Preliminary results from a study related to the subduction of the Juan De Fuca Plate beneath the North American Plate off the coast of Washington and British Columbia indicate that the potential mega-thrust earthquake could strike closer to the Seattle-Tacoma area than previously thought. I believe this project is the same one where I posted their press release about two years ago. According to the article, the average return period for these mega-thrust earthquakes is 400 to 500 years with a range between 300 and 800 and the estimated magnitude of around 9.0. The last mega-thrust earthquake along the Cascadia Subduction Zone was in January of 1700. Previously, scientists predicted the earthquake would be centered just off the coast, they now think it could be 30 miles or more inland, under the Olympic Peninsula to the west of the Seattle-Tacoma area. Source: Physorg.com via Geology.com.
The Seattle Times (hat tip to ASCE SmartBrief) has reported that seven voids have been discovered above the Beacon Hill Tunnel with one opening up at the ground surface. The tunnel is being constructed by Sound Transit, the area’s transportation agency as part of a roughly $2.6 billion (yep, billion) light-rail project connecting downtown Seattle with the University of Washington and SEA-TAC airport. The voids were a result of running sand pockets in the otherwise stable clay units that were encountered by the tunnel boring machine or TBM. These voids migrated up like a chimney with one reaching the surface, almost 160-ft above the tunnel. This void was apparently 21-ft deep and opened up in a resident’s front yard and could have easily swallowed her up as she noticed it while gardening. The other voids were discovered at a depth of 20- to 65-ft below the ground surface. More after the break. (Illustration from Seattle Times)