Return of the Atacama deep Slow Slip Event: The 5-year recurrence confirmed by continuous GPS

Abstract

Four years ago, using survey GPS measurements, the first deep slow slip event (SSE) was detected in Chile (near Copiapó, Atacama region), unrelated to any major earthquake. It was located between 40 and 60 km depth on the subduction interface, lasted approximately 18 months (2014–2016.5) and reached an equivalent magnitude of Mw 6.9. The single permanent station operating in the region between 2002 and 2015 revealed that similar events had occurred at least twice before around 2006 and 2010, suggesting a 5-year repeat time. In anticipation of the next event expected for 2020, we densified the existing continuous GNSS network in the region with 5 new stations in early 2019. Here we show that the SSE occurred in 2020 as expected with the 5 year recurrence time. The event started around March 2020 and developed during 6 months, before it was perturbed by the 2020 Atacama seismic sequence that occurred nearby. During those initial 6 months, the 2020 event had the same characteristics as the 2014 SSE. It occurred in the same area and at the same depth, repeating a similar pattern of surface deformation. Before the occurrence of the nearby seismic sequence of September 2020, it had reached a third of the total amplitude of the 2014 SSE which had lasted three times longer. Whether the 2020 SSE was aborted when the nearby seismic sequence occurred or continued in the background is unknown but this will be resolved with longer times series.