1. A method of identifying passive seismic events in seismic data, the seismic data comprising at least first seismic data traces acquired at a first seismic receiver and second seismic data traces acquired at a second receiver spatially separated from the first receiver, the method comprising: determining an overall measure of similarity for a pair of passive seismic events in the seismic traces, the overall measure of similarity being indicative of similarity between the events acquired at the first seismic receiver, and of similarity between the events acquired at the second seismic receiver.