Postseismic response of repeating earthquakes around the 2011 Tohoku‐oki earthquake: Moment increases due to the fast loading rate

Abstract

Abstract We examined the temporal variation of the size of repeating earthquakes related to the 2011 Tohoku‐oki earthquake ( M 9.0) in the northeastern Japan subduction zone for the period from July 1984 to the end of 2011. The repeaters ( M 2.5–6.1) show postseismic magnitude increases for most sequences located in the area of large postseismic slip at the downdip extension of the M 9 source region. The magnitudes of the first events after the M 9 earthquake increased by an average of about 0.3 for sequences having three or more earthquakes over the 9 months following it. We also examined the slip area in detail for Kamaishi repeaters whose magnitudes had been M 4.9 ± 0.2 but which increased by about 1 after the M 9 earthquake. Waveform modeling shows that the slip area for the post‐ M 9 Kamaishi earthquakes overlaps with that before the Tohoku‐oki earthquake but enlarged by about 6 times. Until the occurrence time of the last event (September 2011) in the analysis period, the rupture area remained larger than before but appeared to shrink over time. The enlargement of the rupture area suggests that an aseismic‐to‐seismic transition occurred in the region surrounding the pre‐ M 9 repeaters and is most likely related to fast loading of the repeaters due to rapid postseismic slip estimated to have occurred in the area. The existence of conditionally stable regions around the repeating earthquakes and/or patches slightly larger than the earthquake nucleation sizes may explain such behavior. The temporal change of loading rate is an important factor in determining earthquake size in this case.