Misconception of Waveform Similarity in the Identification of Repeating Earthquakes

• Published in: Geophysical research letters Vol. 48; no. 13
• Main Authors: Gao, Dawei, Kao, Honn, Wang, Bei
• Format: Journal Article
• Language: English
• Published: 16.07.2021
• Subjects: differential‐traveltime double‐difference (DTDD) method; equivalent rupture radius; interevent separation; neighboring earthquakes; repeating earthquakes; waveform similarity
• ISSN: 0094-8276; 1944-8007
• DOI: 10.1029/2021GL092815

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. Plain Language Summary Repeating earthquakes (repeaters) are events that occur repeatedly on the same fault patch with the same fault directional motion and similar amount of slip. They provide important insights into a variety of geophysical subjects such as fault behavior, subsurface structure change, inner core rotation, and nucleation process of earthquakes and landslides. The identification of repeaters is usually solely based on waveform similarity, but the criteria can vary significantly from one case to another. With both synthetic and real data, we find that waveform similarity is controlled by many factors, in addition to interevent distance. Therefore, higher degree of waveform similarity does not necessarily imply a smaller hypocenter separation, and vice versa. Our results undoubtedly suggest that waveform similarity alone is insufficient for repeater identification. To more reliably identify repeaters, we should rely on the overlap of the source areas. Quantitatively, we define a repeating pair if their interevent distance is smaller than the rupture area of the larger event. Our results imply that previously identified repeaters and associated interpretations/hypotheses may be unreliable and hence need a systematic reexamination. Key Points There is no simple relationship between cross‐correlation coefficient (CC) and interevent separation CC is affected by many factors and thus lacks the resolution to determine two events as true repeating or just neighboring earthquakes To reliably identify repeating earthquakes, we should rely on the precise estimation of both source dimension and interevent distance