Long-range Earthquake Fault Models

U the physics of complex phenomena such as earthquakes is a formidable challenge. Because we are constrained by our inability to do experiments on earthquake faults, we turn to computers as our earthquake laboratories. Computer simulations of earthquakes help us to test theoretical models and allow us to generate catalogs of synthetic earthquake events. In this column, we discuss several models and algorithms for simulating strike-slip faults with nearest-neighbor and long-range interactions. We begin by briefly reviewing the essential physics of earthquakes ~see Ref. 1 for a more complete introduction and Ref. 2 for a more advanced exposition!. A strike-slip or transform fault, such as the San Andreas fault in California, delineates the boundary between two crustal regions that move primarily along the earth’s surface and opposite to each other. Although earthquakes occur at other types of faults, we shall concentrate solely on strike-slip earthquakes, which have their initial rupture sites near the earth’s surface, because their mainly one-dimensional motion makes them easier to simulate than other types of faults. Without the earth’s relatively rigid upper layer, called the lithosphere, these shallow-focus quakes would not occur. The lithosphere, an approximately 100-km-thick layer, which is made of the crust and a portion of the upper mantle, consists of two principal parts that are coupled. Although the upper part can sustain tremendous shear