Real-time back azimuth for earthquake early warning

The incentive to speed up real-time location has motivated previous researchers to go beyond standard location procedures and use not only P-wave arrival at some network stations but also its nonarrival at others. In addition to being sensitive to velocity model and picking uncertainties, this approach is also highly dependent on time delays due to unknowns network latencies, processing, and packet size. Thus, seeking ways to add independent real-time constraints on earthquake location are important for earthquake early warning applications. In this study, we assess the robustness of three independent real-time back-azimuth (BAZ) determination schemes, using offline records of southern California earthquakes.We find that BAZ values computed by the three methods provide equivalent levels of accuracy. By sending the three BAZ estimates to a screening module that checks for coherency and signal-to-noise ratio criteria, we show that accurate BAZ estimates are obtainable in real time, with a standard deviation of 13°. Through examination of two earthquake scenarios that use offline data, we show that the inclusion of BAZ estimates into real-time location schemes improves the performance of real-time hypocenter determination, by cutting the time it takes to obtain well-constrained hypocenters.