Seismic Hazard Resulting from Aftershock Activity Following a Cascadia Subduction Earthquake

Abstract eng:
Great (M>8.0) subduction interface earthquakes, or megathrust earthquakes, are typically followed by hundreds or even thousands of aftershocks, several of which may have magnitudes exceeding M7.0. While the seismic hazard resulting from a Cascadia megathrust earthquake is well recognized and discussed, little attention has been given to quantifying the subsequent aftershock activity and its potential impact on communities in the region. The probability of the next Cascadia megathrust earthquake occurring within the next 100 years is estimated to be 17 percent. Based on an analysis of recorded aftershock sequences at Cascadia-like subduction zones, this paper offers preliminary estimates of ground shaking probabilities resulting from Cascadia megathrust earthquake aftershocks for 22 communities in southwestern British Columbia, Canada and the Pacific Northwest, USA. Calculations estimate the likelihood of an aftershock exceeding each of three intensity levels (MMI V - widely felt; MMI VI threshold for non-structural damage; and, MMI VII - threshold of structural damage). Results presented in this paper are intended to enable community officials and the general public to better understand the Cascadia earthquake threat and to encourage a more comprehensive discussion of the next great Cascadia megathrust event.

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