Active Faulting, Submarine Surface Rupture, and Seismic Migration Along the Liquiñe‐Ofqui Fault System, Patagonian Andes

Abstract

The intra-arc Liquiñe-Ofqui Fault System (LOFS) is an active transpressive fault zone located in the Patagonian Andes of Chile. In 2007, a seismic sequence occurred in the Aysén Fjord region of Chilean Patagonia along the LOFS, with a M w 6.2 main earthquake that triggered dozens of landslides, some of which induced tsunami waves that caused severe damage and casualties. Through the analysis of high-resolution seismic reflection and bathymetric data, we identify six submarine faults cutting the Late Quaternary postglacial sedimentary infill of the fjord. The most conspicuous are the dextral-normal NE-SW striking Quitralco fault (QF) and the N-S striking strike-slip Río Cuervo (RCF) and Punta Cola faults (PCF). Our paleoseismological analysis reveals at least seven paleolandslide events buried in the fjord sediments that were triggered by local paleoearthquakes, which occurred since local ice sheet retreat, that is, circa 12 kyr. By combining tectonic observations with local seismicity data, we propose a seismotectonic model for the evolution of the 2007 seismic sequence where three structures were progressively activated from the depth toward the upper continental crust, causing surface rupture along the PCF and with earthquakes, suggesting only partial ruptures along other faults. Because the other faults did not rupture to the seafloor they remain important sources of seismic hazard. Thus, the last seismic sequence was a consequence of a stress transfer from the lower-ductile toward the upper-brittle continental crust, close to the triple junction of the Nazca, South American, and Antarctica Plates. Our results emphasize on the potential synergies between multiple geological and geophysical methods to assess complex events. Plain Language Summary When crustal faults rupture, the energy released is the earthquakes we feel at the surface of the Earth. Recent studies along strike-slip faults demonstrate that these phenomena are often not only related to a single fault but instead take place along several faults like was seen in the 2016 Kaikoura earthquake in New Zealand. Using novel high-resolution seismic reflection imagery and bathymetric data together with the reanalysis of local seismicity, we show (i) multiple active faults in the Aysén Fjord area along the Liquiñe-Ofqui Fault System in the Patagonian Andes; (ii) several of these faults were activated during the last 2007 seismic sequence and one of them (the Punta Cola fault), ruptured to the surface causing a M w 6.2 earthquake that generated massive landslides and local tsunami; and (iii) mapping of similar landslides in the fjord sediments demonstrates similar events occurred at least seven times since the last ice sheet retreat in the area, that is, in the last 12,000 yr. This demonstrates the necessity for including the possibility of complex ruptures involving multiple faults regarding seismic hazard assessment along crustal faults, especially for faults near populated areas.

Liquiñe-Ofqui’s fast slipping intra-volcanic arc crustal faulting above the subducted Chile Ridge

“…Our results also support the Kanamori and Rivera 24 M w 7.7 crustal strike slip dextral event in 1960 with clear long-term dextral displacements consistent with surface faulting as would be expected after a dextral strike slip surface rupture. This is important because until our study, no clear evidence of terrestrial LOFZ surface faulting was reported to support this idea, although submarine evidence suggest repeated events within the past 12 ka 25 . Perhaps then, the Chile Margin in Aysén is one end member of the oblique subduction systems on Earth with significant partitioning from the subduction zone to the surface faulting along the LOFZ and is thus similar in nature and in regards to rates to the Philippines fault (20 to 25 mm/year 16 ) and in stark contrast with the San-in shear zone in Japan in the upper plate of the Nankai subduction zone where no surface faults are known 24 ?…”

“…The fast slipping main trace of the LOFZ ruptures and causes major earthquakes (at least Mw 6.2 up to at least Mw 7.7; e.g. 24 , 25 ) and these surface ruptures cut all crustal rock units and Quaternary materials including bisecting volcanoes located along the LOFZ. The high-intensity and shallow upper crustal strong ground motions (i.e.…”

“…Kanamori and Rivera 24 reevaluated source characteristics of a June 6, 1960 M s 6.9 earthquake within the 1960 Valdivia aftershock sequence, where they revised this event to instead a shallow dextral M w 7.7 event along the LOFZ (although no surface rupture was ever documented). Analysis of submarine data after the 2007 event suggested that the main trace of the LOFZ did not rupture to the seafloor (which they refer to as the “Rio Cuervo fault”), and with subtle evidence, which in places is obscured by submarine landslides, to suggest instead that seafloor rupturing occurred along a minor fault parallel to the main LOFZ 25 . Because slip rates provides a key parameter to how fast a fault system is loaded and thus can provide insight into timing between major earthquakes, geological characterisation can help with hazard models as well.…”

Seismicity in a Transpressional Volcanic Arc: The Liquiñe‐Ofqui Fault System in the Puyuhuapi Area, Southern Andes, Chile (44°S)

“…Most other normal mechanisms are consistent with NE or WNW orientations (e.g., Events 3, 5, 9, and 11). Numerous NE oriented normal and dextral-normal faults have been observed along the SVZ (e.g., Cembrano & Lara, 2009;Pérez-Flores et al, 2016;Rosenau et al, 2006;Vargas et al, 2013;Villalobos et al, 2020), and similar focal mechanisms were detected during the 2007 Aysen swarm (Legrand et al, 2011;Russo et al, 2011). In particular, NE oriented faults and NE oriented lineaments have been found to be kinematically coupled with volcanoes and some of the minor eruptive centers located in the SVZ (Cembrano & Lara, 2009).…”

“…Crustal deformation in the SVZ is accommodated by two groups of faults: the LOFS and the ATF, as shown in Figure 1 (e.g., Cembrano et al, 1996; Cembrano & Lara, 2009; Melnick et al, 2006; Pérez‐Flores et al, 2016). The LOFS is an active fault system composed of NS oriented master faults with dextral and dextral‐reverse kinematics accompanied by NE to ENE oriented dextral and dextral‐normal faults (e.g., Arancibia et al, 1999; Iturrieta et al, 2017; Lange et al, 2008; Lavenu & Cembrano, 1999; Pérez‐Flores et al, 2016; Rosenau et al, 2006 ; Villalobos et al, 2020). Geological evidence indicates that the LOFS is a continental‐scale shear zone dominated by a transpressional regime, with a NS oriented dextral‐slip deformation that partially accommodates the margin‐parallel component of the plate‐convergence vector (see Figure 1).…”