Stress before and after the 2011 great Tohoku‐oki earthquake and induced earthquakes in inland areas of eastern Japan

Abstract

Stress fields in inland areas of eastern Japan before and after the Tohoku‐oki earthquake were estimated by inverting focal mechanism data. Before the earthquake,σ1axis was oriented EW in Tohoku but NW‐SE in Kanto‐Chubu. The stress fields changed after the earthquake in northern Tohoku and in southeastern Tohoku near Iwaki city, where the orientations of the principal stresses became approximately the same as the orientations of the static stress change associated with the earthquake. This indicates that differential stress magnitudes in these areas before the earthquake were smaller than 1 MPa. The stress field did not change in central Tohoku, even though the stresses loaded after the earthquake had nearly reversed orientations, which indicates that the differential stress magnitudes there were significantly larger than 1 MPa. In Kanto‐Chubu, stresses having nearly the same orientations as the background stresses were loaded after the earthquake, and the stress fields did not change as expected. This may have caused very high induced seismicities in Kanto‐Chubu.