In this study, we investigated temporal variations in stress drop and b-value in the earthquake swarm that occurred at the Yamagata-Fukushima border, NE Japan, after the 2011 Tohoku-Oki earthquake. In this swarm, frictional strengths were estimated to have changed with time due to fluid diffusion. We first estimated the source spectra for 1,800 earthquakes with 2.0 ≤ MJMA < 3.0, by correcting the site-amplification and attenuation effects determined using both S waves and coda waves. We then determined corner frequency assuming the omega-square model and estimated stress drop for 1,693 earthquakes. We found that the estimated stress drops tended to have values of 1–4 MPa and that stress drops significantly changed with time. In particular, the estimated stress drops were very small at the beginning, and increased with time for ~50 days. Similar temporal changes were obtained for b-value; the b-value was very high (b ~ 2) at the beginning, and decreased with time, becoming approximately constant (b ~ 1) after ~50 days. Patterns of temporal changes in stress drop and b-value were similar to the patterns for frictional strength and earthquake occurrence rate, suggesting that the change in frictional strength due to migrating fluid not only triggered the swarm activity but also affected earthquake and seismicity characteristics. The estimated high Q−1 value, as well as the hypocenter migration, supports the presence of fluid, and its role in the generation and physical characteristics of the swarm.
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