Regional Seismic Travel Time (RSTT) Tomography

The Regional Seismic Travel Time (RSTT) tomography model has been developed to improve travel time predictions for regional phases (Pn, Sn, Pg, Lg/Sg) in order to increase seismic location accuracy. In the past, the use of regional, along with teleseismic, phases in routine seismic location has been shown to degrade the overall accuracy of the resulting event. The RSTT model is specifically designed to permit the use of these regional phases for location, especially when combining them with teleseismic arrivals. Previously, the RSTT model has been calibrated for specific geographic regions (North America, Eurasia) only. In a cooperative project with the Comprehensive Test-Ban-Treaty Organization (CTBTO), we are currently expanding the RSTT model to be as globally calibrated as data will allow for all regional phases. Using a base set of ~3 million arrivals, we create summary rays (806,796 Pn, 193,786 Sn, 78,152 Pg, and 39,204 Lg/Sg) that are used to define a global tomographic model for predicting travel times. We fully describe the tomography methodology required to create the RSTT model, including smoothing and damping constraints. The RSTT model is defined on a tessellated grid, with a velocity profile at each node from the crust into the upper mantle. An upper mantle gradient is also defined allowing for the presumption of diving rays in a velocity gradient. The tomographic method for Pn and Sn includes solving for laterally-varying upper mantle slowness and gradient, while Pg and Lg/Sg are similarly determined by solving for a mid-crustal slowness, allowing for downgoing/upgoing components of the rays in the crust. We also solve for an overall crustal modifier that accounts for small variations in the crust beyond the starting velocity model.