© johanssonclark.com
This view from Bainbridge Island towards Seattle looks out across Puget Sound. The active Seattle Fault Zone runs through this area and poses a significant threat to the region.
About two weeks ago, a seismic swarm just west of the city of Seattle, Washington, and southwest of Bainbridge Island started. It began with a M=3.3, which was followed a week later by a M=3.4 and a M=3.5.
In total, the USGS has recorded 72 earthquakes in the area over the last two weeks. While there has only been one earthquake in the last few days (a M=1.1 at 2:13 a.m. this morning), we thought we'd take a look at the activity and the regional tectonics, with help from the people at the Pacific Northwest Seismic Network.
The city of Seattle sits right on top of the Seattle Fault zone, an east-west-striking system of reverse faults within the Puget Lowland. While active, the Seattle Fault Zone is largely concealed as it lies at the southern end of the Seattle Basin, which is covered by surface deposits, water and dense vegetation. Nonetheless, by using LiDAR (light detection and ranging) the faults can be clearly seen (See image below). This complex system of reverse faults formed due to regional compression on the order of 0.5 cm/yr.
© PNSN
This LiDAR image from the Pacific Northwest Seismic Network clearly shows traces of the active Seattle Fault. If this fault were to rupture in a large earthquake like it did 1,100 years ago (M=7.0) it would have devastating effects on the region.
