Crustal heterogeneity around the Nagamachi-Rifu fault, northeastern Japan, as inferred from travel-time tomography

Abstract

An M=5 earthquake occurred on September 15, 1998, in the Nagamachi-Rifu fault (NRF), northeastern Japan. In the aftermath of this event, many seismograph stations were constructed temporarily around the fault, forming a dense network of stations with a spatial separation of 5 km. We report here our estimation of the threedimensional velocity structures of the P and S waves using arrival-time data recorded at these stations with the aim of understanding the heterogeneous structure around the NRF. Low-Velocity and high Poisson's ratio anomalies are imaged in the lower crust beneath the volcanic area, which are probably associated with the partially molten materials conveyed through the upwelling flow in the mantle wedge. A distinct low-velocity anomaly, which is explainable by the existence of H 2 O-filled pores, is observed in the mid crust at the deeper extension of the NRF. Two low-velocity anomalies that are probably associated with the remnants of magmatic activity that formed the Shirasawa caldera and with the existence of thick late-Cenozoic sedimentary layers are observed at depths shallower than 10 km in the hanging wall of the NRF. Our results successfully characterize the major features of the complex velocity structure around the NRF, with implications for the existence of fluid-rich regions in the mid to lower crust.

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

“…7, while the region beneath the Kuresaka-Toge fault shows moderately high V p /V s (moderately high PR) as shown by Anomaly (2). Our interpretation is that the low velocities and low V p /V s values of the shallow crust beneath the Hijima fault may be attributed to the existence of water-filled pores of relatively large aspect ratio, while the low velocities and moderate high V p /V s values beneath the Kuresaka-Toge fault may be due to the existence of water-filled pores of a relatively small aspect ratio (e.g., Takei, 2002;Matsubara et al, 2004;Kato et al, 2006;Nakajima et al, 2006).…”

“…7 and 8. Such anomaly features have been commonly imaged in the lower crust in northeastern Japan from previous travel-time tomographic studies (e.g., Nakajima et al, 2001Nakajima et al, , 2006Nakajima and Hasegawa, 2003;Kato et al, 2006;Matsubara et al, 2008) , 5, 7, 10, 12, 15, 17, and 20 and they are interpreted to be possibly related to partially molten materials. Although Quaternary, or active, volcanoes are not found around the Yamasaki fault zone, Yoshida (2001) showed that temperatures in the lower crust generally exceed the wet solidus of basalt and andesite.…”

“…One plausible explanation is the existence of anhydrous plutonic rocks in this region (Yoshida, 2001), however, this interpretation needs further investigation. The presence of partial melt or fluids beneath the seismogenic zone in the lower crust structure can change the deformation of the fault zone and affect its stress accumulation (Zhao et al, 1996;Hasegawa et al, 2000Hasegawa et al, , 2005Nakajima et al, 2006).…”

“…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.…”

Local Earthquake Tomography in the Broader Area of Western Corinth Gulf

“…Some interesting features of this study are revealed through anticorrelation of higher P and lower S velocities and as a consequence high Vp/Vs ratio values (1.90-2.06) parallel to the mean direction of mapped faults and thrusts in which are mainly connected to tectonic grabens filled with a thick layer of recent sedimentary deposits or fracturation and fluid penetration along the fault plane such as the case of AFZ. The presence of high Vp/Vs ratio values at 15 km depth in AFZ case indicates that the rupture area consists of relatively small aspect ratio water-filled pores of the basement rock under overpressured conditions, as similar cases has shown (Takei, 2002;Nakajima et al, 2006). On the other hand, the unfractured parts of the crust are clearly expressed as high-velocity and low Vp/Vs ratio structures such as Antirrion and Nafpaktos basins in the northern coasts of Patraikos and Western Corinth Gulf respectively.…”