Temporal Changes in Stress Drop, Frictional Strength, and Earthquake Size Distribution in the 2011 Yamagata‐Fukushima, NE Japan, Earthquake Swarm, Caused by Fluid Migration
Temporal Changes in Stress Drop, Frictional Strength, and Earthquake Size Distribution in the 2011 Yamagata‐Fukushima, NE Japan, Earthquake Swarm, Caused by Fluid Migration
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DOI：
10.1002/2017jb014334
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发表时间：
2017-12
期刊：
影响因子：
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通讯作者：
Keisuke Yoshida;Tatsuhiko Saito;Y. Urata;Y. Asano;A. Hasegawa
中科院分区：
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作者：
Keisuke Yoshida;Tatsuhiko Saito;Y. Urata;Y. Asano;A. Hasegawa
在本研究中，我们调查了2011年东北冲大地震后发生在日本东北部山形-福岛边界的震群中应力降和B值的时间变化。在这个群中，摩擦强度估计已随着时间的推移，由于流体扩散。我们首先估计了2.0 ≤ MJMA < 3.0的1，800个地震的震源谱，通过校正使用S波和尾波确定的场地放大和衰减效应。然后我们确定了假设Ω平方模型的拐角频率，并估计了1,693次地震的应力降。我们发现，估计的应力降往往具有1-4 MPa的值，并且应力降随时间显著变化。特别是，估计的应力下降在开始时非常小，并随着时间的推移增加约50天。B-值也出现了类似的时间变化; B-值在开始时非常高（B ~ 2），并随着时间的推移而下降，在约50天后变得近似恒定（B ~ 1）。应力降和B-值的时间变化模式与摩擦强度和地震发生率的时间变化模式相似，表明流体迁移引起的摩擦强度变化不仅触发了震群活动，而且影响了地震和地震活动特征。估计的高Q−1值，以及震源迁移，支持流体的存在，以及它在群的生成和物理特性中的作用。
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.