Seismic imaging of arc magma and fluids under the central part of northeastern Japan

Junichi Nakajima, Toru Matsuzawa, Akira Hasegawa, Dapeng Zhao

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66 Citations (Scopus)

Abstract

A dense seismic network was temporarily deployed during 1997-1999 in the central part of northeastern Japan by Japanese university research groups. Using the local earthquake arrival time data recorded by the network, we have determined high-resolution three-dimensional images of P-wave velocity (Vp), S-wave velocity (Vs) and Vp/Vs ratio in this area. Our results show that low Vp, low Vs and high Vp/Vs areas are extensively distributed in the uppermost mantle along the volcanic front. In the lower crust, low Vp, low Vs and high Vp/Vs areas are visible but they are confined to the individual volcanic areas. In contrast, the upper crust of volcanic areas shows low Vp, low Vs and low Vp/Vs rather than high Vp/Vs. These observations suggest that partial melting materials exist in the uppermost mantle along the volcanic front and they spread up to the midcrust of the volcanic areas. Low Vp, low Vs and low Vp/Vs in the upper crust of the volcanic areas suggest the presence of H2O (rather than melt) there. Deep low-frequency microearthquakes, perhaps caused by the rapid movement of fluids, are located mostly at the edge of partial melting zones, particularly at their upper surface. Distinct S-wave reflectors (bright spots) with large reflection coefficients are distributed in the midcrust with slightly low Vp/Vs, suggesting that most of them are filled with H2O.

Original languageEnglish
Pages (from-to)1-17
Number of pages17
JournalTectonophysics
Volume341
Issue number1-4
DOIs
Publication statusPublished - 2001

Keywords

  • Active volcano
  • Fluids
  • Northeastern Japan
  • Seismic velocity structure
  • Travel-time tomography

ASJC Scopus subject areas

  • Geophysics
  • Earth-Surface Processes

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