Faults within the North American plate are another source of earthquakes in the Cascadia region. They result from residual stresses left over from what isn’t released on the subduction interface. These have shallow depths of 0 to about 25 km.
The largest crustal earthquakes include:
- Two earthquakes on Vancouver Island (M7.3 in 1946 and M7.0 in 1918);
- The M 6.8-7.4 North Cascades earthquake in 1872; and
- An earthquake on the Seattle Fault approximately 1,100 years ago.
Crustal earthquakes typically have a mainshock followed by a sequence of aftershocks, and because they may break close to the surface, can generate ground shaking that is locally stronger than that from larger interface or intraplate earthquakes.
CREW published an earthquake scenario for emergency-planning purposes that describes a crustal earthquake.