Temporal Variations in Seismic Attenuation: A 3D Pre- and Post-Event Tomography of the region around the 2020 Río Loa Mw 6.2 Earthquake (Chile)

The Río Loa earthquake (Mw 6.2), which occurred at a depth of ~56 km in northern Chile on September 11, 2020, was inferred to have happened within the South American upper plate although it was located in the direct vicinity of the plate interface. This was mostly due to its strike-slip focal mechanism, which is distinct from the typical megathrust seismicity observed along the subduction interface. According to Tassara et al. (2022), this earthquake may reflect the fluid-driven rupture of a fault zone and the release of megathrust fluids into the upper plate, a process similar to that observed in the aftershock sequence of the 1995 Antofagasta earthquake (Mw 8.0).

In this study, we aim to perform a comprehensive analysis of temporal variations in seismic attenuation by conducting a comparative 3D tomography of the region before and after the Río Loa earthquake. This approach aims to detect potential changes in the attenuation structure, which could provide insights into stress redistribution, fluid migration, and fault zone evolution triggered by the event.

Seismic attenuation is highly sensitive to temperature variations, fluid presence, and the degree of fracturing within the crust and mantle. Changes in attenuation following a significant seismic event can indicate perturbations in these properties, reflecting enhanced permeability or increased pore fluid pressure in the surrounding rock. This study leverages data from a recent extension of the seismicity catalog of Sippl et al. (2023), which comprises over >200,000 events recorded between 2007 and 2023, with dense station coverage from the Integrated Plate boundary Observatory Chile (IPOC) and temporary deployments.

By applying the coda normalization method and the Multi-Resolution Attenuation Tomography (MuRAT) algorithm (Sketsiou et al., 2021), we obtain high-resolution attenuation models of the forearc region surrounding the Río Loa earthquake. Our inversion process uses ray paths traced through the 3D velocity model of Hassan et al. (2024) to estimate total-Q values. A key focus is the analysis of anomalies in attenuation that may coincide with the mainshock rupture plane or regions exhibiting aftershock clustering.