Changes in the b value in and around the focal areas of the M6.9 and M6.8 earthquakes at off-Miyagi prefecture, Japan, in 2021

We investigated changes in the b value in and around the focal areas of earthquakes on March 20 and on May 1, 2021, with magnitude (M) 6.9 and 6.8, respectively, which occurred on the Pacific coast of Miyagi prefecture, northeastern Japan. We reveal that the b value in these focal areas had been noticeably low, especially within a few years before the occurrence of the M6.9 earthquake in its vicinity, indicating that the focal areas had been highly stressed. The high-stress state inferred by the b-value analysis is concordant with the characteristics of notable interplate slip events, which the co-seismic slip of the 2011 Tohoku earthquake stopped just short of the east side of the focal areas and the afterslip of the earthquake was relatively small in these areas compared to the surrounding regions. These events suggest that the slip deficit that accumulated in the focal areas had remained. We find that the M6.8 earthquake on May 1 occurred near an area where the b value remained low even after the M6.9 quake. The ruptured areas by the two earthquakes now seem to almost coincide with the low-b-value region that had existed before their occurrence. The b value on the east side of the focal areas of the M6.9 and M6.8 earthquakes which corresponds to the eastern part of the source region of the 1978 off-Miyagi prefecture earthquake had been consistently high, while the seismicity enhanced by the two earthquakes also shows a high b value, implying that the stress in the region has not been very high.