Improving event location accuracy at the IDC using RSTT-based travel time corrections

Prediction of seismic travel times at the International Data Centre (IDC) of the Comprehensive Nuclear-Test Ban Treaty Organization (CTBTO) has been based until recently on the one-dimensional IASPEI91 travel-time curves for teleseismic and regional phases, with the addition of some local or regional models for regional and local phases in some areas (North America and Eurasia). Since IASPEI91 is not universally applicable in a heterogeneous Earth, travel-time predictions are further corrected to account for, among others, the Earth’s ellipticity, station elevation, and source-specific effects, including regional geology.

In order to improve travel time predictions, especially for regional phases for which the prediction error is most prominent, the IDC is now using travel time corrections based on the Regional Seismic Travel Time (RSTT) velocity model first introduced by Lawrence Livermore National Labs to account for the source-specific effects. The RSTT velocity model is a global model that approximates a 3D crust and upper mantle and is based on ground truth (GT) events recorded globally.

Examination of one year (August 2020 until August 2021) of the Reviewed Event Bulletin (REB) shows that the use of these RSTT-based travel time corrections has improved the precision of event location as measured by a) travel time residuals of regional phases, b) the number of defining regional phases according to the stringent IDC event definition criteria and c) comparison of events similar in magnitude and location in the periods before and after the application of the RSTT-based corrections. Although the improvement is seen worldwide, it is more prominent for stations in areas such as Australia and Africa, where previously the travel time corrections were based only on the IASPEI91 curves, that is, there were no local or regional velocity models available.