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

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Emilie Klein , Bertrand Potin, Francisco Pasten-Araya , Roxane Tissandier , Kellen Azua, Zacharie Duputel, Carlos Herrera , Luis Rivera , Jean-Mathieu Nocquet , Juan-Carlos Baez, Dimitri Zigone , Raul Madariaga , Jean Paul Ampuero , Sergio Ruiz, Christophe Vigny


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.



Earth Sciences, Geophysics and Seismology, Physical Sciences and Mathematics


Atacama, GPS, Chilean subduction zone, earthquake sequence, post seismic


Published: 2021-07-01 04:48


CC BY Attribution 4.0 International

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Conflict of interest statement:

Data Availability (Reason not available):
Seismic data collected are available through the Incorporated Research Institutions for Seismology (IRIS) Data Management Center. The results of the W-phase analysis are available on the Supporting information. The relocated catalog will be made available at final publication. \\ The coseismic offset tables extracted from daily and HRGPS presented in the study are in the supporting information. Position time series of the sequence can be made available upon request.

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