Model and Methods for Regional Travel-Time Calculation

摘要

This project develops a model and methods for routine computation of regional travel times for crustal events anywhere on the globe. To improve on existing methods, the travel time calculations must capture the effect of the three-dimensional (3D) earth, yet the computation must be exceedingly efficient. We achieve global coverage by defining a seamless global tessellation of nodes with spacing of approximately 1 degree. Three-dimensional crustal structure is captured by interpolating P- and S-velocity depth profiles at each node. Mantle structure is approximated by a linear velocity gradient (as a function of depth) at each node. The linear gradient parameterization in the mantle enables an analytical approximation for the diving Pn/Sn ray that allows computation of travel times in approximately 1 millisecond. Regional Pg and Lg propagation are approximated with a ray traveling horizontally along a mid-crustal layer. At local distance, P and S travel times are computed using the layered velocity structure under the station. The starting model is a hybrid of the Lawrence Livermore National Laboratory (LLNL)/Los Alamos National Laboratory (LANL) unified model, which is a 3D geophysical compilation spanning Eurasia and North Africa, and CRUST2.0 elsewhere. These 3D models are adapted to the linear gradient parameterization using mantle velocities at the Moho and at 130-km depth. At 130-km depth the velocity of is predominantly derived from 2SMAC. We refine the Eurasian and North African portion of the model using a tomographic formulation that adjusts the average crustal velocity, Pn and Sn velocity, and the mantle gradient at each node. Our tomographic data consists of approximately 700,000 validated regional arrivals from events with known locations or locations meeting accuracy criteria.