Spatiotemporal Seismic Hazard and Risk Assessment of Aftershocks ofM 9 Megathrust Earthquakes

Abstract

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.

Mainshock‐aftershock state‐dependent fragility curves: A case of wood‐frame houses in British Columbia, Canada

“…The developed aftershock fragility curves of Houses 1 to 4 can be employed to estimate the DSs implementing a spatiotemporal risk assessment for a M w 9.0 mainshock triggering both crustal and subduction‐zone aftershocks (eg,) in British Colombia, Canada. The evaluated PGV and real MSAS sequences facilitate the estimation of cumulative damage of wood‐frame houses.…”

Spatiotemporal seismic hazard and risk assessment of M9.0 megathrust earthquake sequences of wood‐frame houses in Victoria, British Columbia, Canada

“…28 Rupture widths are simulated to capture the locations of different down-dip edge models from the 2014 USGS national seismic hazard model. In terms of the aftershock decay outside the CSZ rupture area, the same procedures as in the Tohoku case 15 are applied to build a spatial kernel function using 1D and 2D power laws. Figure 4 shows the probability density distribution of the aftershock spatial distribution outside the rupture area of the M9.0 event with a rupture length of 1,100 km and width of 130 km.…”

“…Figure 4 shows the probability density distribution of the aftershock spatial distribution outside the rupture area of the M9.0 event with a rupture length of 1,100 km and width of 130 km. Additional features including the depth, earthquake type, and focal mechanism are assigned to each event in synthetic catalogues based on Zhang et al 15 These additional features allow simulating the rupture plane of large crustal and subduction-zone aftershocks (M ≥ 6.5) and evaluating seismic IMs using GMPEs. Depths for earthquakes with M < 8 are sampled from empirical cumulative distribution functions (ECDFs) of depths obtained from past observations in the CSZ.…”

“…Due to the plate motion of the subduction zone, the crustal and subduction-zone earthquakes tend to have similar strike directions as the subduction plane. 15 Following that, the strike and dip angles of the subduction and crustal aftershock are assumed to be similar to the strike and dip angles of the subduction plane. The ECDFs of strike and dip angles for crustal and subduction earthquakes are evaluated from the global Centroid Moment Tensor (gCMT) catalogue, and the sampled angles are assigned to the large aftershocks with M ≥ 6.5.…”

“…13,14 All above-mentioned studies, however, focussed on shallow crustal seismicity, whereas regional spatiotemporal seismic risk assessments in subduction zones are rarely carried out. Recently, Zhang et al 15 applied the ETAS model to subduction-zone regions and developed a new simulation framework to assess spatiotemporal seismic hazard and risk due to aftershocks triggered by M9.0 events. Their case study of Tohoku-like events has shown that synthetic catalogues from the new simulation framework are in good agreement with the observed M9.0 Tohoku sequence.…”