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

Junichi Nakajima, Akira Hasegawa, Shigeki Horiuchi, Kazuo Yoshimoto, Takeyoshi Yoshida, Norihito Umino

Research output: Contribution to journalArticlepeer-review

17 Citations (Scopus)


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.

Original languageEnglish
Pages (from-to)843-853
Number of pages11
Journalearth, planets and space
Issue number7
Publication statusPublished - 2006
Externally publishedYes


  • Active fault
  • Caldera
  • Fluids
  • Magma
  • Nagamachi-Rifu fault
  • Northeastern Japan
  • Seismic velocity structure

ASJC Scopus subject areas

  • Geology
  • Space and Planetary Science


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