Changes in In-Situ Rock Stress Before and After the Major 2011 Tohoku-Oki Earthquake

To examine the change in in-situ stress between before and after the 2011 Tohoku-oki earthquake, we performed stress measurements after the earthquake in the Kamaishi mine, located near the northern termination of the mainshock rupture. The in-situ stress measurement period was from 1991 to 2016. The results showed that the magnitudes of the three-dimensional principal stresses and the vertical stress drastically increased during the mainshock and, at one year after the earthquake, were more than double those before the earthquake. The principal stress magnitudes then decreased with time, and returned almost to the pre-earthquake levels at about five years after the earthquake. The increasing and decreasing trends in stress in the Kamaishi mine can be interpreted in terms of the effects of coseismic rupture behavior of the Tohoku-oki earthquake mainshock and the occurrence of aftershocks in the Sanriku-oki low-seismicity region (SLSR), where the Kamaishi mine is located.