The role of anti-repeating earthquakes in seismic sequences and swarms

Repeating earthquakes have overlapping rupture patches, similar focal mechanism and magnitudes. They are often detected based on highly similar waveforms using template matching techniques, which help to reconstruct complex sequences and swarms. Here, we investigate earthquakes with highly anti-correlated waveforms. Such poorly known observation implies the occurrence of reversed seismogenic processes at close hypocentral locations. We introduce the terms true and quasi anti-repeating earthquakes to denote cases affecting the same rupture patch or neighboring patches, respectively. We report about a number of observations of anti-repeating earthquakes in different environments, such as volcano, induced and intermediate-depth seismicity, and then review conceptual models to explain them. Some of these observations occurred during seismicity unrests, in the form of seismic sequences and swarms. Both true and quasi anti-repeating earthquakes are indicators for stress perturbation transients or local stress heterogeneities, often controlled by fluid migration processes. Therefore, their analysis may help the identification and tracking of fluids in the subsurface.