Earthquake Mechanisms and Tectonics

Abstract

Seismology has played a key role in the development of the plate tectonic theory, which describes and explains the motions of lithosphere plates. The global distribution of earthquakes outlines the plate boundaries, the occurrence of deep earthquakes indicates regions where plates are recycled back into Earth’s mantle, and earthquake fault plane solutions provide information about the relative sense of motion at plate boundaries. Three different types of plate boundaries exist: convergent, divergent, and transform. Relative motion of two plates toward each other occurs at convergent, away from each other at divergent, and parallel to the boundary at transform boundaries (Fig. 1). Convergent plate boundaries are subduction zones where oceanic lithosphere of one plate dives beneath another plate. The overriding plate can be either oceanic lithosphere, such as in the Philippine Sea, or continental lithosphere, such as in South America. Continental collision zones are another form of convergent boundary. Continental lithosphere material, in particular continental crust, is not dense enough to sink into the asthenosphere and where two continents collide, such as in the Himalayas where the Indian plate is actively colliding with the Eurasian plate, compressional mountain belts are formed. Divergent plate boundaries in the ocean are mid-ocean ridge spreading centers where new oceanic lithosphere is generated. The Mid-Atlantic Ridge and the East Pacific Rise are examples. Mature divergent plate boundaries are always associated with seafloor spreading; divergent plate boundaries in continents, such as the East African Rift System, indicate rift initiation, young or failed rifting. At transform plate boundaries, new material is neither created nor destructed, rather two plates move sideways (laterally) past each other. The motion sense along a transform fault can be either right lateral (dextral) or left lateral (sinistral). The best-known transform plate boundaries occur on continents, e.g., the San Andreas Fault in California where the North American and Pacific plates slide past each other in a right-lateral sense. Transform faults are also abundant in oceanic lithosphere where they often connect two spreading segments. Most earthquakes occur at the boundary between two tectonic plates. These are interplate earthquakes. Fewer earthquakes occur inside plates, and these are intraplate earthquakes. The depth of earthquakes within the lithosphere is usually restricted to the shallow part of the lithosphere that deforms in a brittle fashion. In the continental crust, the depth cutoff is often associated with the 350 C isotherm which is reached at mid-crustal depth; the actual cutoff depth (between about 10 and 25 km) depends on the temperature gradient. At temperatures above 350 C,