Evidence consistent with a multiple‐event mechanism for large earthquakes

Abstract

We analyzed the short‐period codas of 418 small events (NOS mb ≤ 5.8) and 148 large‐events (NOS mb, NOS Ms, or mb from Pasadena or Berkeley ≥7.0). The normalized large‐event codas are found to be significantly larger at any given time after the first‐arrival onset than corresponding values for small‐event codas. This suggests that large events are multiple events, and that the occurrence of later events in a given sequence retards coda decay and elevates the relative amplitudes of the codas above those expected for a small (single) event. Large events are also found to be emergent, displaying a 0.2 to 0.3 mb increase in amplitude between 5 and 30 seconds after the P wave arrival over that observed in the first 5 seconds after the arrival. Coda decay characteristics for large events may be used to estimate the length of the fault zone. We suggest that multiple events are a more common phenomenon than has been generally suspected, and that most if not all events of high or intermediate Ms are probably multiple events.