11:15 AM - 11:30 AM
[SSS09-03] Estimation of M7-class ground motion using dense seismic observation records for the 2021 Northwestern Chiba Earthquake (Mw5.9)
In the capital and its surrounding areas, magnitude 7-8 class earthquakes have occurred repeatedly in the past, and the probability of an M7 class earthquake occurring within 30 years is over 70%. The Cabinet Office has published damage estimates for these earthquakes.
LONGLIFE AEDGiS, an IoT disaster prevention information system developed and operated by Asahi Kasei Corporation, Asahi Kasei Homes Corporation, and NIED (Koyama et al. (2020)), observed 2021 Northwestern Chiba Earthquake (Mw 5.9) at 93 locations, mainly in the 23 wards.
This earthquake recorded a maximum shaking of 5 strong in the 23 wards. An earthquake of the same magnitude (Mw 6.0) occurred in this epicenter in 2005. It has been pointed out that the 1923 Ansei-Edo earthquake may have occurred in this area. (Sato (2016)).
In this study, future predictions of multiple cases of M7-class earthquake ground motion with an epicenter in the Northwestern Chiba Prefecture were performed using the Empirical Green's Function method (EGF) using observation records obtained at 93 locations.
The magnitude of the earthquake is M7.3, and M6.8, a smaller earthquake but with a higher probability of occurrence, is targeted. For the source parameters, a "recipe" method and a case with a doubled stress drop were also set up to indicate the uncertainty of the earthquake in terms of width. In addition, the analysis was performed with 9 different the asperity arrangement on the fault plane.
The records observed at the surface were used as engineering base waves by inverse amplification analysis using the S-wave velocity structure from the small-scale microtremor array. Using this as an element earthquake, an M7-class engineering base wave was created by applying the EGF. Furthermore, we predicted surface shaking by performing a nonlinear analysis based on iterative integration.
The analysis results were arranged using the response spectrum (h=0.05) and instrumental seismic intensity as indexes. When compared with the spectrum specified by the Building Standards Act, the responses were similar at M7.3 and tended to be slightly lower at M6.8. On the other hand, the periodic characteristics tended to be dominant around 1 second when the damage to the building was large. The effect of the double difference in stress drop was a difference of about 0.6 in the seismic intensity, and the tendency of the seismic intensity distribution was the same. It was confirmed that the difference due to the asperity arrangement is about 0.2 in the seismic intensity, and the distribution is slightly different. The validity of the analysis results was confirmed by the GMPE (Morikawa and Fujiwara (2013)). Among the multiple cases examined this time, the setting of M7.3, the maximum shaking, and double the amount of stress drop was the same level as the seismic intensity assumed by the Cabinet Office.
It is necessary to analyze the seismic motion prediction results of 93 points individually and improve the accuracy. In particular, it has been reconfirmed that the nonlinear effect of the surface layer has a large effect on the seismic motion at points where soft ground is deposited. In the future, we plan to expand the prediction results of 93 locations to the entire 23 wards using the method of the AEDGiS. In addition, if new records are obtained, we will continue to examine them to elucidate the seismic image of Tokyo inland earthquakes.
Acknowledgments:This research was conducted by Asahi Kasei Corporation, Asahi Kasei Homes Corporation, and the NIED as part of their research on estimating earthquake ground motions directly under the Tokyo metropolitan area using the AEDGiS. We would like to express our deepest gratitude to all those who provided observation data and valuable comments during the execution of this study.
LONGLIFE AEDGiS, an IoT disaster prevention information system developed and operated by Asahi Kasei Corporation, Asahi Kasei Homes Corporation, and NIED (Koyama et al. (2020)), observed 2021 Northwestern Chiba Earthquake (Mw 5.9) at 93 locations, mainly in the 23 wards.
This earthquake recorded a maximum shaking of 5 strong in the 23 wards. An earthquake of the same magnitude (Mw 6.0) occurred in this epicenter in 2005. It has been pointed out that the 1923 Ansei-Edo earthquake may have occurred in this area. (Sato (2016)).
In this study, future predictions of multiple cases of M7-class earthquake ground motion with an epicenter in the Northwestern Chiba Prefecture were performed using the Empirical Green's Function method (EGF) using observation records obtained at 93 locations.
The magnitude of the earthquake is M7.3, and M6.8, a smaller earthquake but with a higher probability of occurrence, is targeted. For the source parameters, a "recipe" method and a case with a doubled stress drop were also set up to indicate the uncertainty of the earthquake in terms of width. In addition, the analysis was performed with 9 different the asperity arrangement on the fault plane.
The records observed at the surface were used as engineering base waves by inverse amplification analysis using the S-wave velocity structure from the small-scale microtremor array. Using this as an element earthquake, an M7-class engineering base wave was created by applying the EGF. Furthermore, we predicted surface shaking by performing a nonlinear analysis based on iterative integration.
The analysis results were arranged using the response spectrum (h=0.05) and instrumental seismic intensity as indexes. When compared with the spectrum specified by the Building Standards Act, the responses were similar at M7.3 and tended to be slightly lower at M6.8. On the other hand, the periodic characteristics tended to be dominant around 1 second when the damage to the building was large. The effect of the double difference in stress drop was a difference of about 0.6 in the seismic intensity, and the tendency of the seismic intensity distribution was the same. It was confirmed that the difference due to the asperity arrangement is about 0.2 in the seismic intensity, and the distribution is slightly different. The validity of the analysis results was confirmed by the GMPE (Morikawa and Fujiwara (2013)). Among the multiple cases examined this time, the setting of M7.3, the maximum shaking, and double the amount of stress drop was the same level as the seismic intensity assumed by the Cabinet Office.
It is necessary to analyze the seismic motion prediction results of 93 points individually and improve the accuracy. In particular, it has been reconfirmed that the nonlinear effect of the surface layer has a large effect on the seismic motion at points where soft ground is deposited. In the future, we plan to expand the prediction results of 93 locations to the entire 23 wards using the method of the AEDGiS. In addition, if new records are obtained, we will continue to examine them to elucidate the seismic image of Tokyo inland earthquakes.
Acknowledgments:This research was conducted by Asahi Kasei Corporation, Asahi Kasei Homes Corporation, and the NIED as part of their research on estimating earthquake ground motions directly under the Tokyo metropolitan area using the AEDGiS. We would like to express our deepest gratitude to all those who provided observation data and valuable comments during the execution of this study.