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  • Article
  • Open Access

The seismicity, fault structures, and stress field in the seismic gap adjacent to the 2004 Mid-Niigata earthquake inferred from seismological observations

Earth, Planets and Space200658:BF03351988

https://doi.org/10.1186/BF03351988

  • Received: 22 July 2005
  • Accepted: 17 February 2006
  • Published:

Abstract

We investigate the seismicity, fault structures, and stress field in the southern seismic gap adjacent to the source region of the 2004 Mid-Niigata earthquake based on precise earthquake locations and focal mechanism solutions. The double-difference earthquake locations reveal detailed fault structures including five discrete clusters. A stress tensor inversion method indicates that, at the southern ends of the Muikamachi and the Tokamachi faults, the stress state changes from thrust faulting to a strike-slip faulting regime. We show two possibilities for the deep-fault geometry of the southern part of the Muikamachi fault. Our preferred fault geometries of the Muikamachi fault differ from that of the mainshock, especially for the geometry of the dip direction. It is likely that the Muikamachi fault is divided into two segments whose boundary corresponds to the southern end of the source region of the 2004 event, and that the rupture area was terminated due to this fault segment boundary. The Coulomb failure stresses that are induced by the mainshock and the largest aftershock increase at the southern part of the Muikamachi faults. Furthermore, the angle between the maximum principal stress and the preferred fault planes implies that the shear stress resolving along the planes are large. This evidence suggests a high seismic risk in the seismic gap.

Key words

  • The 2004 Mid-Niigata earthquake
  • seismicity
  • fault structure
  • stress field
  • seismic gap
  • temporary seismic observation
  • microearthquake

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