Local earthquake tomography with flexible gridding

Abstract

In local earthquake tomography (LET) problems, the spatial resolution of velocity structure varies significantly in a model due to the naturally heterogeneous 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 node `linking' to allow for finer-scale resolution where warranted by the data, to provide improved model stability where resolution is relatively poor and to allow for the incorporation of a priori information on structure, for example the location of significant discontinuities. Flexible gridding has been incorporated into a widely-used LET inversion algorithm. The inversion approach is documented along with a discussion of other recent advances and improvements to the algorithm. We discuss various situations in which flexible 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 time improving the overall quality of the model and the fit to the data.