In-situ rock stress measurement was carried out in the Kamaishi mine of lwate prefecture to quantify the variation of the shallow crustal stress field associated with the 2011 Tohoku-oki earthquake. Stress measurement was conducted by the Compact Conical-ended Borehole Overcoring (CCBO) technique. Measurement period is from 1991 to 2015. Main results obtained in this study can be summarized as follows. The magnitudes of the three-dimensional principal stresses and the vertical stress at one year after the earthquake were more than double those before the earthquake.
The principal stress magnitudes measured at two and three years after the earthquake decreased relative to those at one year after the earthquake.
The increasing and decreasing trends in crustal 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. The consistency between the change in measured stress and the change in seismicity in the Kamaishi regions suggests that the results of stress measurements, even those at a much shallower depth than the earthquake source fault, can be useful for understanding rupture propagation behavior.
In-situ rock stress is an important consideration in engineering fields that involve the Earth's crust, such as exploration for oil and natural gas, the extraction of geothermal energy, CCS and deep mining at depths of several km. The Tohoku-oki earthquake that occurred on March 11, 2011 was in the largest class of earthquakes. The Tohoku region experienced crustal movement of up to 5.3 m in the horizontal direction and up to 1.2 m in the vertical direction (subsidence) within a short period. This crustal disturbance with a large displacement rate is likely to have had a major impact on the crustal stress field at a relatively shallow depth; i.e., at several hundreds of meters.