Low-frequency Earthquakes and 3-D Velocity Structure in the Bungo Channel and Shikoku Area, Japan

We used a tomographic inversion to determine P and S wave structures in the western Shikoku and Bungo Channel region, for comparison with locations of low-frequency earthquakes. The velocity model clearly images the high velocity subducting slab and we can see the spatial relation to the low-frequency earthquakes. Under western Shikoku the low-frequency earthquakes occur at depths close to the plate interface, but under the adjacent Bungo channel region to the west, there is a clear depth separation. Our interpretation is that the low- frequency events are occurring in a region of high Vp/Vs that is located above the subducting slab. We used local P and S phases to determine the three-dimensional velocity structure for the region of Bungo channel and western Shikoku. We used the program SIMULPS12 (Thurber and Eberhart-Phillips, 1993) which provide a local earthquake algorithm inversion to calculate the 3-D Vp and Vp/Vs structures. The iterative damped least-squares algorithm simultaneously calculates the velocity model and hypocentral adjustments. The data were P and S arrival times compiled by the Japan Meteorological Agency (JMA) for 1998 to 2003. 2981 events with 655, 503 arrival times from 208 stations in the area were used for inversion. Travel time data of the low-frequency earthquakes (LFE) from the JMA catalogue were used for July 2000 to December 2003 to relocate the events. Because of relatively large uncertainties in the arrival time of the LFE, these data were not included in the inversion, but were relocated using the 3-D structure determined from the ordinary earthquakes. Our tomographic inversion for the P and S wave structure for the subduction zone in the western Shikoku and the Bungo channel clearly identify the subducting slab from local velocity increases of about 10%. The associated seismicity of ordinary earthquakes appears to be located within the slab. In contrast, the depths of the low-frequency events are 5 to 20 km shallower than the ordinary earthquakes with the difference increasing toward the west. In the Bungo channel area, the LFE events are located significantly shallower than the imaged slab and we conclude that these events are occurring in a region of high Vp/Vs that is located above the subducting