Upper-plate fault activity in northern Chile: An integrated paleoseismological and numerical modeling approach Joaquín Cortés A.* (1) , Gabriel González L. (1) , Steven A. Binnie (2), Ruth Robinson (3), Joseph Martinod (4) and Dominique Remy (4) (1) Departamento de Ciencias Geológicas, Universidad Católica del Norte, Avenida Angamos 0610, Antofagasta, Chile. (2) Institut fur Geologie und Mineralogie, Universitat zu Koln, Greinstra βe 4-6, Koln, D-50939, Deutschland. (3) Department of Earth Sciences, University of St Andrews, St Andrews, KY16 9AL, UK. (4) Géosciences Environnement Toulouse [GET], Université Paul Sabatier, Toulouse, 31400, France. * email: [email protected] Abstract. We characterize the paleoseismological activity distribution of those superficial areas prone to experience of two upper-plate plate faults in the Antofagasta Region, tension (positive CSC values). northern Chile. These correspond to the Mejillones and Salar del Carmen faults, located in the eastern Mejillones Peninsula and eastward of Antofagasta Town, respectively. 2 Seismotectonic setting Both structures have experienced Mw 6.5-7 earthquakes since at least late Pleistocene times. The recurrence of The convergence between Nazca and South America plates earthquakes on upper-plate faults is on the order of ca. 4-5 occurs in northern Chile at a rate of 65 mm/y along a ka, and is not synchronized with the cycle of mega vector oriented N75°E (Angermann et al., 1999). Interplate subduction earthquakes in northern Chile. The activity of elastic strain build up occurs during the interseismic along upper-plate faults appears to result from contributions of a locked portion of the seismogenic zone, which defines a both coseismic and interseismic subduction stages over plane inclined 18°E. The seismogenic zone extends from repeated seismic cycles. near the trench to 38-55 km depth (e.g. Husen et al., 2000; Key words: Paleoseismology, Coulomb stress change Khazaradze and Klotz, 2003). Coupling at this subduction models, upper-plate faults, northern Chile. seismogenic zone and others worldwide is heterogeneous along strike and dip, being in some regions locked more 1 Introduction strongly than in others (e.g. Chlieh et al., 2011), while in some areas aseismic creep is reported (e.g. Victor et al, The tectonic scenario of northern Chile is mainly 2011). Earthquake distribution in northern Chile has controlled by the convergence between the Nazca and allowed definition of an along strike segmentation of the South America plates. This process is responsible for the subduction seismogenic zone (e.g. Delouis et al., 1997). In elastic strain accumulation in the interplate locked zone, this sense, it has been proposed that the seismogenic zone which is mostly balanced between the stages of the beneath the Mejillones Peninsula is the border between the subduction cycle (e.g. Bevis et al. 2001). Nevertheless, ruptures of the Iquique 1877, Antofagasta 1995 and part of this deformation is transferred to the upper-plate in Tocopilla 2007 earthquakes (e.g. Motagh et al., 2010). a permanent way. This is manifested, in the Coastal Victor et al. (2011) conclude that 90% of the slip deficit Forearc of the Antofagasta Region, by the occurrence of since 1877 has been accommodated aseismically beneath several kilometric NS normal faults (e.g. Arabasz, 1971; it. If true, the role of a seismic barrier attributed to the González et al., 2003; Allmendinger and González, 2010). Mejillones Peninsula may be related to a change in the Despite their remarkable manifestations in the local frictional properties along the strike of the seismogenic topography, a detailed characterization of their recent zone. More recently, Chlieh et al. (2011) determined that activity is still lacking in this tectonic context. In this the peninsula is a transition between a shallower locked contribution, we first present the results of area (> 75% coupling) and a deeper locked are northward, paleoseismological studies performed for two of these reflecting the along dip heterogeneities of the subduction normal faults, the Mejillones and Salar del Carmen faults. interface in the area. These and other normal faults in the area present similar geometrical properties (azimuth/dip) and in most of the 3 Structural framework cases promote the uplift of the block positioned westward The Coastal Forearc between 22°35’S and 24°S is marked of their trace. Because of this condition, we suspect that by the existence of the Mejillones Peninsula, an anomalous their origin and activity must be controlled by the morphotectonic feature that interrupts the straight trend of convergence, more specifically by the sum of the effects of the coastline (Figure 1). This peninsula has experienced interseismic and coseismic periods over repeated extension and uplifting during the Quaternary (e.g. subduction seismic cycles. In order to explore this Marquardt, 2005). From a geological standpoint, the relationship we performed Coulomb Stress Change (CSC) Mejillones Peninsula is spatially related to two models. Particularly, we focus on the nature of the conspicuous characteristics linked to the existence of interplate contact beneath the Mejillones Peninsula and the upper-plate faults: i) The curvature of the Atacama Fault 188 System, specifically the Salar del Carmen Segment (Figure (Cortés et al., 2012). 1); and ii) The larger number if NS faults accommodating extension at this latitude (Figure 1). In general, all these The Salar del Carmen Fault has promoted the faults are subvertical and dip to the east (e.g. Niemeyer et abandonment of one alluvial surface. This occurred at a al., 1996). Classically, their origin and activity has been maximal age between 550 +/- 234 ka and 351 +/-181 ka related to the convergence process (e.g. Niemeyer et al., (González et al., 2006). Since then, this fault has generated 1996). at least three Mw 6.5-6.7 earthquakes, as indic ated by three colluvial wedges identified at the base of the fault scarp. The first identified earthquake occurred close to 12 ka, after which the f ault apparently experienced a quiescence period. Because this age was obtained by OSL in the basal wedge we think that it could be more appropriate to define the scarp age than the 21 Ne data previously reported (González et al., 2006). The quiescent stage was interrupted by an earthquake at ca 2.2 ka, after which a further seismic event occurred at ca. 0.48 ka. Apparently, no younger Mw 6.5-6.7 earthquakes have taken place on the Salar del Carmen Fault. If the scarp construction began at ca. 12 ka, we can estimate a slip rate of 0.22 + 0.04 m/ka for this time span. Moreover, the recurrence for Mw 6.5- 6.7 earthquakes on this fault would be 4.23 + 0.80 ka. This estimation is in agreement with that made for the Mejillones Fault, and thus also differs with the frequency Figura 1. Seismotectonic and structural setting of the study area. of mega subduction earthquakes. Faults are yellow lines. Ellipses are the ruptures of the largest earthquakes that have struck the northern Chile since 1877. White 5 Coulomb Stress Change model approach arrow indicates the convergence. AFS= Atacama Fault System; Coulomb Stress Change (CSC) models have been MF= Mejillones Fault; SCF= Salar del Carmen Fault; MP= successfully used to explore aftershock distribution and the Mejillones Peninsula. The white square limits the study area potential reactivation of faults after a large earthquake (e.g. showed in the zoom at the right. Freed, 2005). In this contribution we consider coseismic and interseismic CSC models to explore the dynamic 4 Paleoseismology results relationship between the subduction earthquake cycle and By combining surface field observations, 2D trench the upper-plate fault activity at the Coastal Forearc of the logging, OSL dating of colluvial deposits accumulated at Antofagasta Region. Model principles and parameters the base of the scarps, and cosmogenic exposure ages 10 21 follow previously published works (e.g. Loveless, 2007). ( Be- Ne) of alluvial surfaces displaced by the faults; we Coseismic models evaluate two main scenarios. First, we characterized the late Pleistocene-Holocene modeled a rupture similar to the 1877 Iquique Earthquake, paleoseismology of the Mejillones and Salar del Carmen stopping under the central Mejillones Peninsula. Second, faults. we considered a rupture passing beneath the peninsula. Both types of simulations emulate ~Mw 8.5 earthquakes. For the Mejillones Fault we constrained that it has From these we noted that ruptures like the one associated promoted the uplifting and abandonment of two alluvial to the 1877 Earthquake would have only partially surfaces at ca. 35 ka and ca. 14 ka (Cortés et al., 2012). facilitated tension and thus normal reactivation of faults Based on the identification of colluvial wedges (sensu like Mejillones and Salar del Carmen (northern segments). McCalpin, 1996) we interpreted that between ca. 14 ka and On the other hand, a rupture passing beneath the ca. 7 ka the fault experienced two Mw 7 earthquakes. Mejillones Peninsula would be able to enhance tensional Subsequently, between ca. 7 ka and ca. 3 ka, the fault conditions along the whole strike of these upper-plate influenced and deformed overlying hillslope deposits, faults. Moreover, we observed that coseismic models suggesting minor discrete displacements occurred during promote tension in the entire forearc over and eastward of this time. These were accommodated seismically and/or a given rupture. This is not in agreement with the aseismically. At ca. 3 ka, the fault produced a Mw 6.6 distribution of upper-plate faults in the area, which occupy earthquake, as deduced from the identification of another a narrow NS fringe between the coast and ~50 km inland colluvial wedge. After this age the fault has not triggered (transparent yellow area in figure 1). Interseismic models major seismic events. Over the last 35 ka, the Mejillones considered three settings. We first simulated a completely Fault has acted with a slip rate of 0.61+0.26 m/ka.