Anti-repeating earthquakes in North Chile: a very rare observation

Repeating earthquakes are an expression of repeated activation of the same fault patch (asperity), with full or significant rupture area overlap. They can be used to locally characterize slip behavior, dynamics, and fine structure of faults at depth which are inaccessible with other methods. Very recently, the enigmatic observation of so called anti-repeating earthquakes has resurfaced, which is the observation of repeater-like waveforms that bear opposite amplitude signs. Observations of such a phenomenon are rare, and the known examples are today limited and in parts disputable.
This is partly because identification and characterization of repeaters requires a sound analysis in order to discriminate between neighboring earthquakes and repeaters which truly activate identical fault-patches.

Here we used a recently compiled repeating earthquake catalog for North Chile using comparatively strict repeater identification criteria, such as the utilization of a time window that covers both p- and s-phase and a cc≥0.95 for a passband of 1-8Hz at at least two stations. It consists of 10,706 repeating earthquakes that are part of 3,179 sequences. We have explicitly searched for anti-repeating sequences, which are sequences, that include at least one event with flipped waveforms at all seismic stations. Among all the repeater sequences, we find only 4, which show such a behavior. Of those, 3 are doublets (pairs of two) and only one group of 34 events contains multiple anti-repeaters. We show detailed analysis of that group, applying highly precise relative relocation and clustering methods.

The existence of anti-repeating earthquakes raises the question on the in-situ nucleation and rupture conditions required to produce the phenomenon. While apparently being an absolute exception, better understanding their circumstances will potentially improve our understanding of earthquake source mechanisms and the subduction system.