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fault rupture within the Australian plate caused several moderate earthquakes (76), and geologic reconnaissance has uncovered evidence of surface rupture associated with other intraplate earthquakes there within the past several thousand years. Paleoseismic investigations of the surficial fault ruptures associated with the earthquakes reveal that the fault had not moved for 50,000 to 100,000 years or more before the recent event (77).
Though they can occur far from plate boundaries, most intraplate earthquakes are still caused by plate-tectonic forces. The patterns of the tectonic stress that drive intraplate seismicity have been mapped using a variety of indicators—wellbore breakouts, volcanic alignments, and earthquake focal mechanisms—and their orientations are coherent over distances of 400 to 4000 kilometers (Figure 3.22). These observed trajectories generally match the predictions of intraplate stress from dynamic models of plate motions in which the primary driving forces are ridge push (compression due to gravitational sliding of newly formed lithosphere away from mid-ocean ridge highs) and slab pull (tension due to the gravitational sinking of the old subducting slabs). The spatial and temporal patterns of intraplate seismicity remain poorly understood, however. In some cases, the stress that causes these earthquakes may come from nontectonic sources, such as the withdrawal of large continental ice sheets. Reservoir loading and subsequent water infiltration are significant factors in generating some intraplate earthquakes.
3.4ESTIMATING EARTHQUAKE RISK
Earthquake loss estimates are forecasts of damage and human and economic impacts that may result from future earthquakes. Seismic retrofitting and earthquake-resistant design involve substantial investments (Section 1.1), so it is necessary to measure the consequences of earthquakes in economic terms (i.e., dollars) that allow rational trade-offs between the known costs of preparation and the anticipated losses. The methods for constructing loss estimates have thereby become important tools for disaster preparation and decision making. Communities have begun to use such estimates in setting priorities for mitigation efforts (e.g., identifying specific structures for seismic retrofits) and developing contingencies for earthquake emergencies (e.g., alternative transport routes). Governments and insurance companies employ such estimates to anticipate the financial impact of earthquake damage. Rescue and response organizations are devising systems that make and revise damage projections in near real time based on seismic information received immediately after an earthquake, so they can focus their postseismic response where it will be most needed.