Improving sparse network seismic location with Bayesian kriging and teleseismically constrained calibration events

Monitoring the Comprehensive Nuclear-Test-Ban Treaty will require improved seismic location capability for small-magnitude events, The International M onitoring System (IMS) is well suited to locate events that are large enoug h to be recorded at teleseismic distances. However, small events are likely to be recorded on a sparse subset of IMS stations at regional- to upper-ma ntle distances (less than 30 degrees), and sparse-network locations can be strongly effected by travel-time errors that result from path-specific velo city model inaccuracies. In an effort to improve sparse network location ca pability, we outline a procedure that applies empirical corrections to trav el times determined with an appropriate velocity model. More specifically, Bayesian kriging and calibration events (constrained with a global network) are used to estimate epicenter-specific travel-time corrections. For a tes t (sparse) network of stations, we calculate travel-time residuals for the calibration events relative to the ak135 velocity model. Travel-time residu als are assigned to the respective calibration epicenter, forming a set of spatially varying travel-time correction points. The spatial set of correct ion points is declustered to reduce the dimension of the observations with minimal reduction in accuracy of the travel-time corrections. We then use t he declustered set of calibration points and Bayesian kriging to form conti nuous travel-time correction surfaces for each station of the test network. The effectiveness of travel-time correction surfaces is evaluated by locat ing, with and without corrections, a subset of the 1991 Racha earthquake se quence (Caucasus Mountains), for which we have accurate locations that were independently determined with a dense local network. When no travel-time c orrection is applied, the mean horizontal distance between the local and te st network locations is 42 km, and there is a distinct bias in sparse-netwo rk locations toward the north-northwest. The mean difference between local and sparse network locations is cut to 13 km when corrections are applied, and the bias in location is significantly reduced. When calibration events in the Racha vicinity are not used to make the correction surfaces, there i s still a significant improvement in location, with mean mislocations of 15 km, When corrections are not applied, only one of the locally determined l ocations lies within the associated 90% coverage ellipse determined with th e test (sparse) network. However, by using traveltime corrections and estim ates of model uncertainty determined using kriging, representative error el lipses are obtained. This study demonstrates that kriging correction surfac es based on global-network-constrained calibration events can improve the a bility to accurately locate lower magnitude events while providing represen tative coverage ellipses.