5:15 PM - 6:30 PM
[SSS08-P21] Spatial Distribution of Stress Drop for Aftershocks of the 2016 Kumamoto Earthquake Estimated by the Coda Spectral Ratio Method
Keywords:The 2016 Kumamoto Earthquake, S-wave spectral ratio method, source scaling, stress drop
The 2016 Kumamoto earthquake sequence, which involved the foreshock (MJMA 6.5) on April 14 followed by the mainshock (MJMA 7.3) on April 16, caused heavy damage on residential area by two strong motions with seismic intensity of 7 in the Japanese Metrological Agency (JMA) scale. Since the mainshock, many aftershocks have occurred in Kumamoto, Oita Prefecture and Aso District of Kumamoto Prefecture.
In order to understand the rupture characteristics of earthquakes, we need to investigate essential parameters such as seismic moment M0, corner frequency fc and stress drop Δσ. We estimated M0 and fc for 406 aftershocks of the 2016 Kumamoto Earthquake by using a S-wave coda spectral ratio method. The parameters for events were simultaneously estimated by an inversion processing that are based on data of S-wave coda spectral ratios between the observed seismograms of event pairs. The ratio data for each event pair was obtained from stacked among 16 stations to stabilize it. Throughout the processing, we obtained seismic moment ratio to the largest event among events of a group and corner frequencies of event pairs. Then we calculated stress drop based on a circular crack model (Eshelby, 1957; Brune, 1970, 1971), and discussed the spatial variation in stress drops.
The results showed the seismic moment and the corner frequency approximately followed the relationship of M0∝fc-3.6. By adopting seismic moment of the largest earthquake among the aftershocks analyzed in this study as one in F-net catalog and assuming a circular clack fault model, we obtained the stress drop distribution. The stress drops of the events varied between 0.30 and 32 MPa (geometric mean: 3.6 MPa, −1σ/+1σ: 1.7/7.4 MPa). The values were within the range of stress drop for inland earthquake in Japan by previous study (Somei et al., 2010, 2014).
In this study, we examined the accuracy of the estimated stress drop in detail and found significant dependence on the magnitude of earthquake and the focal depth in the epicentral region of the Kumamoto earthquake. These results are important to consider the mechanism of earthquake occurrence in the epicentral region.
Acknowledgement: Hi-net and F-net were provided by National Research Institute for Earth Science and Disaster Resilience (NIED), Japan. Seismogram data were provided by Hi-net (NIED) and moment tensor solutions were provided by F-net (NIED). The unified earthquake catalog was produced by Japan Meteorological Agency (JMA).
In order to understand the rupture characteristics of earthquakes, we need to investigate essential parameters such as seismic moment M0, corner frequency fc and stress drop Δσ. We estimated M0 and fc for 406 aftershocks of the 2016 Kumamoto Earthquake by using a S-wave coda spectral ratio method. The parameters for events were simultaneously estimated by an inversion processing that are based on data of S-wave coda spectral ratios between the observed seismograms of event pairs. The ratio data for each event pair was obtained from stacked among 16 stations to stabilize it. Throughout the processing, we obtained seismic moment ratio to the largest event among events of a group and corner frequencies of event pairs. Then we calculated stress drop based on a circular crack model (Eshelby, 1957; Brune, 1970, 1971), and discussed the spatial variation in stress drops.
The results showed the seismic moment and the corner frequency approximately followed the relationship of M0∝fc-3.6. By adopting seismic moment of the largest earthquake among the aftershocks analyzed in this study as one in F-net catalog and assuming a circular clack fault model, we obtained the stress drop distribution. The stress drops of the events varied between 0.30 and 32 MPa (geometric mean: 3.6 MPa, −1σ/+1σ: 1.7/7.4 MPa). The values were within the range of stress drop for inland earthquake in Japan by previous study (Somei et al., 2010, 2014).
In this study, we examined the accuracy of the estimated stress drop in detail and found significant dependence on the magnitude of earthquake and the focal depth in the epicentral region of the Kumamoto earthquake. These results are important to consider the mechanism of earthquake occurrence in the epicentral region.
Acknowledgement: Hi-net and F-net were provided by National Research Institute for Earth Science and Disaster Resilience (NIED), Japan. Seismogram data were provided by Hi-net (NIED) and moment tensor solutions were provided by F-net (NIED). The unified earthquake catalog was produced by Japan Meteorological Agency (JMA).