An image of a magma intrusion process inferred from precise hypocentral migrations of the earthquake swarm east of the Izu peninsula
An image of a magma intrusion process inferred from precise hypocentral migrations of the earthquake swarm east of the Izu peninsula
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DOI：
10.1046/j.1365-246x.2003.01892.x
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发表时间：
2003-04-01
影响因子：
2.8
通讯作者：
Morita, Y
中科院分区：
文献类型：
--
作者：
Hayashi, Y;Morita, Y
日本中部伊豆半岛以东地区的地震群在经历了40年的平静之后，自1978年以来一直活跃且间歇性地重复发生。这些活动总是伴随着地壳变形，岩脉侵入很好地模拟了地壳变形。为了研究堤坝侵入的过程，本文利用波形相似性获得了1998年发生的最新活动的精确震源，从而成功地获取了多座火山的火山过程新图像。重新定位的震源主要分布在深度为3～7公里的圆形薄垂直面上，中心存在地震区。平面法线方向与震源区周围构造拉应力方向吻合较好，与理论模型吻合较好。在活动开始时，一小部分事件发生在更深的地方，震源在垂直线上对齐并以 1 km h(-1) 的速率向上迁移。该阶段的运移速率与浮力驱动堤坝扩展的理论模型非常吻合。 1天后，地震区周围开始发生地震，震源分布在上述薄圆平面上。在此活动期间，震源似乎从中心向下和向上迁移。地震是由堤坝顶端的剪切断裂引起的，震源的迁移反映了堤坝从位于群中心的中性浮力层向下和向上扩展。本文还提出了基于震源精确迁移的岩浆侵入过程，并表明1998年的活动是由几个简单的火山过程组成：岩浆受浮力上升，停留在中性浮力点，并在内部过剩岩浆压力的作用下以弹性断裂方式从中性点向外扩展。
Earthquake swarms in the area east of the Izu Peninsula, Central Japan have been active and have been repeated intermittently since 1978 after 40 years of quiescence. The activities were always accompanied by crustal deformations, which were well modelled by dyke intrusions. To study the process of a dyke intrusion, precise hypocentres of the latest activity occurring in 1998 are obtained in this paper using waveform similarity, by which new images of volcanic processes have been successfully acquired at several volcanoes. The relocated hypocentres are mainly aligned on a thin vertical plane with a circular shape at a depth of 3-7 km, and there is an aseismic area at the centre. The normal direction to the plane coincides well with the direction of tectonic extensional stress around the hypocentral area and matches theoretical models well. At the beginning of the activity, a small fraction of the events occurred at greater depth, where hypocentres align on a vertical line and migrate upward at a rate of 1 km h(-1). The migration rate in this stage agrees well with a theoretical model of buoyancy-driven dyke propagation. After 1 day from the beginning, earthquakes began to occur around the aseismic area, and hypocentres spread on the thin circular plane described above. During this activity, the hypocentres seemed to migrate downwards and upwards from the centre. The earthquakes are caused by shear fracture at the tip of the dyke and the migration of hypocentres reflects that the dyke expands downward and upward from the neutrally buoyant layer that is located at the centre of the swarm. In this paper, we also propose a process of magma intrusion based on the precise migration of hypocentres, and show that the 1998 activity is composed of a few simple volcanic processes: magma rising by buoyancy, it staying at a neutral buoyancy point and spreading outward from the neutral point with elastic fracture by an inside excess magma pressure.