Seismic velocity, attenuation, and thermal structures in the Shikoku and Kyushu area of Japan: Relationship to low-frequency earthquakes

Andri Nugraha

We have used a high resolution tomography inversion to determine P-and S-wave velocity structures in the western Shikoku and Bungo channel regions to investigate the subduction zone structure and the source region of low-frequency earthquakes (LFEs). The velocity models clearly images the high velocity subducting Philippine Sea slab and other features in the western Shikoku and Kyushu regions.

We also carried out a high resolution Q-tomography inversion for this region. We carefully analyzed high dynamic range waveform spectra from Hi-net high sensitivity borehole seismic stations to determine t* values for both P-and S-waves from small earthquakes. The t* values were used to invert for a 3-D frequency-independent attenuation structures. The obtained attenuation structure clearly shows the sharp contrast between the subducting slab with relatively a low attenuation (Qp and Qs of ~450 - 850), and the mantle region above the slab. We extended our investigation of the subduction zone structures by estimating the thermal structure. We applied the experimental results about the relation between temperature and to our attenuation model, using the high-temperature background method. The obtained thermal structures clearly exhibit low temperature features of the subducting slab (6600C - 6800C) compared with the hotter region just above the slab (6800C - 7400C). Our study is a new result for the estimation attenuation and thermal structures around source region of the LFEs in western Shikoku and Bungo channel regions.

We relocated the LFEs using a combination of waveform cross-correlation and the 3-D velocity structure. Under western Shikoku, the LFEs occur at depths close to the plate interface, but under the Bungo channel area the LFEs are located significantly shallower than the imaged slab and we conclude that the LFEs are occurring in a region of high Vp/Vs (~1.84 - 1.88) that is located above the slab. For the high Vp/Vs region of the LFEs, there are large changes in the Vs than in Vp, so is relatively high (~1.8). This moderately high value implies conditions that are not texturally equilibrated and suggest that fluid movements may be a cause of the LFEs. The LFEs are occurring in a region of high attenuation (Qp and Qs of ~200 - 250) close to and above the slab under western Shikoku and Bungo channel area. Correspondingly in our temperature model, the LFEs are occurring in a region of moderate high temperature (6800C to 7000C) above the subducting slab.