Earthquake and Tsunami Hazards in the Inland Sea of the San Juan Archipelago, Salish Sea of Washington State

Date: 4/24/2019

Time: 02:15 PM

Room: Vashon

Extensive seismic-reflection profiling and bathymetric fault mapping in the inland sea of the San Juan Archipelago has enabled the mapping of local earthquake and tsunami hazards in the region. This inland sea comprises the central part of the Salish Sea and is part of the upper tectonic plate associated with the Cascadia subduction complex. The region is composed of subducted, accreted, and transported blocks of rocks that have been heavily glaciated, forming deep fiord channels, and sounds. The Archipelago is bounded by two E-W trending active strike-slip fault zones, the Devils Mountain Fault Zone in the south and the newly mapped Skipjack Island Fault zone in the north, both of which extend for over 55 km where mapped offshore but may have a total length of over 125 km if connected to onland faults. Similar fault zones to the south (e.g., Seattle, Tacoma fault zones) have produced M_w 6.5-7.5 earthquakes in the last 15 ka.

The Skipjack Island Fault Zone is especially significant as it crosses between the islands of the Archipelago and is near potentially unstable seafloor features. The close proximity of the fault zone to Orcas Island, composed of fractured Mesozoic basement rocks with a steep (locally near vertical) and high (732 m) NE facing side, could trigger rockfalls, which would produce impact tsunamis that could inundate the low-lying coastal areas of islands and the mainland of the US and Canada to the N and NE. In addition, the distal southern edge of the Fraser River Delta of Canada lies just north of the Skipjack Island Fault Zone. A moderate to large magnitude earthquake could mobilize mass wasting of deltaic sediment, thus producing a tsunami directed toward the islands of the San Juan Archipelago and low coastal areas of mainland US.

Presenting Author: H. Gary Greene

Authors