Interplay of seismic and a-seismic deformation during the 2020 sequence of Atacama, Chile.

Abstract

An earthquake sequence occurred in the Atacama region of Chile throughout September 2020. The sequence initiated by a mainshock of magnitude Mw6.9, followed 17 hours later by a Mw6.4 aftershock. The sequence lasted several weeks, during which more than a thousand events larger than Ml 1 occurred, including several larger earthquakes of magnitudes between 5.5 and 6.4. Using a dense network that includes broad-band, strong motion and GPS sites, we study in details the seismic sources of the mainshock and its largest aftershock, the afterslip they generate and their aftershock, shedding light of the spatial temporal evolution of seismic and aseismic slip during the sequence. Dynamic inversions show that the two largest earthquakes are located on the subduction interface and have a stress drop and rupture times which are characteristics of subduction earthquakes. The mainshock and the aftershocks, localised in a 3D velocity model, occur in a narrow region of interseismic coupling (ranging 40%-80%), i.e. between two large highly coupled areas, North and South of the sequence, both ruptured by the great Mw~8.5 1922 megathrust earthquake. High rate GPS data (1 Hz) allow to determine instantaneous coseismic displacements and to infer coseismic slip models, not contaminated by early afterslip. We find that the total slip over 24 hours inferred from precise daily solutions is larger than the sum of the two instantaneous coseismic slip models. Differencing the two models indicates that rapid aseismic slip developed up-dip the mainshock rupture area and down-dip of the largest aftershock. During the 17 hours separating the two earthquakes, micro-seismicity migrated from the mainshock rupture area up-dip towards the epicenter of the Mw6.4 aftershocks and continued to propagate upwards at ~0.7 km/day. The bulk of the afterslip is located up-dip the mainshock and down-dip the largest aftershock, and is accompanied with the migration of seismicity, from the mainshock rupture to the aftershock area, suggesting that this aseismic slip triggered the Mw6.4 aftershock. Unusually large post-seismic slip, equivalent to Mw6.8 developed during three weeks to the North, in low coupling areas located both up-dip and downdip the narrow strip of higher coupling, and possibly connecting to the area of the deep Slow Sleep Event detected in the Copiapo area in 2014. The sequence highlights how seismic and aseismic slip interacted and witness short scale lateral variations of friction properties at the megathrust.

Interplate Coupling and Seismic Potential in the Atacama Seismic Gap (Chile): Dismissing a Rigid Andean Sliver

“…Recently, Klein et al. (2021) analyzed the interplay between a seismic sequence and afterslip triggered by a M w 6.9 mainshock that struck at the Barranquilla LCZ during 2020. Their seismic and aseismic slip areas are located within a region where our coupling evaluations range from 0.45 to 0.6, and where afterslip extends to an updip limit delineated by a transition to larger coupling values along the megathrust (Figure 3).…”

“…Seismicity from National Seismological Center catalog with M ≥ 2.5 for period 2010–2016. Colored contour lines indicate the slip of the 1995 ( M w 8.0) Antofagasta earthquake (Pritchard et al., 2006), Copiapo slow slip event (Klein, Meneses, et al., 2018) and 2020 seismic sequence and afterslip from (Klein et al., 2021). Fore‐arc faults traces from SERNAGEOMIN (2003) in black dashed lines.…”

A 20 year-long GNSS solution across South-America with focus in Chile

“…Throughout Chile, the national data set is also complemented by local dense networks aiming at studying specific questions in targeted areas (e.g. Klein et al, 2021). In addition and in order to realise a continental scale processing, we include data of a selection of stations from the Argentine RAMSAC network (Piñón et al, 2018) and of the Brazil RBMC network.…”

“…GPS, sometimes associated with InSAR, also revealed transient deformation at different time and space scales, associated with seismic swarms and/or a-seismic deformation (Pritchard and Simons, 2006, Klein et al, 2018, Klein et al, 2021 and the post-seismic deformation that immediately follows an earthquake which can reach several millimeters per year, as far as thousands of kilometers away from the epicenter and last for decades or even longer (Khazaradze et al, 2002, Suito and Freymueller, 2009, Trubienko et al, 2013, Klein et al, 2016, Hu et al, 2016. The latter results from the relaxation of stress in the underlying viscous mantle and takes place at the continent scale.…”

Along‐Dip Segmentation of the Slip Behavior and Rheology of the Copiapó Ridge Subducted in North‐Central Chile

“…Large megathrust earthquakes also occurred in the past, the largest one in 1922 (Mw ∼ 8.5–8.8), and several moderate earthquakes have occurred more frequently for example, 1796, 1918, 1983 (Beck et al., 1998; Kanamori et al., 2019; Ruiz & Madariaga, 2018). The last event, with Mw 6.9, happened on 1 September 2020 and was part of a remarkable seismic and a‐seismic sequence (Klein et al., 2021) (Figure 1). Most ruptures of moderate magnitude earthquakes (<7.5) that initiated north of the CoR stopped at the CoR, although the large megathrust earthquake of 1922 appears to ruptured through it.…”