The Quaternary Active El Arrayan Fault, Santiago, Chile

Understanding the location and nature of Quaternary active crustal faults is critical to reduction both fault rupture strong ground motions hazards in built environments. Recent work along San Ramón Fault (SRF) demonstrates that seismic sources are important within Santiago Metropolitan region Chile. In this paper, we present evidence a second likely (the El Arrayán Fault, EAF) runs through northeastern part Santiago, Lo Barnechea area. The EAF strike-slip zone at least 13 km-long, has steep dip (mean 77º NNE), WNW-ESE general trend (~110º) cuts across folded rocks Abanico Formation deposits. Sequences meter-wide area coincident with tectonic geomorphology EAF, scarps, deflected streams, saddles, “island hills”, sag ponds linear valleys La Dehesa strike from bedrock exposures. Although direct dating not yet available, exposures tied fault-related geomorphic features crosscut Late-Quaternary morphologies deposits (including hillslopes, ridges, landslides alluvial fans), provides seismogenic fault. Sinistral slickensides left-lateral streams indicates kinematics (with slight up north reverse motion) for EAF. Due location, geometry, activity structure interpreted as an arc-oblique transfer West Andean Thrust (WATS). If stick-slip behaviour, it potentially source could be responsible earthquakes Mw 6.4 based on earthquake scaling laws microseismicity observations around Santiago. lack avoidance criteria (i.e., do build faults) Chilean law, requires immediate enhanced mapping, legislation implementation areas reduce risk associated structures environment.