DEEP-CRUSTAL EARTHQUAKES ASSOCIATED WITH CONTINENTAL RIFTS

Deep (> 20 km) crustal earthquakes have occurred within or along the m argins of at least four continental rift zones. The largest of these d eep crustal earthquakes (M greater-than-or-equal-to 5.0) have strike-s lip or oblique-slip mechanisms with T-axes oriented similarly to those associated with shallow normal faulting within the rift zones. The ma jority of deep crustal earthquakes occur along the rift margins in reg ions that have cooler, thicker crust. Several deep crustal events, how ever, occur in regions of high heat flow. These regions also appear to be regions of high strain, a factor that could account for the observ ed depths. We believe the deep crustal earthquakes represent either th e relative motion of rift zones with respect to adjacent stable region s or the propagation of rifting into stable regions.