Dec 30, 2015 10:37am
The earthquake was widely felt in Victoria (home of the Pacific Geoscience Centre of the Geological Survey of Canada), Vancouver, and Seattle.
The 50 km (30 mi) depth places the shock on or near the megathrust surface. This is the same fault surface on which the 26 January 1700 M=9.0 Cascadia earthquake struck, rupturing for about 1,000 km (600 mi) along the Oregon, Washington, and British Columbia coastline. These giant quakes result from ‘subduction’ of the Juan de Fuca slab as it descends beneath the Pacific Northwest coastline.
But the focal mechanism for the M=4.9 event identifies it as a ‘normal’ fault rupture on a steeply inclined plane, which results from local tension or perhaps bending of the plate. So, both the cause of this Victoria, Canada earthquake, and any potential role in ratcheting up the stress on nearby portions of the megathrust, are currently uncertain. If the focal mechanism, location, and depth are all correct, this event occurred near—but not on—the megathrust.
Ross Stein and Volkan Sevilgen, Temblor
Data from the USGS, Pacific Northwest Seismic Network (University of Washington), Pacific Geoscience Centre (Geological Survey of Canada), and McCrory et al (2012).
You can check the aftershocks at http://temblor.net