Deep low-frequency tremor in the southwest Japan subduction zone

摘要

Non-volcanic deep low-frequency tremors detected in the southwest Japan subduction zone are distributed in the forearc side along the strike of the descending Philippine Sea plate. The source depth of the tremor corresponds to the transition zone on the plate boundary or the Moho discontinuity. The time sequence of the tremor activity is characterized by the long duration from hours to weeks and the mobility and the occurrence of the tremor is related to the existence of fluid liberated from the slab by dehydration process. The spatial distribution of the tremor is not homogeneous in the narrow belt but spatially clustered. The major activity of the tremor with relatively long time duration 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, the tremor is sometimes triggered by local earthquakes or teleseismic waves. The periodicity of the tremor activity represents the stable accumulation of fluid with the stable subduction process and the triggering phenomena might be reflected the unstable condition of the occurrence of tremors.