Evidence of a Transient Aseismic Slip Driving the 2017 Valparaiso Earthquake Sequence, From Foreshocks to Aftershocks

Abstract

Following laboratory experiments and friction theory, slow slip events and seismicity rate accelerations observed before mainshocks are sometimes interpreted as evidence of a nucleation phase. However, such precursory observations still remain scarce and are associated with different time and length scales, raising doubts about their actual preparatory nature. We study the 2017 Valparaiso Mw = 6.9 earthquake, which was preceded by aseismic slip accompanied by an intense seismicity, suspected to reflect its nucleation phase. We complement previous observations, which have focused only on precursory activity, with a continuous investigation of seismic and aseismic processes from the foreshock sequence to the post‐mainshock phase. By building a high‐resolution earthquake catalog and searching for anomalous seismicity rate increases compared to aftershock triggering models, we highlight an over‐productive seismicity starting within the foreshock sequence and persisting several days after the mainshock. Using repeating earthquakes and high‐rate GPS observations, we highlight a transient aseismic perturbation starting 1‐day before the first foreshock and continuing after the mainshock. The estimated slip rate over time is lightly impacted by large magnitude earthquakes and does not accelerate towards the mainshock. Therefore, the unusual seismic and aseismic activity observed during the 2017 Valparaiso sequence might be interpreted as the result of a slow slip event starting before the mainshock and continuing beyond it. Rather than pointing to a possible nucleation phase of the 2017 Valparaiso mainshock, the identified slow slip event acts as an aseismic loading of nearby faults, increasing the seismic activity, and thus the likelihood of a large rupture.

Migrating Foreshocks Driven by a Slow Slip Event Before the 2021 MW 6.1 Yangbi, China Earthquake

“…In other words, the regions outlined by the foreshocks or aseismic slip can be much larger than the eventual nucleation zone of the mainshock. As mentioned before, there are growing number of observations showing that foreshocks and slow slip can migrate over certain distances before megathrust earthquakes (Brodsky & Lay, 2014; Kato et al., 2012; Moutote et al., 2023; Ruiz et al., 2014; Schurr et al., 2014). GPS and passive seismic recordings on the Whillans Ice Plain in West Antarctica also revealed migratory pre‐slip and foreshocks over tens of kilometers right before the daily ice‐stream earthquake (Barcheck et al., 2021).…”

“…The primary difference is that the pre‐slip model would require that a prolonged aseismic slip process restricted by a critical nucleation length (Lc) at the eventual hypocenter of the mainshock (i.e., self‐nucleation) (Barcheck et al., 2021). In our migratory slow‐slip model, an aseismic slip would start days or weeks before, migrate along the strike, and drive both the foreshock and the mainshock (likely including aftershocks) (Moutote et al., 2023). In this case, the mainshock would likely initiate at a different location, rather than at the same spot where initial slow‐slip or foreshocks have started.…”

“…However, such precursory slip typically migrates over tens of kilometers, rather than near the hypocenter (i.e., self-nucleation). Moutote et al (2023) also argued that a slow-slip event drove the entire sequence (i.e., foreshocks, the mainshock and aftershocks) associated with the 24 April 2017, M6.9 earthquake west of Valparaiso, Chile, rather than simply acting during the nucleation phase of the mainshock.…”