Modeling future Cascadia tsunamis: Don’t prepare only for the rarest and biggest one

Abstract:

Preparing for destructive tsunamis involves estimating how large future earthquakes and the resulting tsunamis are likely to be and how often they may occur. The largest tsunami that can potentially hit North America's west coast is likely to come from the Cascadia Subduction Zone (CSZ), which has a long geologic record of megathrust earthquakes. CSZ earthquakes have been the primary subject of tsunami scenarios along the Pacific Northwest due to the large population centers in the coastal area. However, the geographic extent of potential earthquake ruptures in Cascadia, and thus the resulting tsunamis, are poorly known. Consequently, a full-length rupture of the CSZ (equivalent to M_w=9.2) is often used as the “worst-case” tsunami scenario. However, preparing only for this maximum, but rarest, tsunami may hamper mitigation strategies. Such strategies may be so costly as to not be implemented, or implemented slowly, causing underpreparation for more frequent smaller events. Hence, including the tsunami hazard from smaller earthquakes by using a range of reasonable scenarios will help communities identify an appropriate level of mitigation.

To obtain a set of physics-based tsunami scenarios in Cascadia, we model M_w 7.5—9.2 earthquakes using realistic ruptures of different lengths that are derived from geodetic models and dynamic simulations. These simulations take into account the recurrence intervals inferred from the geologic records in Cascadia. We then apply the shallow water approximation and incorporate coastal bathymetry to generate tsunami amplitudes along the coast. By varying the seismic moment thresholds of simulated earthquake ruptures, we find that regional maximum coastal amplitudes are not unique for a given rupture size. In many cases and especially in central Cascadia, smaller earthquakes can generate relatively large tsunamis up to several meters high on the coast. Numerical experiments show that these phenomena are mostly due to the special coastal geometry, rather than simply the particular slip partitioning of the elongated north-south rupture. Beyond a magnitude of M_w≈8.5, increasing the rupture size will not significantly vary the tsunami hazard. Hence, mitigation plans for the Pacific Northwest should consider a range of potential tsunamis from smaller event.