Imaging faults in three dimensions from earthquakes, with examples from southern California and Taiwan.

摘要

Knowledge of the 3-D shape of active faults is of fundamental importance in many fields, from earthquake hazard analysis, to oil exploration, to regional tectonics and seismotectonics. Yet, until recently, detailed 3D fault modeling was limited to the shallow subsurface due to a lack of both basic data and tools.{09}Several new techniques in 3-D fault modeling are described, whose applications range from models of single fault surfaces or small groups of faults, which can be used in earthquake hazard evaluation, to regional tectonic models.; An example of how these new techniques can be applied to single, small structures is that of the Northridge thrust, western Transverse Ranges, southern California. The 3-D geometry of the fault that generated the M 6.8, 1994 Northridge earthquake was determined from the aftershocks of this event. It was also possible to determine the geometry of several nearby faults, some of which were previously unknown and are capable of producing damaging earthquakes.; Larger fault systems can be modeled too, and interactions between faults studied in detail. This thesis presents as an example the case of the San Andreas fault system near San Gorgonio Pass, eastern Transverse Ranges, southern California. The fault network was imaged using over 43,000 relocated small earthquakes. Knowledge of the fault geometry was then applied to the study of possible earthquake triggering scenarios to determine the likelihood of a major rupture of the San Andreas from the Salton Sea to east of Los Angeles.; A study of the 3-D structure of the crust in central Taiwan, where the M 7.6, 1999 Chi-Chi earthquake produced a large number of aftershocks, is an example of modeling applied to regional tectonics, and mountain building in particular. For the first time the large detachment beneath Taiwan was imaged. This detachment had been postulated by several authors, but never seen before. Most faults capable of producing major earthquakes are connected to this detachment at depth. The results obtained allowed us a new test of critical-taper wedge mechanics, and suggest the shape of the detachment controls the reversal of topographic slope across Taiwan.