Misconception of Waveform Similarity in the Identification of Repeating Earthquakes

Identification of repeating earthquakes (repeaters) usually depends on waveform similarity expressed as the corresponding cross-correlation coefficient (CC) above a prescribed threshold, typically ranging in 0.70-0.98. However, the robustness and effectiveness of such a strategy have rarely been fully examined. In this study, we examine whether CC is a valid proxy for repeater identification through both synthetic and real earthquake experiments. We reveal that CC is controlled by not only the interevent distance but also many other factors, including station azimuth, epicentral distance, and velocity structure. Consequently, CC lacks the resolution in identifying true repeaters. For reliable repeater identification, we should consider the interevent overlap. Specifically, we define an event pair to be true repeaters if their interevent separation is smaller than the source dimension of the larger event. Our results imply that a systematic recheck of previously identified repeaters and associated interpretations/hypotheses may be important and necessary.