3:30 PM - 4:30 PM
[J05-P-11] Seismic velocity structure in the lower crust beneath the seismic belt in the San-in district, Japan
In the San-in district in southwest Japan, a linear distribution of epicenters is seen along the Japan Sea coast. The linear distribution is called the seismic belt in the San-in district. Large earthquakes also occurred in the seismic belt. What localizes the earthquakes in the San-in district far from the plate boundary? We thought that the model proposed by Iio et al. (2002, 2004) could answer the question. The model is as follows. A part of the lower crust has low viscosity. The low viscous part is called ‘weak zone'. The stress and strain in the upper crust are concentrated right above the weak zone, and earthquakes occur there. To verify whether the weak zone exists in the lower crust beneath the seismic belt, we estimated the seismic velocity structure there by seismic travel time tomography.
We carried out the tomography with the program, FMTOMO (Rawlinson et al., 2006). FMTOMO implements wavefront tracking, which can trace rays robustly. The area of the velocity model contains San-in, Sanyo, Shikoku and a part of Kinki district. The depth of the model extends 0-81 km. We used travel times picked by Japan Meteorological Agency (JMA) for earthquakes that occurred in the study area. In addition, we used the travel times manually picked for earthquakes that occurred within the Philippine Sea Slab, because seismic waves from the earthquakes to stations in the San-in district pass through the lower crust beneath the San-in district. We expect that the data can improve the resolution in the lower crust.
The tomography revealed that the lower crust beneath the seismic belt has low velocity anomalies and might have low viscosity, since velocities of rocks decrease with temperature and/or water content. Therefore, the results of this study support the model proposed by Iio et al. (2002, 2004).
Acknowledgement: We used JMA's earthquake catalogs. We also used waveform data from permanent stations of National Research Institute for Earth Science and Disaster Resilience.
We carried out the tomography with the program, FMTOMO (Rawlinson et al., 2006). FMTOMO implements wavefront tracking, which can trace rays robustly. The area of the velocity model contains San-in, Sanyo, Shikoku and a part of Kinki district. The depth of the model extends 0-81 km. We used travel times picked by Japan Meteorological Agency (JMA) for earthquakes that occurred in the study area. In addition, we used the travel times manually picked for earthquakes that occurred within the Philippine Sea Slab, because seismic waves from the earthquakes to stations in the San-in district pass through the lower crust beneath the San-in district. We expect that the data can improve the resolution in the lower crust.
The tomography revealed that the lower crust beneath the seismic belt has low velocity anomalies and might have low viscosity, since velocities of rocks decrease with temperature and/or water content. Therefore, the results of this study support the model proposed by Iio et al. (2002, 2004).
Acknowledgement: We used JMA's earthquake catalogs. We also used waveform data from permanent stations of National Research Institute for Earth Science and Disaster Resilience.