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[SSS30-P14] The 3-stage earthquake generation process observed during 3 months before the 2011 Tohoku earthquake
Keywords:2011Tohoku earthquake, F-net, GNSS
Various phenomena were observed before the 2011 Tohoku earthquake. As for the broad band seismic network; F-net, its availability degraded. The 1st degradation occurred from December 22, 2010 to January 18, 2011, then 2nd one occurred from February 16 to March 2, after the part of the first degradation recovered to normal status. The main shock occurred on March 11, after the part of the second degradation returned to normal again.
Remarkable improvement of such measuring instrument as GNSS in recent years gives useful information about movement of crusts. So, seismic activities and measuring results of GNSS etc. are added to further check the degraded situation of the F-net.
It seems that the period of approximately 3 months before the earthquake was consisted of 3 stages indicated below.
Period: End of December, 2010 to End of January, 2011.
Analysis: Accumulation of strain in the continental plate reached maximum limit in the Tohoku and Chubu regions. Possibly as a result of such situation, the wide area in Japan showed vibration or slip. Then the Pacific ocean plate stopped its advancement. Since such movements are on several day-basis, this slipping is the one like Creep. They occurred in the area far from the epicenter, the epicenter have not been formed at this moment.
Period: Middle of February to early March
Analysis: As a result of the first stage, restless increase of stresses by the Pacific ocean plate can not be accepted any more. Then, small breakage was formed near the initial rupture point of the earthquake.
Period: Several days before to the day of the main shock on March 11.
Analysis: Slipping of the continental plate started, and it reached the main shock.
On March 8, eastward movement was recorded by GNSS.
The Sanriku-oki earthquake (M7.3) occurred on March 9. The earthquakes with magnitude of 6 followed.
On March 11, The main shock of the Tohoku earthquake occurred.