DEEP-CRUSTAL EARTHQUAKES ASSOCIATED WITH CONTINENTAL RIFTS

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