Anti-repeating earthquakes and how to explain them

Repeating earthquakes, or repeaters, affecting overlapping rupture patches with a similar focal mechanism, have important implications to track fault slip rates, aseismic deformation, slow earthquakes and earthquake nucleation processes. They are often detected based on highly similar waveforms. Here, we discuss earthquakes with highly anti-correlated waveforms, denoting a reversed seismogenic process at the same or a neighbouring location, which we refer to as true and quasi anti-repeaters. We first report a range such observations in different environments, including volcano seismicity, intermediate depth seismicity and injection-induced microseismicity. Then, we review conceptual models proposed to explain them. True and quasi anti-repeaters can be robustly identified via a three-component single station or distributed network data. They are key indicators for stress perturbation transients or local stress heterogeneities. Since most of these observations were explained as the response to fluid migration processes, they may help to identify and track fluid movements in the subsurface.