Rock stress was measured in situ in the Kamaishi mine in Iwate Prefecture to quantify the variation in the shallow crustal stress field associated with the 2011 Tohoku-oki earthquake. Stress was measured by the Compact Conical-ended Borehole Overcoring (CCBO) technique, which is one of the suggested methods of the ISRM. The measurement period was from 1991 to 2014. The main results can be summarized as follows.
The direction of the maximum principal stress did not change much between before and after the earthquake. However, the directions of the intermediate principal stress and the minimum principal stress at one year after the earthquake were different than those before the earthquake; i.e., the intermediate principal stress was in the East-West direction horizontally, while the minimum principal stress was in the vertical direction. This is a type of reverse fault stress, and was maintained for three years after the earthquake.
The magnitudes of the principal stresses and the vertical stress at one year after the earthquake were more than double those before the earthquake. However, the magnitudes of the maximum principal stress at two year after the earthquake were less than those at one year after the earthquake. Furthermore, the magnitudes of the intermediate principal stress and the minimum principal stress at two years after the earthquake were almost the same as those before the earthquake, and the vertical stress at two years after the earthquake was almost the same as the overburden pressure. Moreover the magnitudes of the principal stresses at three year after the earthquake was almost same as those before the earthquake. These findings suggested that the changes in the magnitudes of stresses were caused by upheaval (the velocity of upheaval in particular) of the Earth's crust after the earthquake.
The ratio of shear stress to the mean stress μm was dramatically decreased at one year after the earthquake. In addition, the value of μm showed an increasing trend after the earthquake. It is possible that the shear stress had an increasing trend and returned to its previous level following the earthquake.
