Crustal faults in the Chilean Andes: geological constraints and seismic potential

Abstract

The Chilean Andes, as a characteristic tectonic and geomorphological region, is a perfect location to unravel the geologic nature of seismic hazards. The Chilean segment of the Nazca-South American subduction zone has experienced mega-earthquakes with Moment Magnitudes (Mw) >8.5 (e.g., Mw 9.5 Valdivia, 1960; Mw 8.8 Maule, 2010) and many large earthquakes with Mw >7.5, both with recurrence times of tens to hundreds of years. By contrast, crustal faults within the overriding South American plate commonly have longer recurrence times (thousands of years) and are known to produce earthquakes with maximum Mw of 7.0 to 7.5. Subduction-type earthquakes are considered the principal seismic hazard in Chile, with the potential to cause significant damage to its population and economy. However crustal (non-subduction) earthquakes can also cause great destruction at a local scale, because of their shallower hypocentral depth. Nevertheless, the nature, timing and slip rates of crustal seismic sources in the Chilean Andes remain poorly constrained. This work aims to address the seismic potential of the crustal faults in Chile, contributing to the estimation of key fault parameters for the seismic hazard assessment. We have examined the main parameters involved in the magnitude of an earthquake, including length, width and mean displacement of some case studies crustal faults and their morphotectonic settings, exposing the parametrical similarities in longitudinal domains (N-S stripes) and disparity from W to E, across latitudinal domains. The maximum hypocentral depths for crustal earthquakes vary across margin parallel tectonic domains aligned parallel, from 25-30 km in the outer forearc to 8-12 km in the volcanic arc, thus allowing for a first-order approach for seismic potential assessment. Current structural, paleoseismological and geodetic data, although sparse and limited, suggest that slip rates of Chilean crustal faults range from 0.2 mm/yr (in the forearc region) to up to 7.0 mm/yr (in the intra-arc region). The different tectonic modes for crustal fault reactivation and their wide range of slip rates complicates the estimation of seismic hazard. A rigorous seismic hazard assessment must therefore consider the different tectonic settings, timing and slip rates of Andean crustal faults. Understanding the nature of these faults will allow a better evaluation of the associated seismic hazard, and better constraints to be placed on their relationship with the subduction seismic cycle.

Comment on “Crustal faults in the Chilean Andes: geological constraints and seismic potential” by Santibáñez et al. (2019), Andean Geology 46 (1): 32-65

“…Demonstrating where active faults are located through careful mapping, and to determine how fast they accommodate tectonic deformation and their seismic and fault rupture hazards are key questions in neotectonics. Recently Santibáñez et al (2019) explore active fault systems in the Chilean Andes. In their paper they outline active and potentially seismogenic (i.e., earthquake producing) fault systems in the Chilean Andes through a review of the literature, seismicity, case studies (earthquakes), and modeling data and then they define potential tectonic domains for subdivision of Chile.…”

“…In their paper they outline active and potentially seismogenic (i.e., earthquake producing) fault systems in the Chilean Andes through a review of the literature, seismicity, case studies (earthquakes), and modeling data and then they define potential tectonic domains for subdivision of Chile. These domains were suggested to allow "a first-order approach for seismic potential assessment" (Santibáñez et al, 2019). The three subdivisions they suggest, i.e., domains are the External Forearc, Inner Forearc and Volcanic Arc, were proposed based on several fault parameters (e.g., fault length), case studies, the morphotectonic setting and seismicity.…”

“…Their paper generates a great foundation to build upon for both the active tectonics and geological hazards community, in addition to being useful for potential end users such as the Chilean local and national government from a planning perspective. Although the Santibáñez et al (2019) paper takes steps in the right direction, and should be considered an important contribution to the scientific community, this comment addresses three potential issues with their analysis and conclusions that should be reflected upon by the seismic hazard and active tectonics community. These ideas are summarized below and expanded on in detail thereafter.…”

Marine terraces of the last interglacial period along the Pacific coast of South America (1° N–40° S)

“…Valparaíso (33°S, Quintay and Valparaíso faults), near the Arauco Peninsula (36°-39°S, Santa María and Lanalhue faults), and in between (Topocalma, Pichilemu, Carranza, and Pelluhue faults) (Ota et al, 1995;Melnick et al, 2009;Santibáñez et al, 2019;Maldonado et al, 2020;Melnick et al, 2020 https://doi.org/10.5194/essd-2020-385 In the 50-to 180-km-wide coastal area of the Ecuadorian Andes, where the Western Cordillera is flanked by a structural depression, relief is relatively low (< 300 m asl). The Gulf of Guayaquil (3°S) and the Dolores-Guayaquil megashear separate the northern from the southern forearc units.…”

“…We compared the deformation patterns of marine terraces along the coast of South America with proxies that included crustal faults, bathymetric anomalies, trench-sediment thickness, and distance to the trench. control of climatic parameters in the morphology of marine terraces, we compared our data set with wave heights, tidal range, mean annual precipitation rate, and the azimuth of the coastline (Schweller et al, 1981;Bangs and Cande, 1997;von Huene et al, 1997;Collot et al, 2002;Ceccherini et al, 2015;Hayes et al, 2018;Santibáñez et al, 2019; GEBCO Bathymetric Compilation Group, 2020) ( Fig. 1).…”