[S-SS11] 地震活動とその物理

[SSS11-06] Aperiodicity Parameters Estimated From the Recent Repeating Earthquake Catalogs and Implication for Seismic Hazard Assessment

*石辺 岳男¹、松浦 律子¹ (1.地震予知総合研究振興会)

キーワード：Brownian Passage Time (BPT) distribution、Aperiodicity Parameter、Repeating Earthquake Catalog、Seismic Hazard Assessment

The Earthquake Research Committee, Headquarters for Earthquake Research Promotion (ERC, HERP) adopts two probabilistic density functions (i.e., Poisson and Brownian Passage Time (BPT)) for seismic hazard assessments. The homogeneous Poisson distribution assumes that earthquakes randomly occur with the temporally-constant rate, while the BPT has two parameters (average recurrence interval and aperiodicity parameter (AP)). The BPT is applied for faults with relatively well-known recurrent histories. As for AP, HERP adopted 0.24 depending on the recurrent histories for four major active faults (ERC, HERP, 2001). The setting of AP strongly influences, for example, 30-year probability and is of great importance for seismic hazard assessments. Here, we examined AP for the BPT distribution by using two recently-developed repeating earthquake catalogs to explore the appropriate setting of AP. The one is Igarashi (2020), who constructed a long-term catalog of repeating earthquakes in central Japan since 1981 and throughout the Japanese Islands since 2001. The other is by Waldhauser and Schaff (2021), who carried out a comprehensive search for repeating earthquakes in northern California between 1984 and 2014. As for repeating earthquakes in Japan, the 2011 off the Pacific coast of Tohoku earthquake (M9.0) occurred and the aperiodicity has been influenced due to the stress perturbation and/or temporal changes in plate convergence rate. Furthermore, the catalog includes burst activities following the 2011 event. Therefore, we estimated AP from repeating earthquakes during the pre-Tohoku earthquake period by using the maximum likelihood method. The resulting AP for the sequence with the number of events >= 3 has a peak of approximately 0.20, and the value is consistent with the setting by ERC, HERP (0.24). The APs have a peak of 0.20, 0.14, and 0.32 for the sequence with the number of events of >= 3, >= 5, and >= 7, respectively. Furthermore, estimated APs show a negative correlation between the average recurrence intervals of repeating earthquakes; i.e., the shorter the average recurrence intervals are, the larger the APs are. The APs for the repeating earthquakes in northern California are also consistent with the setting by ERC, HERP (2001); the histogram of APs has a peak of approximately 0.30. These investigations would help determine an appropriate AP for seismic hazard assessments, whereas the maximum likelihood estimate for sequences with a small number of events possibly underestimates AP (e.g., Nomura, 2015).

Acknowledgements
This study has been supported by the Headquarters for Earthquake Research Promotion (HERP) of the Ministry of Education, Culture, Sports, Science, and Technology (MEXT) of Japan, and MEXT of Japan, under its The Second Earthquake and Volcano Hazards Observation and Research Program (Earthquake and Volcano Hazard Reduction Research).

References
Earthquake Research Committee, the Headquarters for Earthquake Research Promotion (2001). Regarding methods for evaluating long-term probability of earthquake occurrence (In Japanese), 46pp.
Igarashi, T. (2020). Catalog of small repeating earthquakes for the Japanese Islands. Earth, Planets and Space, 72(1), 1-8.
Nomura, S. (2015). Point Process Models for Recurrent Earthquakes at Active Faults and Their Long-term Forecast, Proceedings of the Institute of Statistical Mathematics Vol. 63, No. 1, 83–104.
Waldhauser, F., and Schaff, D. P. (2021). A Comprehensive Search for Repeating Earthquakes in Northern California: Implications for Fault Creep, Slip Rates, Slip Partitioning, and Transient Stress. Journal of Geophysical Research: Solid Earth, 126(11), e2021JB022495.