Onset of October 9, 1995 Manzanillo, Mexico tsunami. For many local tsunamis, the ocean initially recedes before inundation. Photo from Tsunami Field Survey Photographs site maintained by Tsunami Hazard Mitigation Program (Costas Synolakis, Director) at USC.
In the past century, several damaging tsunamis have struck the Pacific Northwest coast (Northern California, Oregon, and Washington). All of these tsunamis were distant tsunamis generated from earthquakes located far across the Pacific basin and are distinguished from tsunamis generated by earthquakes near the coasttermed local tsunamis.
(Figure is taken from online edition of This Dynamic Earth)
The Pacific Northwest is the site of the Cascadia subduction zone, where an oceanic tectonic plate (the Juan de Fuca plate) is being pulled and driven (i.e., subducted) beneath a continental plate (the North American plate). Earthquakes along the fault that is the contact between the two plates, termed the interplate thrust or megathrust, may generate significant local tsunamis in the Pacific Northwest.
Except for the M=7.2 1992 Cape Mendocino earthquake at the southernmost part of the subduction zone, there have been no major earthquakes on the megathrust in historic time. Does this mean that the two plates are sliding past each other freely without generating earthquakes? This would make the Cascadia subduction zone unlike most other subduction zones around the world. Rather, geologic evidence is accumulating that the Cascadia subduction zone is poised between major earthquakes. Therefore, the possibility exists that local tsunamis may someday accompany a major earthquake along the Cascadia megathrust. Some Native American tribal nations in the Pacific Northwest have legends of large waves striking the coast. Do these legends refer to tsunamis?
An excellent introduction to tsunamis, covering topics such as the physics of tsunamis to mitigation efforts, can be found at Tsunami! (where the image by Hokusai, shown above, can be found). That site is operated by the University of Washington Geophysics Program. Also, see our Tsunamis and Earthquakes links page.
How do various parameters that describe an earthquake influence the resulting local tsunami? This question was the focus of a study conducted by the Pacific Coastal and Marine Science Center (formerly the Western Coastal and Marine Geology Team). The research progressed in three phases:
Phase 3: Local Tsunami Hazards in the Pacific Northwest from Cascadia Subduction Zone Earthquakes (USGS Professional Paper 1661-B)