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[SCG49-03] Recent Studies on Advanced Seismic Hazard Assessment in Japan
Keywords:Seismic hazard assessment, Offshore active faults, Seismic activity model, Response spectra
This paper provides an overview of studies on seismic hazard assessment in Japan after the publish of the 2020 edition of the National Seismic Hazard Maps for Japan.
[Modeling of offshore active faults]
Long-term evaluations of offshore active faults have been initiated by the Headquarters for Earthquake Research Promotion (HERP), and published for the southwestern part of the Sea of Japan in March 2022 and for off the northern coast of Hyogo Prefecture to the Joetsu region of Niigata Prefecture in August 2024. Based on these evaluations, modeling of seismic activity for the PSHA is currently constructed. However, it is also known that many offshore active faults exist along the eastern margin of the Japan Sea and in the Nansei (Ryukyu) Islands, and modeling of these faults should also be conducted in the future. The 2024 Noto Peninsula earthquake occurred on a long active fault in the offshore area, and we are also currently verifying a strong-motion prediction method “recipe” for this earthquake.
[Revisions of seismic activity model for background earthquakes]
The ratio of the number of earthquakes between inter-plate and intra-plate earthquakes for subduction earthquakes was examined with respect to background earthquakes. For earthquakes on the Pacific Plate, we previously determined whether an earthquake was inter-plate or intra-plate based only on the hypocenter location, but the HERP determined it based on the source mechanism solution and aftershock distribution. Using the HERP’s determination, the ratio of intra-plate earthquakes was higher than the current model. On the other hand, for earthquakes on the Philippine Sea Plate in the southern Kanto region, the current model used a model with a higher ratio of intra-plate earthquakes based on the research result that at least four of the five M7-class earthquakes that have occurred in about 100 years are estimated to have been intra-plate earthquakes. However, the number of inter-plate earthquakes was larger based on HERP’s determinations on the recent small tom moderate-size earthquakes (M5 class or smaller). For South Kanto, the model allows both to be considered in a logic tree. In addition, as the JMA's earthquake catalog has been maintained until the end of 2022, we will update the earthquake frequency model, adding it to the previous data, which was available until the end of 2017.
[Integrated shallow and deep velocity structure model for Tokai region]
Based on a large number of microtremor observations, the “Integrated Shallow and Deep Velocity Structure Model for the Tokai Region” was constructed (Senna et al. 2023), and not only the deep ground model for the region but also the Vs30 model for each 250m square mesh was improved to high accuracy. This ground structure model will be used to update not only the PSHA but also the scenario earthquake shaking maps.
[Development of seismic hazard maps of response spectra]
In addition, hazard maps for peak acceleration and 5%-damped acceleration response spectra have been developed (Miyakoshi et al., 2024). For the evaluation, we use the National Seismic Hazard Map for Japan, 2020 edition (Earthquake Research Committee, 2021) as a seismic activity model and Morikawa and Fujiwara (2013) as a ground motion prediction equation. The results are being prepared for release from the Japan Seismic Hazard Information Station, J-SHIS. This is expected to improve the accuracy of damage estimation and expand the use of the results for engineering purposes such as seismic design.
[Future works]
We have also begun to develop models that account for epistemic uncertainties for both seismic activity models and ground motion prediction models.
[Modeling of offshore active faults]
Long-term evaluations of offshore active faults have been initiated by the Headquarters for Earthquake Research Promotion (HERP), and published for the southwestern part of the Sea of Japan in March 2022 and for off the northern coast of Hyogo Prefecture to the Joetsu region of Niigata Prefecture in August 2024. Based on these evaluations, modeling of seismic activity for the PSHA is currently constructed. However, it is also known that many offshore active faults exist along the eastern margin of the Japan Sea and in the Nansei (Ryukyu) Islands, and modeling of these faults should also be conducted in the future. The 2024 Noto Peninsula earthquake occurred on a long active fault in the offshore area, and we are also currently verifying a strong-motion prediction method “recipe” for this earthquake.
[Revisions of seismic activity model for background earthquakes]
The ratio of the number of earthquakes between inter-plate and intra-plate earthquakes for subduction earthquakes was examined with respect to background earthquakes. For earthquakes on the Pacific Plate, we previously determined whether an earthquake was inter-plate or intra-plate based only on the hypocenter location, but the HERP determined it based on the source mechanism solution and aftershock distribution. Using the HERP’s determination, the ratio of intra-plate earthquakes was higher than the current model. On the other hand, for earthquakes on the Philippine Sea Plate in the southern Kanto region, the current model used a model with a higher ratio of intra-plate earthquakes based on the research result that at least four of the five M7-class earthquakes that have occurred in about 100 years are estimated to have been intra-plate earthquakes. However, the number of inter-plate earthquakes was larger based on HERP’s determinations on the recent small tom moderate-size earthquakes (M5 class or smaller). For South Kanto, the model allows both to be considered in a logic tree. In addition, as the JMA's earthquake catalog has been maintained until the end of 2022, we will update the earthquake frequency model, adding it to the previous data, which was available until the end of 2017.
[Integrated shallow and deep velocity structure model for Tokai region]
Based on a large number of microtremor observations, the “Integrated Shallow and Deep Velocity Structure Model for the Tokai Region” was constructed (Senna et al. 2023), and not only the deep ground model for the region but also the Vs30 model for each 250m square mesh was improved to high accuracy. This ground structure model will be used to update not only the PSHA but also the scenario earthquake shaking maps.
[Development of seismic hazard maps of response spectra]
In addition, hazard maps for peak acceleration and 5%-damped acceleration response spectra have been developed (Miyakoshi et al., 2024). For the evaluation, we use the National Seismic Hazard Map for Japan, 2020 edition (Earthquake Research Committee, 2021) as a seismic activity model and Morikawa and Fujiwara (2013) as a ground motion prediction equation. The results are being prepared for release from the Japan Seismic Hazard Information Station, J-SHIS. This is expected to improve the accuracy of damage estimation and expand the use of the results for engineering purposes such as seismic design.
[Future works]
We have also begun to develop models that account for epistemic uncertainties for both seismic activity models and ground motion prediction models.