Dike Inflation Process Beneath Sakurajima Volcano, Japan, During the Earthquake Swarm of August 15, 2015
Dike Inflation Process Beneath Sakurajima Volcano, Japan, During the Earthquake Swarm of August 15, 2015
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DOI：
10.3389/feart.2020.600223
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发表时间：
2021-02
期刊：
影响因子：
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通讯作者：
M. Koike;H. Nakamichi
中科院分区：
文献类型：
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作者：
M. Koike;H. Nakamichi
岩浆侵入通常会引起周围岩石的地震活动和变形，并经常导致火山爆发。2015年8月15日，樱岛火山下方发生了一系列火山构造（VT）地震，这些地震与数小时内的快速岩脉侵入有关。确定了VT震群的震源和震源机制。还得到了机构方位角的压力（P）和张力（T）轴的分布。结果表明，震源和P轴、T轴的分布具有时空变化特征。震源深度分布在0.3-1 km和7：00-10：30 JST，震源深度分布在0.3-3 km和10：30-12：00 JST。12：00-24：00，震源分别在0.2-1 km和1.5- 3.5km处呈浅、深簇状分布。堤坝引起了快速的地面变形，并位于浅，深集群之间。岩墙走向和张开方向分别平行于NE-SW和NW-SE方向，对应区域最大和最小压应力。浅部岩丛的T轴平行于岩墙的开口方向分布。深星系团的P轴呈NE-SW方向，T轴呈NW-SE方向。而在12：00-24：00，在深群也观察到了90°旋转的走滑断层样式，其P轴为NW-SE向，T轴为NE-SW向。这反映了在地震发生过程中由于堤坝膨胀而引起的应力场的变化，并表明由于堤坝膨胀和VT地震而引起的堤坝附近的应力变化是由超过岩石脆性断裂强度的差应力引起的。未来的火山地震和变形观测将验证本研究所示的震源和震源机制的时空变化是否是快速岩脉侵入的独特特征。
Magma intrusion usually causes seismicity and deformation in the surrounding rock and often leads to eruptions. A swarm of volcano-tectonic (VT) earthquakes associated with rapid dike intrusion in hours occurred beneath Sakurajima volcano on August 15, 2015. We determined the hypocenters and focal mechanisms of the VT earthquake swarm. The distributions of pressure (P)- and tension (T)-axes of the azimuths of the mechanisms are also obtained. The results indicate spatiotemporal changes of the distributions of the hypocenters and P- and T-axes. The hypocenters are distributed at depths of 0.3–1 km and 7:00–10:30 JST, and are located at depths of 0.3–3 km and 10:30–12:00 during which the seismic activity is the largest. At 12:00–24:00, the hypocenters are distributed in shallow and deep clusters at depths of 0.2–1 km and 1.5–3.5 km, respectively. The dike induced rapid ground deformation and is located between the shallow and deep clusters. The strike and opening directions of the dike are parallel to the NE–SW and NW–SE directions, respectively, corresponding to the regional maximum and minimum compression stress. The T-axes of the shallow cluster are distributed parallel to the opening direction of the dike. The P-axes of the deep cluster exhibit a pattern corresponding to the NE–SW direction and the T-axes are distributed in the NW–SE direction. In contrast, a 90° rotated pattern of strike-slip faulting is also observed at the deep cluster at 12:00–24:00, where the P-axes are distributed in the NW–SE direction and the T-axes are distributed in the NE–SW direction. This reflects the change in the stress field due to the dike inflation during the earthquake generation, and indicates that the alteration of stress in the vicinity of the dike due to the dike inflation and VT earthquakes are induced by the differential stress exceeding the brittle fracture strength of the rock. Future seismic and deformation observations in volcanoes will verify whether the spatiotemporal changes of the hypocenters and focal mechanism shown by this study are unique features of rapid dike intrusion.