Spatiotemporal Seismic Hazard and Risk Assessment of Aftershocks of M9 Mega-Thrust Earthquakes

Lizhong Zhang, Maximilian J. Werner, & Katsuichiro Goda

Published October 30, 2018, SCEC Contribution #8885

Current national seismic hazard models neglect time-dependent hazard due to triggered earthquakes, although these can certainly generate damaging ground motions. To understand the relative importance of aftershock hazard and risk in the context of a mega-thrust subduction-zone earthquake, we develop a new simulation framework for spatiotemporal seismic hazard and risk assessment of a mega-thrust earthquake and its aftershocks along the plate boundary and in the onshore continental crust. Tohoku region in the northeast Japan is considered as an example to show how the new simulation framework can be implemented to assess the spatiotemporal hazard and risk of aftershocks triggered by a M9 Tohoku-like earthquake. We generate quasi-3D synthetic aftershock catalogs using the Epidemic Type Aftershock Sequences (ETAS) model, modified to characterize aftershocks of large and anisotropic finite mainshock sources. By including the mainshock source model in the new simulation framework, the uncertainty of generating synthetic aftershock catalog is small in comparison with the observation. Therefore, should the mainshock source model is available right after the mainshock, the new simulation framework can be used for the quasi-real time hazard and risk assessments of aftershocks in different regions. For Tohoku region, we assess the relative importance of subduction-zone versus onshore-crustal aftershocks. The results show that the subduction-zone aftershocks tend to dominate hazard with peak ground velocity (PGV) < 60 cm/s (the boundary between VIII (severe) and IX (violent) of Modified Mercalli Intensity). On the other hand, onshore-crustal aftershocks control extreme hazards exceeding PGV of 60 cm/s. Moreover, on the day of the mainshock, aftershocks contribute about 23% of the onshore hazard with PGV > 60 cm/s, and the aftershock hazards remain relatively high for 4-5 days depending on different sites. From a seismic risk viewpoint, the subduction-zone and onshore-crustal aftershocks in the mega-thrust sequence affect buildings differently; both have similar potential to cause minor damage, whilst the latter tend to cause more severe damage.

Key Words
earthquake clustering, time-dependent probabilistic seismic hazard and risk assessment

Citation
Zhang, L., Werner, M. J., & Goda, K. (2018). Spatiotemporal Seismic Hazard and Risk Assessment of Aftershocks of M9 Mega-Thrust Earthquakes. Bulletin of the Seismological Society of America, 108(6), 3313-3335. https://pubs.geoscienceworld.org/ssa/bssa/article/108/6/3313/566078/spatiotemporal-seismic-hazard-and-risk-assessment

Related Projects & Working Groups
CSEP, Earthquake Forecasting and Predictability