5:15 PM - 7:15 PM
[SCG61-P09] Heterogeneous Stress Field in the Nothen Kyushu Island Based on Focal Mechanism Data
Keywords:Kego Fault, Focal Mechanism, Stress field
Several active faults with strike slip type exist in the northern part of Kyushu Island, Japan. The Kego Fault Zone is an active fault system with a strike direction of northwest-southeast that; extends from the Genkai Sea through Hakata Bay into the Fukuoka Plain. It can be divided into two segments: the northwestern segment, which corresponds to the source region of the 2005 West Off Fukuoka Prefecture Earthquake near Shikanoshima in the Genkai Sea, and the southeastern segment, which includes the Kego Fault extending from Hakata Bay to Chikushino City. A previous study by Matsumoto et al. (2012) demonstrated that the stress field in the northwestern part of the Kego Fault Zone is in the strike-slip regime, and that the stress distribution around the fault is heterogeneous. However, similar studies about the southeastern part of the Kego Fault Zone and other faults in northern Kyushu have not yet been conducted. The purpose of this study is to investigate stress field characteristics in the Northern Kyushu Island, including the entire Kego Fault Zone, by utilizing moment tensors derived from focal mechanism solutions, and incorporating data obtained after the study by Matsumoto et al. (2012).
The data used in this study are polarity and phase picked from the seismograms obtained from both permanent and temporary observation stations operated by Kyushu University, the Japan Meteorological Agency, the National Research Institute for Earth Science and Disaster Resilience (NIED), and the Association for the Development of Earthquake Prediction (ADEP), along with the dataset used in Matsumoto et al. (2012). Seismic moment tensors were subsequently calculated from focal mechanisms determined by these polarity data. Average moment tensors were then obtained within spatial grids of 0.1° in latitude and longitude. The results showed that there were differences in the orientation of the principal axes of the average moment tensors in the Kego Fault. The average moment tensors reflect the stress state within each region, and a heterogeneous stress field was also observed in the region of the Kego Fault.
Furthermore, to objectively detect the heterogeneity of stress associated with the fault, we divided the region into several areas. The average moment tensor for each area was used as the reference moment tensor. We calculated the average moment tensors of earthquakes within a certain radius from several points set in each area. The differences between these and the reference moment tensors were analyzed to investigate the scale of stress heterogeneity. As a result, it was found that a stress heterogeneity of approximately 10 km may exist around the Kego Fault. In future studies, we will estimate the stress field based on the scale.
The data used in this study are polarity and phase picked from the seismograms obtained from both permanent and temporary observation stations operated by Kyushu University, the Japan Meteorological Agency, the National Research Institute for Earth Science and Disaster Resilience (NIED), and the Association for the Development of Earthquake Prediction (ADEP), along with the dataset used in Matsumoto et al. (2012). Seismic moment tensors were subsequently calculated from focal mechanisms determined by these polarity data. Average moment tensors were then obtained within spatial grids of 0.1° in latitude and longitude. The results showed that there were differences in the orientation of the principal axes of the average moment tensors in the Kego Fault. The average moment tensors reflect the stress state within each region, and a heterogeneous stress field was also observed in the region of the Kego Fault.
Furthermore, to objectively detect the heterogeneity of stress associated with the fault, we divided the region into several areas. The average moment tensor for each area was used as the reference moment tensor. We calculated the average moment tensors of earthquakes within a certain radius from several points set in each area. The differences between these and the reference moment tensors were analyzed to investigate the scale of stress heterogeneity. As a result, it was found that a stress heterogeneity of approximately 10 km may exist around the Kego Fault. In future studies, we will estimate the stress field based on the scale.