Local earthquake tomography with flexible gridding

In local earthquake tomography (LET) problems, the spatial resolution of ve locity structure varies significantly in a model due to the naturally heter ogeneous distribution of earthquake sources. We present a flexible gridding strategy for tailoring the velocity model parameterization to the dataset being analyzed. The strategy combines regular gridding with two forms of no de 'linking' to allow for finer-scale resolution where warranted by the dat a, to provide improved model stability where resolution is relatively poor and to allow for the incorporation of a priori information on structure, fo r example the location of significant discontinuities. Flexible gridding ha s been incorporated into a widely-used LET inversion algorithm. The inversi on approach is documented along with a discussion of other recent advances and improvements to the algorithm. We discuss various situations in which f lexible gridding can be useful. An example application of flexible gridding to a dataset from the San Andreas fault in central California illustrates how the approach can yield a sharper image of the fault while at the same t ime improving the overall quality of the model and the fit to the data. (C) 1999 Elsevier Science Ltd. All rights reserved.