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We investigated changes in the b value of the Gutenberg-Richter's law in andaround the focal areas of earthquakes on March 20 and on May 1, 2021, withmagnitude (M) 6.9 and 6.8, respectively, which occurred off the Pacific coastof Miyagi prefecture, northeastern Japan. We showed that the b value in thesefocal areas had been noticeably small, especially within a few years before theoccurrence of the M6.9 earthquake in its vicinity, indicating that differentialstress had been high in the focal areas. The coseismic slip of the 2011 Tohokuearthquake seems to have stopped just short of the east side of the focus ofthe M6.9 earthquake. Furthermore, the afterslip of the 2011 Tohoku earthquakewas relatively small in the focal areas of the M6.9 and M6.8 earthquakes,compared to the surrounding regions. In addition, the focus of the M6.9earthquake was situated close to the border point where the interplate slip inthe period from 2012 through 2021 has been considerably larger on the northernside than on the southern side. The high-stress state inferred by the b-valueanalysis is concordant with those characteristics of interplate slip events. Wefound that the M6.8 earthquake on May 1 occurred near an area where the b valueremained small, even after the M6.9 quake. The ruptured areas by the twoearthquakes now seem to almost coincide with the small-b-value region that hadexisted before their occurrence. The b value on the east side of the focalareas of the M6.9 and M6.8 earthquakes which corresponds to the eastern part ofthe source region of the 1978 off-Miyagi prefecture earthquake was consistentlylarge, while the seismicity enhanced by the two earthquakes also shows a largeb value, implying that stress in the region has not been very high.

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