Structural heterogeneity and its implications for the low frequency tremors in Southwest Japan

To better understand the generation of low frequency tremors (LFTs) that occurred in the Nankai subduction zone, detailed 3-D seismic velocity (Vp and Vs) and Poisson's ratio (σ) structures are determined by inverting 181,576 P-wave and 159,517 S-wave arrival time data from 7572 earthquakes. We also estimated the crack density (ε), saturation rate (ζ) and porosity (ψ) structures along the LFT belt using the inverted velocity data. Our results provide geophysical evidence for that the long-duration tremors are closely related to the dehydration reactions of the subducting Philippine Sea slab and in the lowermost crust. Low-Vp, low-Vs, high-σ, high-ε, high-ζ and high porosity anomalies are imaged at depths of 25-45 km along the LFT belt, which may indicate the existence of fluids liberated by the dehydration of the descending slab and/or by the metamorphism in the lowermost crust. The interpretation accords well with the behavior of the subducted hydrous minerals which are expected to dehydrate at the temperature of 350-500 °C at depths of 35-50 km in Southwest Japan. Two prominent LFT gaps between Shikoku and Kii peninsula and between Kii peninsula and Tokai along the tremor belt are imaged with low-σ and low-ζ anomalies due to the lack of dehydration reactions within those gaps. We conclude that the LFTs are caused by the dehydrated fluids migrating in the faults and/or by crack opening, closing and extending in the corner of mantle wedge along the subducting Philippine Sea slab, possibly due to the high pore fluid-pressure and high crack-density under the forearc region of the Nankai subduction zone.