Development of double-pair double difference earthquake location algorithm for improving earthquake locations

Event-pair double-difference (DD) earthquake location method, as incorporated in hypoDD, has been widely used to improve relative earthquake locations by using event-pair differential arrival times from pairs of events to common stations because some common path anomalies outside the source region can be cancelled out due to similar ray paths. Similarly, station-pair differential arrival times from one event to pairs of stations can also be used to improve earthquake locations by cancelling out the event origin time and some path anomalies inside the source region. To utilize advantages of both DD location methods, we have developed a new double-pair DD location method to use differential times constructed from pairs of events to pairs of stations to determine higher-precision relative earthquake locations. Compared to the event-pair and station-pair DD location methods, the new method can remove event origin times and station correction terms from the inversion system and cancel out path anomalies both outside and inside the source region at the same time. The new method is tested on earthquakes around the San Andreas Fault, California to validate its performance. From earthquake relocations it is demonstrated that the double-pair DD location method is able to better sharpen the images of seismicity with smaller relative location uncertainties compared to the event-pair DD location method and thus to reveal more fine-scale structures. In comparison, among three DD location methods, station-pair DD location method can better improve the absolute earthquake locations. For this reason, we further propose a hybrid double-pair DD location method combining station-pair and double-pair differential times to determine accurate absolute and relative locations at the same time, which is validated by both synthetic and real data sets.