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4/11/2012 8:07 PM 

Art Frankel and Craig Weaver of the U.S. Geological Survey gave a presentation to the Seattle City Council on Monday (as noted in this agenda) that has raised some eyebrows – and reaffirms our sensitivity to upcoming seismic disturbances. See their PowerPoint presentation or read on for a few copies of a few of their slides.

In 2014, there will be a regular 6 year update to the National Seismic Hazard Maps. A workshop on the Pacific Northwest Portion of this was held locally on March 21-22, 2012.

Of the sources of earthquakes they presented the following overall probabilities for Seattle:

•Cascadia M9: 10-14%
•Seattle Fault M 6.5: 5% (from slip rate, GR model; 1000 yr return time) ???
•Deep M 6.5: 84% (from 1949, 1965, 2001)
•Random shallow M 6.5 in entire Puget Sound area: 15% (mostly from rate of M 4 since 1963, b=0.8)

Vashon still figures high in the relative PGA maps:

One critical change in upcoming scenic maps will be the inclusion of the fairly newly identified Tacoma fault – the one that runs through southern Vashon island.

According to KIRO news coverage of the presentation, the USGS scientists warned of an “84 percent chance a 6.5 magnitude ‘deep’ earthquake will strike again in the next 50 years”. That's nearly the same size as the 6.8 Nisqually Quake 11 years ago, and other quakes in Puget Sound area in 1965 and 1949.  They also warned of a 10-14 percent chance of a magnitude 9 quake on the Cascadia Fault off the Washington and Oregon coastline, and a 5 percent chance of a 6.5 or larger quake on the Seattle Fault.

West Seattle Be Prepared, our overseas sister organization got some prime footage in that KIRO video report where the USGS's Art Frankel was also quoted as saying that the Tacoma Fault does not have a high likelihood of releasing in the next 50 years. Still we seem to figure high on the overall shake maps in the past decade. Next Wednesday's talk by state experts should prove illuminating!

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Tags: Earthquakes , Seattle , Vashon , USGS , Study , regional , GIS

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