The Process by Which Intraplate Earthquakes Are Generated

I review recent progresses in Japan about the process by which intraplate earthquakes are generated, in particular about the stress accumulation process on intraplate earthquake faults. It is thought that a part of convergent rates between subducting oceanic plates and the inland plate is consumed in the intraplate region. The deformation in the intraplate region in Japan, which occurs mainly in the concentrated deformation zone (NKTZ), results from the weak zone in the lower crust that is characterized by a low viscosity region. Low velocity and high attenuation anomalies are detected in the weak zone. Anomalous stress fields are also estimated around an active fault or fault zone, which are thought to be due to the deformation in the lower crust immediately beneath the active fault or fault zone. These results suggest that intraplate earthquakes are generated by the heterogeneity in the lower crust.