quakes and most of Earth’s seismicity occur at convergent plate boundaries. The subduction of relatively cool oceanic crust increases the depth of the elastic-brittle regime in which earthquakes occur. Moreover, when the subducted slab crosses the elastic-brittle regime at a shallow angle (e.g., in Sumatra, Alaska, and Chile), the seismogenic zone can be hundreds of kilometers wide. The hazard at convergent boundaries is not always commensurate with earthquake size because much of the faulting and the strongest shaking occur underwater. However, the 2004 tsunami (Figure 4.2) illustrates the destructive potential and global reach of large subduction zone earthquakes.
Earthquakes that occur within a tectonic plate account for less than 1 percent of the world’s earthquakes, but they pose a significant seismic hazard and can be quite large. For example, a sequence of strong earthquakes with magnitudes as high as 8 shook New Madrid, Missouri, for eight weeks in 1811-1812, destroying the town and causing widespread destruction across the central United States. Intraplate earthquakes are not readily explained by plate tectonics. Some occur within broad plate boundary deformation zones, such as those across south Asia and western North America, while others are not clearly associated with any plate boundary, such as those in Australia or eastern North America. Half of all intraplate earthquakes occur in failed continental rifts (Johnston and Kanter, 1990), but their underlying cause remains a mystery. Putative explanations for intraplate earthquakes include localized stresses induced by emplacement and crystallization of magma below the surface, postglacial rebound, and weak zones in otherwise strong crust.
Intermediate (70 to 300 km deep) and deep (300 to 700 km deep) focus earthquakes occur at convergent plate boundaries within subducting lithosphere (Figure 2.10). Although they pose less of a threat than shallow, plate-boundary earthquakes, intermediate-depth earthquakes can be quite destructive (Beck et al., 1998). What causes them is unclear because Earth materials are expected to deform plastically at the depths where they occur (Question 4). Candidate mechanisms to explain intermediate and deep earthquakes include elevated fluid pressures, accelerating deformation and frictional heating, and mineral phase changes (Kirby et al., 1996).