted by several groups using data recorded at temporary seismic stations (e.g., Sakai et al, 2005;Kato et al, 2005a;Okada et al, 2005;Shibutani et al, 2005;Matsumoto et al, 2005). The results of the previous studies suggest that the complex structures associated with crustal stretching and folding have the potential to nucleate the mainshock and trigger a sequence of large aftershocks.…”

High resolution seismic velocity structure around the Yamasaki fault zone of southwest Japan as revealed from travel-time tomography

“…They found that the slip is small for the high-velocity regions and large in the shallow low-velocity regions, and concluded that crustal structure heterogeneity could control the rupture process of the mainshock. A tomographic study of the 2004 mid-Niigata prefecture earthquake by Okada et al (2005) concluded that the high-velocity area with low V p /V s values, imaged in the footwall, was considered to be old basement rock, while, the low-velocity areas with high V p /V s values in the hanging wall were associated with sediments. Nakajima et al (2006) investigated the major features of the complex seismic velocity structure around the Nagamachi-Rifu fault in northeastern Japan, with implications for the existence of fluid-rich regions in the mid to lower crust as indicated by low velocity and high Poisson's ratio anomalies.…”