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.
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 [1]. 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. Therefore, it is important that we clarify the change in in-situ stress between before and after the earthquake.
The measurement location is the Kamaishi mine in northeast Japan (Fig. 1). The Kamaishi mine is located about 170 km northwest of the earthquake epicenter. In this study, we selected a measurement station that is about 5 km from the mine opening of the 550 mL site at the Kamaishi mine. The overburden at this measurement station is about 290 m. This measurement station is not affected by galleries or the goaf cavern in this mine. In addition, several stress measurements have been performed at the 550 mL site before the Tohoku-oki earthquake [2–6]. In-situ stress measurements were performed using the Compact Conical-ended Borehole Overcoring (CCBO) technique [7]. The CCBO technique is the one of the suggested methods of the ISRM [5].