Proceedings of the 12th International INQUA meeting on paleoseismology, active tectonic and archaeoseismology

I N T R O D U C T I O N The Mejillones Fault (MF) is a prominent geological structure within the Atacama Fault System (AFS) in northern Chile. It is a major fault influencing the arrangement of geological units intheMejillonesPeninsula (MP) and poses significant seismic hazards, potentially triggering Mw 7.0 earthquakes (Cortés-Aranda et al., 2015; Vargas et al., 2011). Understanding its dynamics, depth, and historical role is crucial for assessing seismic risks, especially given the strategic economic importance of the nearby city of Mejillones and its port. We developed a shear-wave velocity model of the peninsula using the ambient noise tomography (ANT) technique. The present ANT allows us to characterize the geological units that have interacted with the MF and allow us to propose that the MF has been active since the Triassic, reaching depths of at least 5 km. Furthermore, the MF is identified as one of the most significant structures that control the lithological arrangement and morphological evolution of the MP. M E T H O D S A N D D ATA We employed ambient noise tomography (Bensen et al., 2007), a technique that utilizes the crosscorrelation of ambient seismic wavefields to estimate empirical Green’s functions between seismic stations. Using data from the MEJIPE seismological network (Salazar et al., 2013), which recorded continuously from July 2013 to September 2015, phase velocity dispersion curves were calculated to develop a shear wave velocity model of the Mejillones Peninsula. This approach enabled the detailed characterization of the crustal structures associated with the MF. R E S U LT S / D I S C U S S I O N The tomographic maps generated from the ANT revealed clear velocity anomalies correlated with surface geology. The western part of the MF showed lower shear wave velocities (~2.4 km/s) up to a depth of 1.5 km, linked to The Mejillones Metamorphic Complex (Calderón et al., 2017) and marine sediments (Fig. 1). The eastern part of MP exhibited higher velocities (>3.0 km/s) associated with Jurassic igneous rocks (Fig. 1). These differences persisted with depth, suggesting that theMF delineates the boundary between distinct geological zones. We observe intermediate velocities (~2.8 km/s) at a depth of up to 4 km around -23.25°. We propose that the main basin is predominantly filled with La Negra Formation (Fig. 1) based on the observed velocities (Christensen, 1996; Kushnir et al., 2018; Rosalia et al., 2022) and the lateralvertical stratigraphic distribution of La Negra Formation throughout the MP and Coastal Cordillera (Cortés et al., 2007). Several studies (e.g. Allmendinger andGonzález, 2010; González-Alfaro et al., 2018) show that fault activity has been concentrated in the northern part since at least the Miocene and is visible through the Faults scarp (Cortés et al., 2012). The results from this study suggest that recent fault activity would be reflected in the gradual tilting of sediments to the north of the peninsula (Vs < 2.6 km/s), reaching depths of about 1.5 km. The low velocities observed at depths below the MF suggest that the activity of the fault was concentrated in the center of the peninsula during the Triassic-Jurassic, with activity levels close to what is now observed in the northern part of the peninsula. The apparent vertical normal slip rate of the MF from the Upper Triassic to Quaternary is estimated to be ~0.02 m/kyr, which is an order of magnitude less than the slip rates reported from the Miocene to Quaternary (e.g. Cortés et al., 2012; Gonzáles- Alfaro, 2021; Gonzáles-Alfaro et al., 2018). C O N C L U S I O N Based on the results obtained from the ANT and lithological observations at the surface, we concluded that the MF has been active from the Upper Triassic to the Quaternary. Near the city of Mejillones, in the northern part of the MF, a sedimentary thickness of 1500 m depth can be interpreted from the results. The La Negra formation would have a thickness of ~3000 m in the central part of the MP. Our shear wave velocity model will contribute to the evaluation of seismic site effects and the seismic hazard in a strategic area near large cities, airports, mining facilities, and ports. Finally, we provide evidence that the MF has had a significant role in controlling the lithology and morphology of the MP during the last 200 Myr.