Tsunamis endanger communities all along the
Pacific Rim, and can be caused by local
earthquakes or ones thousands of miles across the
ocean. Tsunamis are relatively common in
Japan and thus studied intensely there, but the
west coast of the
United States is also subject to major damage and many deaths from Tsunamis.
The most recent significant Tsunami to hit the west coast was in 1964, following the earthquake in Alaska. Of the 131 deaths caused by that quake, 119 were blamed on tsunamis, and 16 of those deaths were in California and Oregon.
Tsunamis often are the cause of more of the deaths associated with a coastal earthquake than the actual shaking. For example, the "big one" in the Northwest of the US, a predicted catastrophic quake of approximately 9.0 magnitude, is anticipated to kill 2,000 on ground in Oregon, but to kill 3,000 along the Oregon coast due to tsunamis. (I was unable to find parallel statistics for Washington state, but I imagine they would be similar.)
In recent years, state governments along the West Coast have been preparing for tsunamis, in hopes of mitigating the risk. A 1995 Oregon law mandated that no new public service buildings, such as fire stations, schools, or hospitals be built in low-lying coastal areas subject to tsunamis. As part of compliance with this law, maps of the Oregon Coast were drawn with a line at the edge of the tsunami inundation zone. Such maps not only aid in planning the locations of service buildings, but also are distributed to citizens to help them prepare for a tsunami event.
Standardized Tsunami Evacuation Route signs can now be found along the Alaska, British Columbia, Washington, Oregon, California, and Hawaii coasts, guiding the best way to high ground. Pictures of these signs can be found at http://pdxcb.net/gallery/manzanita?page=4 (these pictures are ones I took in Manzanita, Oregon.) The sign was designed in 1994 by an artist at Oregon State University, and the sign was adopted in 1997 by the Tsunami Hazard Mitigation Federal/State Steering Group for use along all Pacific states of the U.S.
Tsunami Warning systems are now in place along the Oregon Coast. In the event of a large earthquake in the Pacific Ocean or any significant motion felt in Oregon, a tsunami warning is issued and tsunami alert sirens are activated. All people are then evacuated to above the inundation line. The recently developed Deep Ocean Assessment and Reporting of Tsunamis system helps in detecting and evaluating tsunamis of distant origin in order to decide whether to activate warning systems.
This system was activated along the Oregon Coast due to the heavy shaking felt during the 2001 Seattle Earthquake (Nisqually Earthquake), and schools were immediately evacuated. Upon feeling any ground motion, all coastal residents should immediately evacuate. Should it became clear that there is no risk of tsunami from a particular event (as was the case with the 2001 quake), the warnings will be canceled and evacuees returned.
The most deadly tsunami on record was caused by a magnitude 9.0 earthquake off the coast of Sumatra, resulting in the deaths of at least 155,000 people.