Crustal Faults in the Chilean Andes: Tectonic significance and implications for geologic hazard

Abstract

The Chilean Andes is one of the best natural laboratories to unravel thegeologic nature of seismic hazards. It has recorded both great subduction earthquakes (e.g. Mw 9.5,Valdivia, 1960) and moderate magnitude crustal earthquakes (e.g., Mw 6.2, Aysen, 2007). At theNazca-South America subduction zone, hundred-kilometer-long segmented megathrust faults canproduce earthquakes of magnitudes greater than 7.5, with recurrence times between 80 to 120 years,and earthquakes of magnitudes greater than 8.5 every 250 to 500 years. Thus, megathrust-typeearthquakes represent the first order seismic hazard in the Chilean Andes, causing the most damageto population and economy.Crustal intra-plate faults, in turn, have longer recurrence times, but can also cause great destruction atlocal scale because of their shallower hipocentral depth. However, the nature, timing and slip rates ofcrustal faults in the Chilean Andes remain poorly constrained. Recent studies have suggested a linkbetween the subduction seismic cycle and activity on crustal faults, but this remains as an openquestion. Some crustal faults –especially those in the outer forearc– have the potential to reactivateco-seismically, when optimally oriented with respect to the instantaneous extension direction arisingfrom elastic rebound of mega-earthquakes. Other faults may activate during the subductioninterseismic period. Among these, are the regional strike-slip faults and thrusts in the main cordillera(e.g. Liquiñe-Ofqui fault, LOF). Although sparse and limited, current structural, paleo-seismologicaland geodetic data suggests that slip rates in Chilean crustal faults range from 0.2 mm/year in theforearc to up to 6.5 mm/year for the LOF. This implies recurrence times in the range of 50.000 to200 years for Mw 7 earthquakes, respectively. The main implication of these very different tectonicmodes for fault reactivation and the wide range of slip rates is that geologic hazard assessment ofcrustal faults is far from trivial: many structures considered active in the traditional sense will notgenerate earthquakes in thousands of years according to their recurrence times, whereas other lesswell-known Quaternary faults, that have no instrumentally recorded seismicity, could trigger Mw 7earthquakes. Furthermore, fault segments that have generated earthquakes independently, mayeventually be capable to merge together into a single rupture zone and generate an earthquake ofgreater magnitude.Our current neotectonic and paleoseismological investigations in Chile are focused into unraveling thespatial distribution, precise geometry, and slip-rates of these faults and their potential link with shortand long-term subduction zone rupture segments. A rigorous seismic hazard assessment must thenconsider the widely different nature, timing and slip rates of Andean faults. Understanding the natureof crustal faults will help us not only to better assessing the geological hazard associated to them, butalso to understand and constrain their link with the subduction zone seismic cycle.