Despite their usefulness as a research tool for illuminating Earth structure, earthquakes are generally considered harmful because of their potential for causing death and destruction. It is therefore unfortunate that the most useful thing that seismologists could do – predict earthquakes – is what they are least able to do. Although many ideas for earthquake prediction have been explored, the sad truth is that reliable prediction of damaging earthquakes is not currently possible on any time scale. In this section we will present some of the terminology and concepts in earthquake occurrence modeling and discuss possible reasons why major earthquakes are so difficult to predict.

The earthquake cycle

The idea that earthquakes represent a sudden release of accumulated stress in the crust was first documented by H. F. Reid, who examined survey lines taken before and after the 1906 earthquake in San Francisco. His results led to the elastic rebound theory of earthquake occurrence, in which stress and strain increase gradually and are then released during an earthquake by sudden movement along a fault (Fig. 10.1). This mechanism is now recognized to be the primary cause of earthquakes in the crust. Earthquakes occur mostly along the boundaries between Earth's surface plates (see Fig. 1.2), releasing the stress that results from the relative tectonic motion between the different plates. Observations of surface deformations, using ground- and satellite-based surveying techniques, can be used to monitor the slow strain changes that are seen in seismically active regions (often termed the secular strain rate) and the sudden change that occurs in the deformation field during earthquakes (these are termed co-seismic changes).