A double-difference earthquake location algorithm: Method and application to the northern Hayward fault, California

We have developed an efficient method to determine high-resolution hypocent er locations over large distances. The location method incorporates ordinar y absolute travel-time measurements and/or cross-correlation P-and S-wave d ifferential travel-time measurements. Residuals between observed and theore tical travel-time differences (or double-differences) are minimized for pai rs of earthquakes at each station while linking together all observed event -station pairs. A least-squares solution is found by iteratively adjusting the vector difference between hypocentral pairs. The double-difference algo rithm minimizes errors due to unmodeled velocity structure without the use of station corrections. Because catalog and cross-correlation data are comb ined into one system of equations, interevent distances within multiplets a re determined to the accuracy of the cross-correlation data, while the rela tive locations between multiplets and uncorrelated events are simultaneousl y determined to the accuracy of the absolute travel-time data. Statistical resampling methods are used to estimate data accuracy and location errors. Uncertainties in double-difference locations are improved by more than an o rder of magnitude compared to catalog locations. The algorithm is tested, a nd its performance is demonstrated on two clusters of earthquakes located o n the northern Hayward fault, California. There it collapses the diffuse ca talog locations into sharp images of seismicity and reveals horizontal line ations of hypocenters that define the narrow regions on the fault where str ess is released by brittle failure.