Double-difference tomography: The method and its application to the Hayward Fault, California

We have developed a double-difference (DD) seismic tomography method that makes use of both absolute and relative arrival times. By reducing systematic errors using the more accurate relative arrival times, the method produces an improved velocity model. Simultaneously, it yields event locations of a quality equivalent to those of the DD location method, while avoiding simplifying assumptions of that method. We test this method on a synthetic dataset and find that it produces a more accurate velocity model and event locations than standard tomography. We also test this method on a Hayward fault, California, earthquake dataset spanning 1984-1998. The earthquakes relocated by this method collapse to a thin line along the fault trace, consistent with previous results. The DD velocity model has sharper velocity contrasts near the source region than the standard tomography model.