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AGU: Geophysical Research Letters

 

Keywords

  • double seismic zone
  • dehydration embrittlement
  • seismicity

Index Terms

  • Seismology: Oceanic crust
  • Seismology: Seismicity and tectonics
  • Seismology: Subduction zones
  • Tectonophysics: Dynamics of lithosphere and mantle: general
  • Tectonophysics: Rheology: crust and lithosphere

Abstract

GEOPHYSICAL RESEARCH LETTERS, VOL. 33, L24310, 5 PP., 2006
doi:10.1029/2006GL028239

Existence of a seismic belt in the upper plane of the double seismic zone extending in the along-arc direction at depths of 70–100 km beneath NE Japan

Saeko Kita

Research Center for Prediction of Earthquakes and Volcanic Eruptions, Tohoku University, Sendai, Japan

Tomomi Okada

Research Center for Prediction of Earthquakes and Volcanic Eruptions, Tohoku University, Sendai, Japan

Junichi Nakajima

Research Center for Prediction of Earthquakes and Volcanic Eruptions, Tohoku University, Sendai, Japan

Toru Matsuzawa

Research Center for Prediction of Earthquakes and Volcanic Eruptions, Tohoku University, Sendai, Japan

Akira Hasegawa

Research Center for Prediction of Earthquakes and Volcanic Eruptions, Tohoku University, Sendai, Japan

We relocated microearthquakes using data obtained via a dense seismic network and systematically detected the characteristic distribution of the upper seismic plane seismicity within the Pacific slab beneath NE Japan. We found a seismic “belt” parallel to the iso-depth contour of the plate interface that is beneath the forearc area at depths of 70–100 km, indicating that the distribution of the upper plane earthquakes is non-uniform. The location of the deeper limits of this belt and seismicity of the upper seismic plane appear to correspond respectively to two facies boundaries where H2O contents change in the slab crust. Events in the upper seismic plane have mainly down-dip compression-type focal mechanisms but several events have normal fault-type (NF-type) versions, whose spatial distribution appears to correspond to these boundaries. These NF events might be induced by the tensional stress field that is caused by volume reduction due to dehydration reactions.

Received 26 September 2006; accepted 14 November 2006; published 23 December 2006.

Citation: Kita, S., T. Okada, J. Nakajima, T. Matsuzawa, and A. Hasegawa (2006), Existence of a seismic belt in the upper plane of the double seismic zone extending in the along-arc direction at depths of 70–100 km beneath NE Japan, Geophys. Res. Lett., 33, L24310, doi:10.1029/2006GL028239.

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