Abstract
We predict, with a model (earthquake stress model) that inverts the displacements documented at 163 GNSS onshore stations of the GEONET, the change of shear and normal stresses on the megathrust near the Japan Trench over the seven years before the 2011 Mw 9.0 Tohoku-Oki earthquake. We find three areas on the megathrust with greater accumulations of shear and normal stresses before the earthquake, which match the ruptured areas of the mainshock and two largest aftershocks (Mw 7.8 and 7.4) that occurred within half an hour after the mainshock. We also find that the change of normal stress on the fault before the earthquake is not uniform but increases in the up-dip portion (shallower depth) of the fault from the hypocenter and decreases in the down-dip portion. We infer that the occurrence of the giant earthquake at the shallow portion of the megathrust may be attributed to the increase of the normal stress there, which leads to an increase of fault shear strength and allows more elastic strain energy to accumulate to prepare for the next big earthquake. Based on these results we propose a new concept of the seismogenic asperity as the area of greater accumulations of shear and normal stresses. The method presented here may be useful for predicting the rupture zone of future large earthquakes.
Highlights
Many attempts have been made to identify the location of the seismogenic zones of oncoming earthquakes
We infer that the tendency for giant earthquakes to include at the shallow portion of megathrusts may be attributed to the increase of the normal stress there, which leads to an increase of fault shear strength and allows more elastic strain energy to accumulate in the shallow portion of the megathrust and to prepare for the big earthquake
We determined the interseismic changes of the normal stress on the fault; it increases in the up-dip portion from the hypocenter and decreases in the down-dip portion
Summary
Many attempts have been made to identify the location of the seismogenic zones of oncoming earthquakes. The systematic mismatch of the model-predicted locked asperities and the actual rupture zone of the 2011 earthquake may be due to the shortcomings of the kinematic models used in the inversion for the fault slip-deficit; www.nature.com/scientificreports/ These may include the lack of consideration of the effect of normal stress on the fault, topography, heterogeneity and anisotropy of the earth media[10]. Using the simulated change of normal stress on the megathrust, we explain why the Tohoku earthquake occurred at the shallow portion of the megathrust and why the predicted locking areas by the back-slip models missed the rupture area of the mainshock Based on these results we propose a new concept of the seismogenic asperity as the area of greater accumulations of shear and normal stresses. The method used in this study may be useful for locating the potential rupture zones of future large earthquakes
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