Seismic research on the western (Sea of Japan) side of Japan is important for mitigating disasters caused by earthquakes and associated tsunamis because the plate boundary between the Okhotsk Plate and Amurian Plate is very close to the western coast of northern Japan. Here, we analyzed stress drops of 88 earthquakes (4.0 \(\le \text{M} \le\) 5.0) that occurred between 2003 and 2021 off the west coast of northern Japan to investigate their spatial pattern and its relationship with historical large earthquakes. Stress drops of the earthquakes ranged from 0.20 to 920 MPa and showed clear spatial heterogeneity. Stress drop values in the source area of the 1983 Central Sea of Japan earthquake were larger than those in the source area of the 1993 off the southwest coast of Hokkaido earthquake, consistent with previous studies on the source characteristics of the two earthquakes that had been estimated from Tsunami data. In addition, the region between the source areas of the two earthquakes showed the greatest stress drop values, suggesting that the high shear strength in that region prevented further rupture propagation during the two large earthquakes. Our results confirm that stress drops of small earthquakes provide important information on the frictional properties of plate boundaries.
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