The 2011 Mega-thrust earthquake off Northeast Japan and multiple earthquake cycles in subduction zones
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Earthquake occurrence is the sudden release of tectonically accumulated stress by faulting. In the case of interplate earthquakes, the stress accumulation is caused by the interseismic gradual increase of slip deficits in potential source regions, and so earthquake occurrence can be regarded as the sudden recovery of the slip deficits. Since the crustal deformation due to interseismic slip-deficit increase is detectable by GPS observations, as well as that due to coseismic slip-deficit recovery, we can now monitor the slip-deficit and -recovery processes at plate interfaces through the inversion analysis of GPS array data. On March 11th of 2011, an Mw 9.0 mega-thrust earthquake occurred at the North American–Pacific plate interface off the Tohoku region, Japan. The inversion analysis of GPS data for an interseismic period (June 1996–May 2000) prior to this event has shown that five remarkable slip-deficit zones are distributed on the plate interface along the southern Kuril–Japan trench. On the other hand, the inversion analysis of coseismic GPS data (March 10–11, 2011) shows that the fault slip of the 2011 mega-thrust earthquake has a bimodal distribution with a northern main peak of 25 m and a southern sub peak of 6 m, corresponding to the Miyagi-oki and Fukushima-oki slip-deficit zones, respectively. In the Miyagi-oki slip-deficit zone, M 7.5-class earthquakes with about 3 m of coseismic slip have repeated every 40 years over the past two centuries. The occurrence of extraordinarily large earthquake with 25 m of coseismic slip in the same slip-deficit zone suggests the possibility of scale-dependent multiple earthquake generation cycles, and leads to the conclusion that the so-called asperity is not a physical substance but a concept representing the spatial irregularity in frictional properties of faults.
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関連論文
- 観測・計算を融合した階層連結地震・津波災害予測システム
- The 2011 Mega-thrust earthquake off Northeast Japan and multiple earthquake cycles in subduction zones