5:15 PM - 7:15 PM
[SCG61-P10] Stress heterogeneity at the bottom of the seismogenic layer in Kyushu, Japan - relation to anelastic deformation of the lower crust -
Keywords:bottom of seismogenic layer, lower crust, stress change, stress field, anelastic deformation
The deformation of the upper crust during interseismic periods in Kyushu is considered to be strongly influenced by anelastic deformation of the lower crust and interplate coupling (Yuasa and Matsumoto, 2023). They estimated the elastic strain rate in the upper crust by the lower crust deformation in Kyushu by GNSS data. They show that anelastic deformation of the lower crust causes stress concentrations in the upper crust directly above, and a large increment of stress at the upper crust by the anelastic deformation of the lower crust coincides with the high seismic activity area in Kyushu. In other words, it is strongly suggested that anelastic deformation of the lower crust is responsible for the increase in inland seismic activity in Kyushu. In this study, we quantitatively investigate the relationship between stress change due to the lower crust and the stress field at the bottom of the seismogenic layer in which stress may directly be affected by the lower crust in Kyushu.
We defined hypocentral area until the 2km above D95 depth, which is the depth of 95% of earthquakes occurring, as “the bottom of the seismogenic layer”. We analyzed the earthquakes in the layer from 1 January 1993 to 31 December 2020. Also, we calculated the stress change tensor at the depth of D95 due to the anelastic strain rate of the lower crust estimated by Yuasa and Matsumoto (2023).
We estimated the focal mechanisms of earthquakes using the HASH algorithm(Hardebeck and Shearer, 2002) and calculated the seismic moment tensors based on focal mechanisms and their magnitude. The moment tensors of earthquakes that occurred within each spatial block with 0.15°× 0.15°distributed in the layer were summed and then obtained normalized stress tensor in the block. To investigate relation between the stress change by the lower crust and stress tensor at the bottom of the seismogenic layer, we calculated the tensor product of the stress change and the stress field in the blocks.
We found areas where the stress change rate by the lower crust is high, such as the area near the Median Tectonic Line and the left lateral shear zone. In those areas, earthquakes tend to be strongly influenced by stress change caused by the lower crust directly below. On the contrary, tensor at the areas where the stress change rate by the lower crust is poor correlation with the stress tensor for earthquakes generation. It can be attributed to dominant tectonic stress field and/or other sources thet cause earthquakes. We considered two possibilities as some kind of stress from other sources: volcanic activity and the Beppu-Shimabara Graben.
We defined hypocentral area until the 2km above D95 depth, which is the depth of 95% of earthquakes occurring, as “the bottom of the seismogenic layer”. We analyzed the earthquakes in the layer from 1 January 1993 to 31 December 2020. Also, we calculated the stress change tensor at the depth of D95 due to the anelastic strain rate of the lower crust estimated by Yuasa and Matsumoto (2023).
We estimated the focal mechanisms of earthquakes using the HASH algorithm(Hardebeck and Shearer, 2002) and calculated the seismic moment tensors based on focal mechanisms and their magnitude. The moment tensors of earthquakes that occurred within each spatial block with 0.15°× 0.15°distributed in the layer were summed and then obtained normalized stress tensor in the block. To investigate relation between the stress change by the lower crust and stress tensor at the bottom of the seismogenic layer, we calculated the tensor product of the stress change and the stress field in the blocks.
We found areas where the stress change rate by the lower crust is high, such as the area near the Median Tectonic Line and the left lateral shear zone. In those areas, earthquakes tend to be strongly influenced by stress change caused by the lower crust directly below. On the contrary, tensor at the areas where the stress change rate by the lower crust is poor correlation with the stress tensor for earthquakes generation. It can be attributed to dominant tectonic stress field and/or other sources thet cause earthquakes. We considered two possibilities as some kind of stress from other sources: volcanic activity and the Beppu-Shimabara Graben.