Plate boundaries in continental crust are generally less sharply defined than in the oceans, with seismicity spread over broad areas. Interplate displacements appear to be largely accommodated by networks of major fault zones. The depth to which microseismic activity extends appears inversely related to regional heat flow and can be satisfactorily modelled as the frictional/quasi-plastic transition for different geotherms using laboratory determined flow laws for quartz-bearing rocks. Larger earthquake ruptures (M > 5.5) tend to nucleate near the base of the seismogenic regime in the region inferred to have the highest shear resistance and concentration of distortional strain energy. Earthquakes from three different sources threaten communities in the seismic regions. These sources are crustal, subduction zone, intraplate, and volcanic earthquakes. The most common are crustal earthquakes, which typically occur along faults, or breaks in the earth’s crust, at shallow depths of 10-20 km below the surface.
Great subduction zone earthquakes occur around the world where the tectonic plates that make up the surface of the earth collide. When these plates collide, one plate slides (subducts) beneath the other, where it is reabsorbed into the mantle of the earth. This dipping interface between the two plates is the site of some of the most powerful earthquakes ever recorded, often having magnitudes of 8 to9 or larger.
Deeper intraplate earthquakes occur within the remains of the ocean floor that is being subducted beneath North America.
