Tsunami trivia… and why tsunamis are not a trivial issue for Washington State

tsunami evacuation route marker
DNR geologists helped map tsunami evacuation routes in many coastal Washington communities.

Ready for a little tsunami trivia to test your knowledge of this threat to many of Washington State’s coastal communities and cities?

Question: What is the difference between a near-field tsunami and a far-field tsunami?

Answer: A tsunami generated close to the shoreline is known as a near-field tsunami. A tsunami generated by a source far from the point of impact is referred to as a far-field tsunami. Near-field tsunamis pose a greater risk for coastal communities because the first waves can move on shore in minutes. Far-field tsunamis will not reach the coast for hours, and allow time for officials to issue warnings and evacuation notices. It is also possible that a tsunami could hit the coast less than an hour after an event at a moderate distance from the coast. In this case, the earthquake may not be felt strongly by residents, so warnings and evacuation notices will be essential for an effective response. In general, it takes a large earthquake (magnitude  7.0 or greater) to generate a damaging tsunami in the near-field and it takes a great earthquake (magnitude over 8.0) to generate a damaging tsunami in the far-field. The earthquake faults that lie off of Washington’s coast are capable of generating earthquakes large enough to cause either type of tsunami.

This information comes from the December 2011 issue of TsuInfo Alert, a newsletter prepared by DNR’s Geology and Earth Resources Division for the National Tsunami Hazard Mitigation Program.

DNR works with federal agencies and local governments to prepare maps of recommended tsunami evacuation routes for many coastal Washington communities. We also map earthquake faults and hazards. Local and state emergency officials rely on these maps, in addition to data and estimates prepared by DNR’s geologists and other sources to plan their responses to earthquakes, tsunamis and other natural disasters.

Sound interesting? Current and back issues of all TsuInfo Alert newsletters are available on  www.dnr.wa.gov