  We investigated changes in the b value of the Gutenberg-Richter's law 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 off the Pacific coast
of Miyagi prefecture, northeastern Japan. We showed that the b value in these
focal areas had been noticeably small, especially within a few years before the
occurrence of the M6.9 earthquake in its vicinity, indicating that differential
stress had been high in the focal areas. The coseismic slip of the 2011 Tohoku
earthquake seems to have stopped just short of the east side of the focus of
the M6.9 earthquake. Furthermore, the afterslip of the 2011 Tohoku earthquake
was 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 M6.9
earthquake was situated close to the border point where the interplate slip in
the period from 2012 through 2021 has been considerably larger on the northern
side than on the southern side. The high-stress state inferred by the b-value
analysis is concordant with those characteristics of interplate slip events. We
found that the M6.8 earthquake on May 1 occurred near an area where the b value
remained small, even after the M6.9 quake. The ruptured areas by the two
earthquakes now seem to almost coincide with the small-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 was consistently
large, while the seismicity enhanced by the two earthquakes also shows a large
b value, implying that stress in the region has not been very high.
