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.

岩浆侵入通常会引起周围岩石的地震活动和变形，并经常导致喷发。2015年8月15日，樱岛火山下方发生了数小时之内与堤防快速入侵相关的火山构造（VT）地震群。我们确定了VT地震群的震源和震源机制。还获得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轴和T轴分布的时空变化。震源分布在JST的0.3–1 km和7：00–10：30深度处，并且在地震活动最大的0.3–3 km和10：30–12：00深度处。在12：00–24：00，震源分别分布在0.2-1 km和1.5-3.5 km的浅层和深层。堤坝引起地面快速变形，位于浅层和深层之间。堤坝的走向和张开方向分别平行于NE–SW和NW–SE方向，分别对应于区域的最大和最小压缩应力。浅簇的T轴平行于堤坝的打开方向分布。深团簇的P轴表现出与NE–SW方向相对应的模式，而T轴则在NW–SE方向上分布。相反，在12：00–24：00的深层团簇中也观察到走滑断层的旋转角度为90°，其中P轴沿NW-SE方向分布，而T轴沿NW-SE方向分布。 NE–SW方向。这反映了在地震发生期间由于堤坝膨胀而引起的应力场变化，并且表明由于堤坝膨胀和VT地震引起的堤坝附近应力的变化是由超过岩石脆性断裂强度的微分应力引起的。未来在火山中的地震和变形观测将证实本研究显示的震源和震源机制的时空变化是否是快速堤防入侵的独特特征。