Abstract
Non-volcanic deep low-frequency tremors detected in southwest Japan are distributed in the forearc side along the strike of the descending Philippine Sea plate. The source depth of the tremor corresponds to the slab interface or the Moho discontinuity. The time sequence of the tremor activity is characterized by long durations from hours to weeks. The mobility and the successive occurrence of the tremor are thought to be related to the existence of fluid liberated from the slab by a dehydration process. The spatial distribution of the tremors is not homogeneous in a narrow belt but is spatially clustered. The major activity of the tremors with relatively long time durations is also clustered periodically, with a period of 2-3 months in the east and middle of Shikoku area and about 6 months in the west of Shikoku. On the other hand, tremors are sometimes triggered by local earthquakes or teleseismic waves. The periodicity of the tremor activity may represent a stable accumulation of fluid with a stable subduction process and the triggering phenomenon implies the unstable condition of the occurrence of tremors.
