In addition to earthquakes on the interface fault between the subducting oceanic and overriding plates, earthquakes occur within the subducting oceanic plates as they are forced into the upper mantle. These can have magnitudes as large as 7.5 and are deeper than ~30 km. These ‘intraplate’ earthquakes break faults within the downgoing Juan de Fuca plate. Descending beneath North America under the influence of plate-driving forces, the Juan de Fuca plate stretches, bends, and compresses, and earthquakes strike when the deformation takes the form of rock shifting along cracks. The bending, stretching, and crumpling stresses within the downgoing plate vary depending on location within the plate, so these earthquakes have a variety of “focal mechanisms”, or faulting types. Some earthquakes in this intraplate class, such as the 2001 M6.8 Nisqually earthquake, have resulted from the failure of normal faults, where overlying rocks move downward along a dipping fault. Others have been strike-slip earthquakes, with horizontal relative motion of each side of a nearly vertical fault.
Although subduction earthquakes are the largest in magnitude, earthquakes that occur within tectonic plates can also threaten lives and cost billions of dollars in damage. These events occur more frequently and could cause substantial damage and loss of life if located beneath a major urban center. CREW published an earthquake scenario for emergency-planning purposes that describes a deep, intraplate earthquake – Cascadia Deep Earthquakes (2008).
Earthquakes also occur within the oceanic plates before they begin to subduct and along their margins, although these typically are not hazardous because of their distance from land and low potential for generating tsunamis.