SOURCE ESTIMATION OF FINITE FAULTS FROM BROAD-BAND REGIONAL NETWORKS

Fast estimation of point-source parameters for earthquakes has progres sed much in recent years due to the development of broadband seismic n etworks. The expansion of these networks now provides the opportunity to investigate second-order effects such as source finiteness for regi onal and local events on a routine basis. This potential motivates the development of methods to quickly generate synthetic seismograms for finite sources. This is possible when the fault dimension is small com pared to the source-receiver distance and when the structure around th e source region is relatively simple. To study the directivity for a f inite source, we discretize the fault region into a set of elements re presented as point sources. We then generate the generalized rays for the best-fitting point-source location and derive for each separate ra y the response for neighboring point sources using power series expans ions. The response for a finite fault is then a summation over rays an d elements. If we sum over elements first, we obtain an effective far- field source-time function for each ray, which is sensitive to the dir ection of rupture. These far-field source-time functions are convolved with the corresponding rays, and the results are summed to form the t otal response. A simple application of the above method is demonstrate d with the tangential motions observed from the 1991 Sierra Madre eart hquake. For this event, we constrain the fault dimension to be about 3 km with rupture toward the west, which is compatible with other more detailed studies.