3D GEOMETRIC MODELS OF THE HAYWARD FAULT FOR USE IN DISTINCT-ELEMENT MODELS OF BAY AREA DEFORMATION AND SEISMICITY

We have developed a number of fault surface models by fitting curvi-planar surfaces to earthquakes on and adjacent to the Hayward fault (HF) in the Richmond, Oakland and Hayward, CA areas. The spatial accuracy of the earthquakes has been improved by applying double-difference relocating techniques. In the study area the Hayward fault is defined by a sparse to relatively dense ‘curtain’ of seismicity that extends from the mapped surface trace of the Hayward fault down to depths of about 15 km. By editing out earthquakes that are clearly off the main trace of the HF we are able to isolate events that define a clear curvi-planar surface that is the main or active trace of the HF. Using a number of different techniques – ranging from ‘eye-balling’ and defining the fault trace at various depths and then linking those lines into a plane, to perhaps more objective computer/numerical surface fitting and subsequent smoothing – we present a few different HF models. Differences between different model attempts are, not surprisingly, most significant in regions where data are most sparse.

These fault models are being used to define a realistic HF geometry for use with distinct-element that are currently in development. The distinct-element method treats a rockmass as a bonded - in tension and shear - assembly of frictional, elastic spheres. Loading the boundary of the assembly forces bonds between particles to fail in a progressive manner, very effectively simulating rock fractures, which subsequently link up to form thoroughgoing faults. The geometry of the Hayward fault, which has been constructed from relocated hypocenter locations, is introduced into the particle assembly as an irregular plane of particles that separates the intact blocks on either side of the fault. Fault surface asperities within the models will be sites of stress concentration and resist slip.

This work is based on the hypothesis that fault surface topography is the major factor that determines the location of earthquakes upon pre-existing faults. These models will predict possible locations of future faulting, both strike- and dip-slip.