Case Studies of Seismic Tomography and Earthquake Location in a Regional Context

We present three case studies of traveltime tomography applied to regional imaging of the earth's crust and upper mantle. The first case study investigates the imaging of Poisson's ratio using measurements of compressional and shear wave traveltimes. Inversions schemes that jointly fit both types of traveltime data and that selectively damp the compressional to shear wave velocity ratio are shown to be very effective. The second case study focuses on the ability of joint inversions of crustal and Moho-reflected wave traveltimes to image both crustal velocity structure and Moho topography. The ability of this kind of tomography to distinguish Moho topography and lower crustal heterogeneity is shown to be poor. The third case study examines the ability of teleseismic tomography to determine the shape of a mantle plume. A new test for departures from an axial shape is proposed and tested. We also present a case study of earthquake location, that compares locations made with traditional P and S wave traveltimes to those based on differential P wave traveltimes (that is, the double-difference method). The underlying tomography software, “raytrace3d” that is used in this study and which is publicly and freely available is described, and the technical elements of traveltime tomography are fully documented in an appendix.