Improved relative locations of clustered earthquakes using constrained multiple event location

A new method for improving relative locations of clustered earthquakes is p resented and applied to a suite of microearthquakes induced by hydraulic fr acturing. The method is based on the assumption that clustering of earthqua ke hypocenters is obscured by the uncorrelated scatter of individual hypoce nters, The method is implemented as an additional constraint in a Joint Hyp ocenter Determination (JHD) scheme. The method shifts event hypocenters tow ard the center of mass of the events within some volume surrounding the eve nt location if the RMS misfit between predicted and measured arrival times does not increase significantly. The method uses the same basic assumption of Jones and Stewart (1997), which is that there is greater clustering in a ctual earthquake locations than there is in locations determined using conv entional techniques, Our method differs in that it is implemented as part o f the JHD process so it operates on raw travel-time data rather than on der ived hypocenters. The method produces hypocenters from a demonstration fiel d dataset that are similar to those obtained by Phillips Et al. (1997), fro m time-consuming precise manual repicking of relative arrival times of even ts. The clustering constraint can easily be incorporated as an additional c onstraint in earthquake location/velocity tomography codes and may lead to improved velocity structure determination and earthquake location pattern i dentification and interpretation.