The east coast of the Tohoku district, Japan has a high seismicity, including aftershocks of the 2011 M9 Tohoku earthquake. We analyzed 1142 earthquakes with $$4.4 \le M_{W} \le 5.0$$ 4.4 ≤ M W ≤ 5.0 that occurred in 2003 through 2018 and obtained spatio-temporal pattern of stress drop on the Pacific Plate that subducts beneath the Okhotsk Plate. Here we show that small earthquakes at edges of a region with a large slip during the 2011 Tohoku earthquake had high values of stress drop, indicating that the areas had a high frictional strength and suppressed the coseismic slip of the 2011 Tohoku earthquake. In addition, stress drops of small earthquakes in some of the areas likely decreased after the 2011 Tohoku earthquake. This indicates that the frictional strength decreased at the areas due to the following aftershocks of the 2011 Tohoku earthquake, consistent with a high aftershock activity. This also supports that the frictional properties on a subducting plate interface can be monitored by stress drops of small earthquakes, as pointed out by some previous studies.

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日本东北地区东部俯冲太平洋板块摩擦特性的时空特征从小地震应力降估计

日本东北地区东海岸地震活动频繁，包括 2011 年 M9 东北地震的余震。我们分析了 2003 年至 2018 年发生的 1142 次地震，其中 $$4.4 \le M_{W} \le 5.0$$ 4.4 ≤ MW ≤ 5.0，并获得了俯冲到鄂霍次克板块下方的太平洋板块应力降的时空模式。在这里，我们展示了在 2011 年东北地震期间大滑移区域边缘的小地震具有高应力降值，表明该区域具有高摩擦强度并抑制了 2011 年东北地震的同震滑移。此外，在 2011 年东北地震后，部分地区小地震的应力降可能会下降。这表明由于 2011 年东北地震的后续余震，该地区的摩擦强度下降，与高余震活动一致。这也支持俯冲板块界面的摩擦特性可以通过小地震的应力降来监测，正如之前的一些研究所指出的那样。