Abstract
Understanding the location and nature of Quaternary active crustal faults is critical to the reduction of both fault rupture and strong ground motions hazards in built environments. Recent work along the San Ramón Fault (SRF) demonstrates that crustal seismic sources are important hazards within the Santiago Metropolitan region of Chile. In this paper, we present the evidence of a second likely Quaternary active fault (the El Arrayán Fault, EAF) that runs through the northeastern part of Santiago, in the Lo Barnechea area. The EAF is a strike-slip fault zone at least 13 km-long, has a steep dip (mean dip 77º NNE), and a WNW-ESE general trend (~110º) that cuts across folded rocks of the Abanico Formation and Quaternary deposits. Sequences of meter-wide fault rocks in El Arrayán area are coincident with the tectonic geomorphology of the EAF, with fault scarps, deflected streams, saddles, “island hills”, sag ponds and linear valleys in the La Dehesa area and coincident with fault strike from the bedrock exposures. Although direct dating is not yet available, fault exposures tied to fault-related tectonic and geomorphic features that crosscut Late-Quaternary morphologies and deposits (including hillslopes, ridges, landslides and alluvial fans), provides strong evidence that the EAF is a Quaternary active and seismogenic crustal fault. Sinistral slickensides and left-lateral deflected streams indicates a left-lateral kinematics (with slight up to the north reverse motion) for the EAF. Due to the location, geometry, kinematics and likely Quaternary activity of the EAF, this structure is interpreted as an arc-oblique transfer zone of the West Andean Thrust (WATS). If the EAF has stick-slip behaviour, it is potentially an important source of fault rupture and strong ground motions and could be responsible for earthquakes up to Mw 6.4 based on earthquake scaling laws and microseismicity observations in and around Santiago. The lack of fault zone avoidance criteria (i.e., do not build in and around active faults) in Chilean law, requires immediate enhanced fault mapping, legislation and the implementation of active fault rupture avoidance areas to reduce the risk associated with active crustal structures in the built environment.
Highlights
The Andes of Central Chile and Argentina is one of the highest active orogens on Earth with peaks reaching up to 7,000 m high (e.g., Mt Aconcagua, Tupungato volcano), partially controlled by a rapid (6-8 cm/year) tectonic plate convergence (Fig. 1A; Pardo-Casas and Molnar, 1987; DeMets et al, 1994; Gripp and Gordon, 2002)
We present the evidence of a second likely Quaternary active fault that runs through the northeastern part of Santiago, in the Lo Barnechea area
Geometry, kinematics and likely Quaternary activity of the El Arrayán Fault (EAF), this structure is interpreted as an arc-oblique transfer zone of the West Andean Thrust (WATS)
Summary
The Andes of Central Chile and Argentina is one of the highest active orogens on Earth with peaks reaching up to 7,000 m high (e.g., Mt Aconcagua, Tupungato volcano), partially controlled by a rapid (6-8 cm/year) tectonic plate convergence (Fig. 1A; Pardo-Casas and Molnar, 1987; DeMets et al, 1994; Gripp and Gordon, 2002). According to historical seismicity record, in September 1958 a strike-slip related moment magnitude (Mw) 6.3 earthquake occurred at a depth of 8 km at Las Melosas, ~60 km to the southeast of Santiago (Fig. 1B; Alvarado et al, 2009). This earthquake caused significant damage to hydroelectric plants, a water-supply aqueduct, accompanied by extensive landslides and rock falls in the region (Sepúlveda et al, 2008). In spite of the high strong ground motions from this event, no surface ruptures were encountered This event demonstrates the importance of shallow crustal earthquakes (from the rupture of crustal faults; see De Pascale, 2021) as sources of seismic hazards in the Principal Cordillera and around Santiago. Geological investigations focussed on mapping and active fault characterization remains major goals in seismic hazard assessments
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