摘要：A cluster of earthquake activity took place beneath the Tanzawa Mountains at a depth of 20 km during the end of January 2012.The activity began at 22:39 UT on January 27 and included 78 earthquakes with magnitudes of 2.0 and greater within the span of 50 h.Five of them had magnitudes greater than 4.0, and the largest one was a M5.4 earthquake.We relocated the hypocenters by using the double-difference method and characterized their migrations away from the first earthquake of the cluster activity.The migration was consistent with fluid diffusion and had a similar speed to that of non-volcanic tremors and of induced earthquakes caused by water-injection experiments.We then analyzed stress drops for 16 earthquakes of M3.5 and greater that occurred from July 2003 to June 2012 in the area of the cluster activity.Earthquakes that occurred before and after the cluster activity had typical and stable values of stress drop.This is consistent with structural studies indicating the existence of little fluid in the region.In contrast, the cluster activity included earthquakes with significantly small stress drops.The leading hypothesis is that the cluster activity was associated with a decrease in the shear strength due to an increase in pore pressure, and this can explain both the migration of hypocenters and the small stress drops associated with the cluster activity.This hypothesis is also supported by the fact that earthquakes before and after the cluster activity had similar values of stress drop, thus suggesting that the activity was triggered by a different mechanism from the other earthquakes in the same region.The most plausible explanation is that there is a little fluid in the closed system beneath the Tanzawa Mountains that is undetectable by structural observations.
关键词：Migration of hypocenters; Stress drop; Cluster activity; Fluid; Tanzawa Mountains
