Reverse-time imaging of a doublet of microearthquakes in the three gorges reservoir region

When a doublet of closely spaced microearthquakes occur nearly simultaneously, their overlapping waveforms complicate the picking of first-arrival times and challenge traveltime-based location methods to estimate the location and origin time of each event in the doublet. The reverse-time imaging (RTI) method locates earthquakes by backpropagating the reconstructed wave fronts of P and S waves to the source locations thereby removing the need to pick first arrivals. Using an elastic RTI method, we successfully estimated the locations and origin times of a doublet of microearthquakes: an M_L0.1 event reported by local earthquake monitoring system and second microearthquake that occurred 0.7 s earlier that was missed by the official monitoring network in the Three Gorges Reservoir region, China. We introduce a time-depth constrain to estimate the origin time and to evaluate how robust the imaged source location is with respect to possible errors in the velocity model. For our case, a model with slightly lower velocities will result in an earlier origin time but will have less effect on the estimated source depth. Results of this study suggest that, by applying the RTI method it is possible to locate doublets with overlapping first arrivals.