Locating nonvolcanic tremors beneath the San Andreas Fault using a station-pair double-difference location method

It has been a challenging task to locate nonvolcanic tremors because of the lack of impulsive wave arrivals. A station-pair double-difference (DD) location method is developed to determine absolute tremor locations by directly using the station-pair travel time differences measured from cross-correlating tremor waveform envelopes. Multiple tremors are located together for inverting for station corrections to take into account velocity model inaccuracy. The new method is applied to tremors in the Parkfield region of central California between 27 July 2001 and 21 February 2009. Compared to the tremor catalog locations determined from a grid search location method, most of the newly located tremors are located at depths between 20 and 35 km, well below the seismogenic zone in the area. The tremors beneath Cholame, CA are more clearly separated into two zones laterally distributed across the San Andreas Fault, with most tremors occurring to the southwest and exhibiting a periodic pattern of occurrence. The new tremor locations help better delineate the spatial and temporal distribution of tremor activity and therefore are helpful for better understanding tremor origin and process. Citation: Zhang, H., R. M. Nadeau, and M. N. Toksoz (2010), Locating nonvolcanic tremors beneath the San Andreas Fault using a station-pair double-difference location method, Geophys. Res. Lett., 37, L13304, doi:10.1029/2010GL043577.