Open Access

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

  • Junichi Nakajima16Email author,
  • Akira Hasegawa16,
  • Shigeki Horiuchi26,
  • Kazuo Yoshimoto36,
  • Takeyoshi Yoshida46 and
  • Norihito Umino16
Earth, Planets and Space200658:BF03351989

Received: 4 November 2004

Accepted: 15 March 2006

Published: 26 July 2006


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 three-dimensional 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 H2O-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.

Key words

seismic velocity structurenortheastern JapanNagamachi-Rifu faultfluidsmagmacalderaactive fault