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Precursory Seismic Activity Surrounding the High‐Slip Patches of the 2011 Mw 9.0 Tohoku‐Oki Earthquake

Abstract Using the Japan Meteorological Agency earthquake catalog since 1923, we investigated the seismic activity prior to the 2011 M w  9.0 Tohoku‐Oki earthquake with reference to the slip distribution, which is characterized by two high‐slip (≥20  m) patches separated by a zone of relatively low slip. The peak of the northern high‐slip patch is located near the trench while the peak of the southern high‐slip patch is situated about 40 km southeast of the mainshock epicenter, about 70 km away from the trench. It is estimated that the mainshock ruptured the southern high‐slip patch first and then extended to a larger adjacent region, including the northern high‐slip patch. The epicenters of foreshock activity that started two days before the mainshock are distributed in the western edge of the northern high‐slip patch where other prominent activity, such as the 1981 event, occurred during the past 90 years. Based on the spatiotemporal seismicity pattern around the two high‐slip patches, we infer that the foreshock activity triggered the 2011 Tohoku‐Oki earthquake because the increased stress from the foreshock activity was able to overcome the strength of the southern high‐slip patch, which had been sufficiently weakened by a series of large surrounding earthquakes since 2003. Other prominent activity, such as the 1981 event, failed to trigger such a great earthquake because similar stress condition had not been established at those times. The doughnut‐shaped seismicity pattern that formed around the peak of the southern high‐slip patch suggests the existence of an extremely strong patch that had not been ruptured by the surrounding large earthquakes for a long period of time.