DNR FIELD NOTES
May 5, 2010
WESTERN WASHINGTON FACES RISKS OF GREAT EARTHQUAKES AND TSUNAMIS
By Tim Walsh, Chief Hazards Geologist
Dave Norman, State Geologist
Washington State Department of Natural Resources
Recent earthquakes that created tsunamis in Chile and Haiti (yes, there was a tsunami in Haiti, too), as well as in Indonesia and Samoa, remind us forcefully that Washington State also is at high risk of these devastating natural catastrophes.
The Cascadia subduction zone, which runs from offshore of Vancouver Island to the northern California shoreline, lies less than 100 miles off the Washington coast. Parts of this earthquake fault run under our state’s coastline.
The Cascadia subduction zone and the Peru-Chile subduction zone share many similarities, including histories of producing great earthquakes and tsunamis like the one that occurred on February 27, 2010, off the coast of Chile.
The last great earthquake and tsunami on the Cascadia subduction zone was in January 1700. It is documented by historical records of a tsunami in Japan, in addition to a large volume of geological evidence and the oral traditions of Northwest Tribes. Geologists have known for more than 20 years that the Cascadia subduction zone is capable of great earthquakes on the scale of the one in Chile, but it is only within the last decade that this threat was fully incorporated into our building codes. Worse, the codes do not recognize the tsunami threat all.
The City of Seattle recently inventoried its unreinforced masonry buildings, which are among the most vulnerable to strong ground shaking. While Seattle has not been able to move forward on a program to retrofit those structures to make them safer, the inventory was at least a great stride forward. It tells us more about how to identify and mitigate our risks of damage and injuries from local earthquake hazards.
Since 1995, the Washington Department of Natural Resources (DNR) and the state’s Emergency Management Division have participated in the National Tsunami Hazard Mitigation Program to identify and mitigate tsunami risks.
DNR also has produced maps of the areas in Washington State most susceptible to earthquake liquefaction, one of the biggest risks to urban residents. It occurs when an earthquake’s shaking causes saturated soil to lose strength, which magnifies the effects of the quake.
Maps showing tsunami and earthquake risks are available to help governments, planners and citizens make better choices in construction, zoning and public safety. DNR posts these maps online at www.dnr.wa.gov so everyone can access them.
The direct link to DNR’s information about tsunamis is: www.dnr.wa.gov/ResearchScience/Topics/GeologicHazardsMapping/Pages/tsunamis.aspx
DNR and the state Emergency Management Division recently began a pilot project to assess seismic vulnerability in Aberdeen and Walla Walla public schools. It’s only a small start and there is still no statewide program or systematic effort to evaluate the vulnerability of our schools in Western Washington to earthquakes and tsunamis. In contrast, the State of Oregon recently evaluated the seismic safety of all of its public schools and emergency facilities, and then initiated a bond-funded grant program to retrofit the most vulnerable of them.
While government clearly has a role, both in planning for recovery after efforts and in informing citizens of potential hazards, citizens also need to take action by understanding their own risks and preparing for earthquakes and tsunamis. It’s part of living in Washington State.
Learn more at www.dnr.wa.gov and at www.emd.wa.gov .
EDITORS: Photos of the authors and a graphic illustrating major earthquake faults in Western Washington are available on request.
Contact: Bob Redling, Senior Communications Manager, 360-902-1149, email@example.com