Friday, April 22, 2011

Raised Beach along the Seattle Fault

Olympic Range from the Bainbridge Island - Seattle Ferry

Seattle skyline from Bainbridge Island - Seattle Ferry

Wednesday morning I crossed Puget Sound on the Seattle to Bainbridge Island Ferry. I often think of the scenes from this ferry run as some of the classic images of western Washington. This may be in part due to some of my initial adventures in the Seattle area involved riding the ferry out to Bainbridge Island.

The landscape viewed from this ferry run has changed a bit from the first time I rode this ferry route. For one thing there are a lot more tall buildings on the Seattle skyline than the Space Needle and what was then called the black box. More recent additions are the two stadiums - one for football and one for baseball. The giant container loading cranes are also a new addition as Seattle remains a very competitive shipping port. And there is the addition of an armed escort from the Coast Guard that is a frequent companion of this ferry run but not very often on the other ferry routes.

Stadiums, cranes and the Coast Guard

Another addition is our understanding of the Seattle Fault zone. The view of Restoration Point at the southeast end of Bainbridge Island has taken on new meaning. The raised bedrock platform beach is a result of uplift within a portion of the fault zone.

Raised bedrock platform beach at Restoration Point southeast Bainbridge Island 

LiDAR image of south end of Bainbridge Island showing Toe Jam Fault raised platform beach. 

The uplifted bedrock platform at Restoration was suspected to be the result of a fault, but finding faults within the forested and glaciated Puget Sound Basin has been a challenge. The are several strands of the Seattle Fault zone expressed on the surface. The proximity of this fault system to population centers poses a significant threat. Surface ruptures as expressed along the Toe Jam Fault are an obvious indicator of seismic risk; however, not all seismic activity will have a surface expression. Someday many of Seattle's structures will be tested by this fault system. It has happened before and will happen again.   

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