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

The Chilean Andes is one of the best natural laboratories to unravel the geologic 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 the Nazca-South America subduction zone, hundred-kilometer-long segmented megathrust faults can produce 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-type earthquakes represent the first order seismic hazard in the Chilean Andes, causing the most damage to population and economy. Crustal intra-plate faults, in turn, have longer recurrence times, but can also cause great destruction at local scale because of their shallower hipocentral depth. However, the nature, timing and slip rates of crustal faults in the Chilean Andes remain poorly constrained. Recent studies have suggested a link between the subduction seismic cycle and activity on crustal faults, but this remains as an open question. Some crustal faults -especially those in the outer forearc- have the potential to reactivate co-seismically, when optimally oriented with respect to the instantaneous extension direction arising from elastic rebound of mega-earthquakes. Other faults may activate during the subduction interseismic 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-seismological and geodetic data suggests that slip rates in Chilean crustal faults range from 0.2 mm/year in the forearc to up to 6.5 mm/year for the LOF. This implies recurrence times in the range of 50.000 to 200 years for Mw 7 earthquakes, respectively. The main implication of these very different tectonic modes for fault reactivation and the wide range of slip rates is that geologic hazard assessment of crustal faults is far from trivial: many structures considered active in the traditional sense will not generate earthquakes in thousands of years according to their recurrence times, whereas other less-well-known Quaternary faults, that have no instrumentally recorded seismicity, could trigger Mw 7 earthquakes. Furthermore, fault segments that have generated earthquakes independently, may eventually be capable to merge together into a single rupture zone and generate an earthquake of greater magnitude. Our current neotectonic and paleoseismological investigations in Chile are focused into unraveling the spatial distribution, precise geometry, and slip-rates of these faults and their potential link with short and long-term subduction zone rupture segments. A rigorous seismic hazard assessment must then consider the widely different nature, timing and slip rates of Andean faults. Understanding the nature of crustal faults will help us not only to better assessing the geological hazard associated to them, but also to understand and constrain their link with the subduction zone seismic cycle.