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Professional Paper 1661-B

Local Tsunami Hazards in the Pacific Northwest from Cascadia Subduction Zone Earthquakes

By Eric L. Geist


map of offshore Pacific northwest from northern California to southern Canada

Computer simulation of tsunami wavefield 20 minutes after a hypothetical magnitude 7.8 earthquake on the Cascadia subduction zone. Topography and wave heights vertically exaggerated at different scales to illustrate wavefield.

In estimating the tsunami hazards for the Pacific Northwest posed by major earthquakes along the Cascadia subduction zone, it is important to identify uncertainties associated with tsunami generation and their effect on near-shore tsunami amplitudes. Earthquake parameters, such as width of rupture and distribution of slip, cause significant variability in local tsunami estimates. These uncertainties are best incorporated in a probabilistic analysis of tsunami hazards.

Since the mid-1980s, there has been accumulating geologic evidence of large local tsunamis generated by earthquakes along the Cascadia subduction zone off the Pacific Northwest. Information from recent geophysical and geological investigations constrains the potential rupture-zone geometry and average repeat time of interplate thrust earthquakes. Based on this information, tsunami hazard maps have been prepared for coastal communities in the Pacific Northwest using sophisticated numerical models to simulate tsunami propagation and inundation. In this study, sources of uncertainty in tsunami hazard calculations are described and a probabilistic hazard analysis for tsunamis, similar to probabilistic seismic hazard analysis (PSHA), is outlined. Though probabilistic tsunami hazard analysis (PTHA) shares many similarities with PSHA, there are specific issues unique to tsunami hazards that need to be accounted for in the probability calculations. For example, the possibility of rupture along splay faults from the Cascadia interplate thrust have a large effect on tsunami generation, but a relatively small impact on peak ground accelerations onshore. The largest sources of uncertainty include the average slip and downdip extent of rupture. Also, it is demonstrated that significant variations in nearshore tsunami amplitude are caused by uncertainty in slip distribution patterns. Correct identification of uncertainties related to tsunami generation and propagation is critical in performing the probability calculations. PTHA has the potential to provide an improved technique for evaluating local tsunami hazards in the Pacific Northwest and elsewhere.

Download Professional Paper 1661-B as a 21-page PDF file (pp1661b.pdf; 1.5 MB)

For questions about the content of this report, contact Eric Geist

This is one of a series of chapters in Earthquake Hazards of the Pacific Northwest Coastal and Marine Regions, USGS Professional Paper 1661, edited by Robert Kayen. The others consist of:

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