FOR IMMEDIATE RELEASE
April 16, 2009
Know your faults: DNR geologists research and alert the public about Washington’s active faults
Celebrate Earth Day by learning about your own back yard OLYMPIA – Geologists with the Washington Department of Natural Resources, Division of Geology and Earth Resources (DNR), use multiple tools to learn more about the numerous active fault systems in Washington—detailed geologic mapping, liquefaction and ground shaking analyses, seismic geophysical surveys, and active fault trenching in high-risk areas of the state. They provide this information to the public to aid engineers, developers and local governments in decision-making, such as where to site a new office building, how to retrofit an older structure, or where to locate a tsunami evacuation route.
According to Dave Norman, Washington’s Geologist and head of DNR’s Division of Geology and Earth Resources, “Since many of the active faults in the state are in heavily populated areas, locating and characterizing these faults is key to public safety. To guide current and future urban development, we need to identify potentially hazardous areas that are susceptible to ground liquefaction, amplified shaking, or even tsunami inundation from a major earthquake”.
Mapping of active faults
New active faults in Washington are discovered every year by DNR geologists, who make detailed geologic maps that locate and identify bedrock and glacial materials, and faults and folds.
In the areas around North Bend, Fall City, and Snoqualmie, numerous potentially active faults of the Rattlesnake Mountain fault zone were found hiding in our wooded backyards during the mapping of topographic quadrangles (See Fig. 1, http://dnr.wa.gov/Publications/em_nr_09057.pdf). These faults likely connect with the Southern Whidbey Island fault zone to the northwest, forming a broad zone containing numerous fault strands that is 4 miles wide by 100 miles long. Geologists determine relationships between these faults and other fault systems like the Seattle fault, through detailed mapping, geophysical modeling, strategic age-dating of rock and soil samples, earthquake data, and cross-section analyses. Additional mapping and characterization of these faults in coming years will add to the knowledge of seismic hazard in the state.
In addition to mapping new areas, DNR has recently compiled active fault data into a statewide fault and fold digital map layer, which is coming in May to the DNR’s online interactive map server—the Washington State Geologic Information Portal. The map layer locates and characterizes all faults in Washington with evidence of movement within the last 1.6 million years.
Tsunami preparedness
Coastal communities like Ocean Shores, Aberdeen, and Long Beach must be prepared for tsunamis generated during large-magnitude subduction zone earthquakes (See Fig. 2, http://dnr.wa.gov/Publications/em_nr_09057.pdf). All of these communities have developed tsunami evacuation plans; but, during large earthquakes, evacuation routes may be completely blocked by ground failure, such as landslides or large cracks in the pavement. DNR geologists are modeling liquefaction and landslide risk during earthquakes in these areas to help city planners relocate their evacuation routes to safer areas.
Active fault investigations
Northern Mason and Grays Harbor Counties contain numerous active faults, many of which have been found in the past few years. Many fault scarps (linear breaks in topography due to active faulting) in the Lake Cushman area were detected by airborne laser techniques used to map heavily forested terrain.
Trenches were dug across some of these scarps (See Fig. 3, http://dnr.wa.gov/Publications/em_nr_09057.pdf) exposing bedrock and young glacial sediments offset by faults. Using the amount of offset of geologic units and radiocarbon ages of charcoal buried in soil layers, DNR geologists were able to generally determine the timing of faulting, the number of earthquakes that produced the scarp, and in some cases, the magnitude of shaking during the events. Such information allows seismologists to more accurately model ground shaking behavior for the state.
Tsunami in Lake Washington?
An earthquake that ruptures the ground surface along the Seattle fault could cause a tsunami in Lake Washington. Next year, DNR geologists plan hope to study the Seattle fault using underwater cameras to understand better the tsunami hazard in the lake. The fault crosses Lake Washington in the vicinity of I-90 and shows up in sonar images (See Fig. 4, http://dnr.wa.gov/Publications/em_nr_09057.pdf).
The images are not precise enough to properly calculate the size of a tsunami, so DNR geologists plan to use a Remotely Operated Vehicle (ROV) to photograph and measure the fault. These photographs and measurements will help them to calculate the size of a tsunami generated by an earthquake that ruptures the ground surface along the Seattle fault. Knowing how big a tsunami might be in Lake Washington will help citizens, engineers and planners know what could be coming their way.
Shaking in Spokane
Currently, DNR geologists are working to map Spokane’s urban areas in greater detail to determine earthquake hazards more accurately. The current project will determine how soils may increase earthquake shaking in particular neighborhoods of the city. DNR’s seismologist is constructing a 3D map of the Spokane Valley that will show the areas at greatest risk during a large earthquake. Knowing what areas will shake strongly during an earthquake will help citizens and planners in Spokane to increase earthquake preparedness.
Seismic hazard maps for all Washington counties
DNR geologists have made maps of seismically hazardous areas for every county in Washington. The maps show what areas may be hazardous during large earthquakes at a county-wide level. They show two types of earthquake hazards–soils that may liquefy and areas that may shake more strongly during earthquakes. Both are used by engineers to design roads and buildings that will withstand earthquake damage. The Liquefaction Susceptibility Maps and National Earthquake Hazard Reduction Program Site Class Maps are available online on the DNR website, www.dnr.wa.gov.
Caring for your natural resources . . . now and forever
As Washington State’s geological survey, the Division of Geology and Earth Resources contributes to the safety and economic well-being of Washington’s citizens by informing the public, government, and industry about the consequences of unpredictable geologic events and the nature of the land around us. It is regarded as a primary source of geological products and services to support decision-making by Washington’s government agencies, its businesses, and the public.
The Division of Geology and Earth Resources provides:
• Evaluation of geologic hazards and advice on their mitigation
• Disaster response and damage assessment
• Surface and subsurface geological mapping
• Inventory and regulation of mineral and oil and gas resources
• Technical support for environmental and forest protection
• A complete library collection on the geology of Washington
• Publications on Washington geology
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Media Contact: Princess N. Jackson-Smith, Senior Communications Manager, 360-902-1066, princess.jackson-smith@dnr.wa.gov
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