Characterization of Swarm and Mainshock–Aftershock Behavior in Puerto Rico

Abstract

The recent Indios, Puerto Rico earthquake sequence has drawn attention, as the increased seismicity rate in this area was unprecedented. The sequence began on 28 December 2019, caused a 6.4 magnitude earthquake on 7 January 2020, and remained active over a year later. This sequence fits the nominal definition of an earthquake swarm in that it had an abrupt onset, a sustained high rate of seismicity without a clear triggering mainshock or evidence for Omori decay, and a lack of adherence to Bath’s law. However, the sequence also had several prominent mainshock–aftershock (MS–AS) sequences embedded within it. We applied three-station waveform cross correlation to the early part of this sequence using the Puerto Rico Seismic Network (PRSN) catalog as templates, which confirmed the mixture of swarm and MS–AS patterns. In an effort to place this intriguing sequence in the context of the previous seismicity in Puerto Rico, we investigated the existence of swarms and MS–AS sequences recorded by the PRSN since 1987 by identifying sequences with increased seismicity rate when compared to the background rate. About 59 sequences were manually verified and characterized into swarms or MS–AS. We found that 58% of the sequences follow traditional swarm patterns and 14% adhere to traditional MS–AS behavior, whereas 29% of the sequences have a mixture of both swarm and MS–AS behaviors. These findings suggest that it is not unusual for the Indios sequence to have a mixture of both the characteristics. In addition, the detection of many swarms distributed over a broad area of the subduction interface indicates stress heterogeneity and low-coupling consistent with prior studies indicating that the potential for a magnitude ∼8 megathrust earthquake along the Puerto Rico trench is unlikely.

Swarms and mainshock–aftershocks sequences are both triggered by fluids in the Ubaye Region (Western Alps)

“…For example, some sequences involve several cascading mainshocks, with a global swarm pattern (Colfiorito sequence, Chiaraluce et al 2003; Ubaye region, Thouvenot et al 2016;Western Bohemia, Hainzl et al 2016, Jakoubková et al 2018Corinth Gulf, Kaviris et al 2021). In the Ubaye Region (De Barros et al 2019) as well as in the Puerto Rico area (Ventura-Valentin and Brudzinski, 2021) or the Corinth Gulf (Kaviris et al 2021), authors observe that a mainshock started a seismic crisis, but with an abnormal number of aftershocks that did not decay according to the expected Omori-Utsu's law and contained small swarm sequences. In such cases, complex processes involving tectonic stress release as well as fluids and/or aseismic-slip are required.…”