Location of Microearthquakes Induced by Hydraulic Fracturing

This paper examines the problem of locating microearthquakes induced by hydraulic fracturing using seismic arrival time data. In addition to the use of absolute arrival times measured for individual events, we consider the use of differential arrival times amongst a set of two or more seismic events as a means of constralning their locations relative to one another. Differential arrival times can be measured very accurately using cross-correlation techniques and are less sensitive than absolute arrival times to subsurface velocity structure. We have developed an algorithm which combines relative event location techniques with conventional absolute location techniques and applied it to a set of 19 microearthquakes recorded during a hydraulic fracturing experiment conducted as part of the Los Alamos Hot Dry Rock project. We find that the events, except for a few outliers, delineate a planar zone 30 meters in dimension, presumably a fracture plane. This example shows that the use of differential arrival times improves the accuracy of locating microearthquake clusters and that the relative locations of events within the cluster are better determined than their absolute locations. The results also suggest the need for directional data from three-component stations or better station geometry to further improve location accuracy.