Real-time seismic warning with a two-station subarray

One of the most basic problems in seismology is earthquake location. In particular, the ability to quickly locate a large, potentially devastating earthquake is of fundamental importance in a real-time warning system where speed is a key factor in determining the level of success of such systems. We have developed a simple method that uses only the two earliest P-wave arrival times in a seismic array. Assuming a simple velocity model, these arrivals are used to construct a hyperbolic curve on which the approximate epicenter of the earthquake is expected to lie. Epicentral location along this hyperbola is further constrained by using the fact that the P waves arriving at the other stations in the array are not first arrivals. When applied to P-wave seismic data from the Hector Mine earthquake in California and a smaller event in the central United States, model results show agreement with actual earthquake locations. Although there is an inherent uncertainty in the subarray method of locating large earthquakes, this may be an acceptable trade-off in an early warning system in view of the time (a few to tens of seconds) saved by not waiting for other P arrivals. Whereas the main goal of this report is to present the location method, we also show that the station closest to the Hector Mine earthquake had recorded about 0.3 and 1 mm of ground motion within 2 and 3 sec, respectively, of the arrival of the P waves, thus indicating that a large event had occurred.