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  • Letter
  • Open Access

Preparatory process reflected in seismicity-pattern change preceding the M=7 earthquakes off Miyagi prefecture, Japan

Earth, Planets and Space200758:BF03352665

  • Received: 16 December 2005
  • Accepted: 6 May 2006
  • Published:


Two M=7 interplate earthquakes recently occurred off Miyagi prefecture, northeastern Japan. These belong to a new sequence of earthquake series. The occurrence of this sequence was forewarned by the government, based primarily on a statistical approach utilizing historical records. Such a process, however, does not account for the present on-going status in the assessment. During the stage when stress approaches a critical level, there is a possibility of stress redistribution caused by a quasi-static slip, which will be reflected in a temporal change in seismic activity. Delineating a spatial map of the seismic-activity rate changes in the seismogenic zone off Miyagi prefecture, we found that there was an activation of microseismicity in the rupture zones several months prior to both M=7 earthquakes. This was interpreted as evidence of a preparatory process preceding each M=7 earthquake, during which a quasi-static slip progressed, driving stress redistribution and resulting in stress concentration on asperities of these earthquakes.

Key words

  • Microseismicity
  • stress redistribution
  • asperity
  • preparatory process