Vashon's Earthquake Risk

Just reading recent blogs with the tag of earthquake paints a useful picture of the rapidly advancing understanding of the geologic risk of earthquake for Vashon-Maury Islands. Conversely they also show that the state of the art still has along way to go. Thus realize that the best available information is far from perfect.

The Pacific Northwest is a seismically active location sitting alongside the Pacific Ring of Fire. See recent earthquake activity as tracked by the Pacific Northwest Seismic Network at UW. There are four types of earthquakes we are exposed to on Vashon, but the most worrisome are:

  1. The huge offshore Cascadia Subduction Zone earthquake similar to the 2011 Japanese Tohoku quake that happen roughly every 300-700 years - most recently in 1700, and
  2. An intraplate earthquake, similar to the 2001 Nisqually 6.8M deep earthquake. These could occur along many of the Puget Sound fault lines (see graphic below). We are most concerned about the Seattle Fault which runs just a few miles north of Vashon. Equally devastating could be the recently discovered Tacoma Fault - as depicted in the ShakeMap to the right. A 2010 USGS report on the Tacoma Fault indicates the potential for a 7.1M shallow earthquake - and thus larger and more violent than the 2001 Nisqually quake.

HAZUS-MH is a FEMA software model used to calculate overall seismic probabilities. Although we're awaiting revised runs in 2012 or so, past runs point to the severe consequences of the Seattle Fault - which runs just north of Vashon and could cause the north end of Vashon to rise some 12 feet and generate significant tsunamis.

Vashon 2005 Hazus Study

Vashon Earthquake Study Fact Sheet

VashonBePrepared has conducted an earthquake study estimating casualties and building damage, as well as needs for food and shelter. The study utilized sophisticated computer modeling techniques managed by a team of geologists, disaster planning specialists, and other volunteers. The study examines a range of possible effects from four earthquake scenarios.

Casualties and Damage

• As many as 70 people on Vashon would require some degree of medical treatment, with as many as ten people critically injured or killed by the most severe of the four studied earthquake scenarios.

• About 100 to as many as 250 Vashon buildings would be so severely damaged that they would be uninhabitable. That’s 2% to 5% of all Vashon building stock.

• Between 600 and 1,100 Vashon buildings would be moderately damaged or worse, requiring extensive repairs or completely destroyed - 22% of all Vashon buildings.

• Depending on the scenario, direct economic losses would be at least $41.2 million. Potentially, losses could reach nearly $83 million.

Food and Shelter

• In the immediate aftermath of a daytime earthquake the Island would need to find shelter for a total of 640 to 1,300 people, including those rendered homeless by destruction of their homes and mainland residents stranded by loss of ferry service.

• People in shelters would require between 1,920 and 3,900 meals per day, a total of between 13,000 and 27,000 meals during the first week after the earthquake.

• If ferry service was out for more than a week, food needs would quickly soar as grocery stores and home pantries were depleted. Assuming a two week ferry outage the Island could require as many as 235,000 emergency meals to feed those in shelters as well as the Island’s own population.

Vashon Stands Alone

A study in early 2005 of a 6.7 magnitude earthquake centered on Bellevue clearly showed Vashon could be on its own during a regional disaster. The Seattle Fault Scenario predicted chaos on the mainland, with all available resources being overwhelmed:

• 1,600 deaths and 24,200 injuries. Most hospital facilities heavily damaged.

• 9,700 buildings destroyed and another 29,000 severely damaged.

• Collapsed bridges cutting all six major highways in the Seattle metropolitan area.

• Downtown and Fauntleroy ferry terminals damaged. Port facilities badly damaged.

• Utilities and other lifeline facilities cut in poor soil areas.

See Full Text of Study

Vashon ShakeMap

Vashon SkakeMap from USGS fact sheet #3023
Possible Vashon ShakeMap from the 2010 USGS fact sheet #3023, modelling the recently discovered Tacoma Fault

Puget Sound Faults

Wikipedia's map of Puget Sound faults The principal Puget Sound faults (approximate location of known extents) and other selected peripheral and minor faults. Southern tip of Vancouver Island and San Juan Islands at top left (faults not shown), Olympic Mountains at center left, Mount Rainer at lower right (near WRZ). Faults north to south: Devils Mountain, Strawberry Point, Utsalady Point, Little River, Sequim, Southern Whidbey Island Fault, Cherry Creek, Rattlesnake Mountain Fault Zone, Lofall, Canyon River, Frigid Creek, Saddle Mountain faults, Hood Canal, Seattle Fault Zone, Tahuya Fault, Tacoma Fault Zone, East Passage, White River (extends east), Olympia Structure, Scammon Creek, Doty (extends west), Western Rainier Zone, Saint Helens Zone (extends south). Also shown: part of the Olympic-Wallowa Lineament.

- from http://en.wikipedia.org/wiki/Puget_Sound_faults