An Elastic Rebound Model For Normal Fault Earthquakes

Abstract

Normal fault earthquakes and interseismic (secular) displacements are generated by a visco-elastic two­ dimensional finite element model. Model generated phases of crustal deformation are consistent with ob­served crustal deformation from repeated precise geodetic levels for the areas of Fairview Peak, Nevada and Hebgen Lake, Montana. The model fit to geodetic measurements is, in most cases, within the limits of random survey error. In this thesis I propose an elastic rebound theory for normal fault earthquakes. Model studies indicate that during the interseismic phase the ground is subject to relative doming in the vicinity of the fault which may have been interpreted by some investigators to result from magma intrusion. The rate at which doming occurs gives an indication of the asthenosphere effective viscosity, which was found to be on the order of 2.0 x 1021 poise for Fairview Peak and about 2.3 x 1021 poise for Hebgen Lake. Furthermore, interseismic model simulated extension rates are 1.36mm/yr, consistent with extension rates of greater than 0.4mm/yr observed within the Great Basin. The co­seismic phase results in the uplift of the footwall block and depression of the hanging wall block as has been reported by other investigators {Savage & Hastie, 1966). The sum of the interseismic and coseismic movements result in a tilt block type of topography, as is observed in the Basin and Range. Based on shear stress recovery the reoccurence interval for these faults is on the order of 10^3 years. Thus, the Fairview Peak and Hebgen Lake faults do not present any current earthquake hazard. However, since these faults do occur in regions where sets of normal faults are common, other faults may be approaching earthquake stress levels and consequently may present current earthquake hazards in these areas.