Abstract
The regional seismic travel time (RSTT) model and software were developed to improve travel-time prediction accuracy by accounting for three-dimensional crust and upper mantle structure. Travel-time uncertainty estimates are used in the process of associating seismic phases to events and to accurately calculate location uncertainty bounds (i.e. event location error ellipses). We improve on the current distance-dependent uncertainty parameterization for RSTT using a random effects model to estimate slowness (inverse velocity) uncertainty as a mean squared error for each model parameter. The random effects model separates the error between observed slowness and model predicted slowness into bias and random components. The path-specific travel-time uncertainty is calculated by integrating these mean squared errors along a seismic-phase ray path. We demonstrate that event location error ellipses computed for a 90% coverage ellipse metric (used by the Comprehensive Nuclear-Test-Ban Treaty Organization International Data Centre (IDC)), and using the path-specific travel-time uncertainty approach, are more representative (median 82.5% ellipse percentage) of true location error than error ellipses computed using distance-dependent travel-time uncertainties (median 70.1%). We also demonstrate measurable improvement in location uncertainties using the RSTT method compared to the current station correction approach used at the IDC (median 74.3% coverage ellipse).
Highlights
For standard seismic event location in real-time, the fastest method for calculating travel times has been the use of one-dimensional (1D) Earth models that vary only with depth
The Regional Seismic Travel Time (RSTT) model and software was developed by the three United States National Nuclear Security Administration (NNSA) National Laboratories (Los Alamos National Laboratory, Lawrence Livermore National Laboratory, Sandia National Laboratories) in order to more accurately predict travel times from regional seismic phases ([ 15°–18°) that typically cause degradation in event location accuracy when combined with teleseismic phases (Z20°)
The assumptions that are built into the RSTT algorithm require that for a given source–receiver distance, the assumed turning-point depth of the ray will depend on the upper mantle velocity and the gradient, and is sensitive to the upper mantle gradient
Summary
For standard seismic event location in real-time, the fastest method for calculating travel times has. Been the use of one-dimensional (1D) Earth models that vary only with depth These 1D models have been used to calculate travel-time lookup tables for various seismic phases that only require the event depth and source–receiver distance as input. RSTT (Regional Seismic Travel Time) is a seismic velocity model and computer software package that captures the major effects of 3D crust and upper mantle structure on regional seismic travel times (Myers et al 2010) This model and software package was designed to be incorporated into real-time event location systems where travel times must be calculated in milliseconds on conventional computer hardware and to work seamlessly with travel-time predictions for teleseismic P-waves that are based on standard 1D models (e.g., iasp (Kennett and Engdahl 1991) and ak135 (Kennett et al 1995)). We describe the formulation of global, pathdependent, travel-time uncertainty estimates for the updated RSTT tomography model using a refined data set of Pn, Pg, Sn, and Lg phases (see companion paper ‘‘Updates to the Regional Seismic Travel Time (RSTT) Model: 1. Tomography’’, this issue)
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