Dynamic lithospheric response to megathrust and precursory seismicity features of megathrust

Temporal variations of seismic properties in inner trench regions before and after three megathrusts with magnitudes greater than or equal to 8.8 since 2004 are investigated to understand the nature of megathrust earthquakes. The seismicity was increased significantly, and the fault-type compositions changed after megathrusts. The seismicity displays characteristic fault-type-dependent distribution on rupture planes. The postseismic thrustal events were populated around the down-dip rupture margins due to the concentration of shear stress after coseismic ruptures. Normal-faulting earthquakes increased after the megathrusts particularly in shallow-depth regions with large slips, which may be associated with lithospheric rebound and development of splay faults. The earthquake occurrence rate (b value) displays a characteristic slip-dependent feature. The earthquake occurrence rates decrease with the slip amount of forthcoming megathrust, which may be caused by continuous accumulation of plate-driven stress and tectonic loading around the future rupture planes on the slab surface. The slip dependency of earthquake occurrence rates is enhanced with time until the occurrence of megathrust. The level of seismicity after megathrust is inversely proportional to that before megathrust, yielding comparable average seismicity over the rupture zone. It was also observed that the dynamic lithospheric response is highly correlated with slip distribution on the rupture plane. The tension axes of the normal-faulting earthquakes for 100 days after the Tohoku-Oki earthquake focus to an apparent pole, suggesting a radial viscoelastic deformation of lithosphere. The temporal changes of slip-dependent b values fit well with an exponential function, suggesting an exponential increase of normal stress on the locked region with time until the occurrence of the megathrust. This observation suggests that the temporal variation of slip-dependent b values may be useful for prediction of forthcoming megathrusts at least several tens years beforehand.