Large earthquakes and the aftershocks they generate cause considerable damage to buildings. As a result, risk management, evacuation planning and rapid seismic loss estimation require good estimates of the cumulative damage likely to arise from an earthquake sequence. This is especially important in geographical regions of extreme risk. For example, wood-framed houses make up 56% of buildings in British Columbia (BC), Canada, with 40% being built before 1970. Since seismic provisions of the National Building Code of Canada were only adopted and enforced in the area after 1973, the seismic resistance of old residential houses is likely below the current local seismic standards. Consequently, in urban areas such as Vancouver and Victoria, a large number of wood-frame houses may be particularly at risk from large (magnitude 9.0 or higher) earthquake sequences.
In a recent paper, LML External Fellow Maximilian Werner and colleagues develop a new simulation framework to assess spatiotemporal seismic hazard and risk due to mainshock-aftershock sequences, and apply it using a realistic building portfolio of wood-frame houses for Victoria. Their innovative simulation framework consists of an epidemic‐type aftershock sequence (ETAS) model for the earthquake process, coupled with a ground‐motion model and state‐dependent seismic fragility model. The latter helps to capture the accumulation of damage to wood‐frame houses due to aftershocks. Werner and colleagues also modified the spatiotemporal ETAS model to characterise aftershocks of large and anisotropic mainshock ruptures.
The results show a significant impact of the variability of mainshock peak ground velocity on the total losses. They also indicate that, on average, aftershocks can cause an additional 10% of losses 1 week after the mainshock, and an additional 20% one year later. Single simulations of mainshock-aftershock results show that the developed simulation framework can capture the subduction and crustal aftershock rates in space and time, and also give estimates of loss distributions for risk management decisions. The authors suggest that this framework will help facilitate the quasi-real-time aftershock hazard and risk assessments.
The paper is available at https://onlinelibrary.wiley.com/doi/abs/10.1002/eqe.3286