Change in stress field after the 2011 great Tohoku-Oki earthquake

Abstract

Temporal change in the stress field after the 2011 great Tohoku-Oki earthquake was observed by using stress tensor inversions of earthquakes near the source region. In the upper plate (hanging wall) before the Tohoku-Oki earthquake, the maximum compressive stress (σ1) axis was oriented in the direction of plate convergence, specifically, in the direction of an estimated large near-trench slip area. The stress field in the upper plate was completely transformed after the earthquake, with the minimum compressive stress (σ3) axis being oriented in the direction of the large near-trench slip area, even near the trench axis. In the near-coast area within the Pacific plate (i.e., in the footwall), the σ1 axis was oriented in the direction of plate convergence prior to the earthquake, as within the upper plate; however, the orientations of the principal stress axes in this area showed no change after the earthquake. After the earthquake, the σ3 axis was oriented in the plate-convergence direction in areas within the Pacific plate below the central to near-trench portions of the mainshock rupture area. These observations provide evidence for large near-trench slip at very shallow depths during the mainshock rupture. Moreover, the trend of the σ1 axis in the upper plate before the Tohoku-Oki earthquake (i.e., in the direction of an estimated large near-trench slip area) strongly suggests that strain accumulation is possible even in the near-trench area at very shallow depths. The observed temporal change in the stress field of the upper plate associated with the Tohoku-Oki earthquake indicates differential stress magnitudes as small as 5–15MPa.