Improving seismotectonics and seismic hazard assessment along the San Ramón Fault at the eastern border of Santiago city, Chile

Abstract

The San Ramón Fault is an active west-vergent thrust fault system located along the eastern border of the city of Santiago, at the foot of the main Andes Cordillera. This is a kilometric crustal-scale structure recently recognized that represents a potential source for geological hazards. In this work, we provide new seismological evidences and strong ground-motion modeling from hypothetic kinematic rupture scenarios, to improve seismic hazard assessment in the Metropolitan area of Central Chile. Firstly, we focused on the study of crustal seismicity that we relate to brittle deformation associated with different seismogenic fringes in the main Andes in front of Santiago. We used a classical hypocentral location technique with an improved 1D crustal velocity model, to relocate crustal seismicity recorded between 2000 and 2011 by the National Seismological Service, University of Chile. This analysis includes waveform modeling of seismic events from local broadband stations deployed in the main Andean range, such as San José de Maipo, El Yeso, Las Melosas and Farellones. We selected events located near the stations, whose hypocenters were localized under the recording sites, with angles of incidence at the receiver\5°and S-P travel times\2 s. Our results evidence that seismic activity clustered around 10 km depth under San José de Maipo and Farellones stations. Because of their identical waveforms, such events are interpreted like repeating earthquakes or multiplets and therefore providing first evidence for seismic tectonic activity consistent with the 123Nat Hazards (2014) 71:243-274 DOI 10.1007 crustal-scale structural model proposed for the San Ramón Fault system in the area (Armijo et al. in Tectonics 29(2):TC2007, 2010). We also analyzed the ground-motion variability generated by an M w 6.9 earthquake rupture scenario by using a kinematic fractal k -2 composite source model. The main goal was to model broadband strong ground motion in the near-fault region and to analyze the variability of ground-motion parameters computed at various receivers. Several kinematic rupture scenarios were computed by changing physical source parameters. The study focused on statistical analysis of horizontal peak ground acceleration (PGAH) and ground velocity (PGVH). We compared the numerically predicted ground-motion parameters with empirical ground-motion predictive relationships from Kanno et al. (Bull Seismol Soc Am 96:879-897, 2006). In general, the synthetic PGAH and PGVH are in good agreement with the ones empirically predicted at various source distances. However, the mean PGAH at intermediate and large distances attenuates faster than the empirical mean curve. The largest mean values for both, PGAH and PGVH, were observed near the SW corner within the area of the fault plane projected to the surface, which coincides rather well with published hanging-wall effects suggesting that ground motions are amplified there.

Kinematics of the active West Andean fold-and-thrust belt (central Chile): Structure and long-term shortening rate

“…[Cornejo and Mahood, 1997;Deckart et al, 2010]). The focal mechanism represented is an average focal mechanism calculated by Pérez et al [2013] for 11 earthquakes with well-constrained locations and depths in this area. The dashed ellipsis represents the uncertainties in average hypocentral location.…”

“…The dip of the faults is constrained from the fold geometries at the subsurface. For comparison, we show in Figure 6 the average focal mechanism calculated by Pérez et al [2013] for 11 earthquakes with wellconstrained locations and depths in this area. Please note that this focal mechanism is not that of a particular seismic event but is rather an average of several earthquakes, all located within an area extending over a 5 km distance and 6-13 km depth range.…”

“…Recent paleoseismological studies have shown that the San Ramón fault (hereafter SRF) hosted two paleo‐earthquakes of M w ~7.5 over the last 17–19 kyr [ Vargas et al ., ]. This emphasizes that the San Ramón fault is an active fault representing a significant seismic threat to the City of Santiago [ Armijo et al ., ; Pérez et al ., ; Vargas et al ., ]. Armijo et al .…”

“…Recent paleoseismological studies have shown that the San Ramón fault (hereafter SRF) hosted two paleo-earthquakes of M w~7 .5 over the last 17-19 kyr [Vargas et al, 2014]. This emphasizes that the San Ramón fault is an active fault representing a significant seismic threat to the City of Santiago [Armijo et al, 2010;Pérez et al, 2013;Vargas et al, 2014]. Armijo et al [2010] proposed a first-order model for the deeper structure of the San Ramón fault and found that it constitutes the frontal expression of a major west vergent fold-and-thrust belt that extends laterally for thousands of kilometers along the western flank of the Andes [see also Armijo et al, 2015].…”

Latitudinal and Longitudinal Patterns of Exhumation in the Andes of North‐Central Chile

“…The eastern topographic front systematically coincides along strike with east vergent active thrust systems that cause intense crustal seismicity along the foreland, (Figure 1a; Alvarado et al, 2005;Alvarado & Ramos, 2011;Costa et al, 2006;Lamb, 2000;. In contrast, evidence of active deformation along the western topographic front is scarce and has been recognized only in northern Chile (Figure 1a; west vergent thrust system; Farías et al, 2005;Garcia & Herail, 2005;Hall et al, 2012;Jordan et al, 2010;Muñoz & Charrier, 1996;Pinto et al, 2007;Victor et al, 2004) and in central Chile (Figure 1a; San Ramón Fault; Armijo et al, 2010;Charrier et al, 2002;Pérez et al, 2013;Rauld, 2011;Vargas et al, 2014). Between 22 and 30°S, structural and thermochronological data suggest that accelerated exhumation along the western topographic front occurred mostly during the Eocene, building the Incaic Range (Figure 1a; Cembrano et al, 2003;Martínez et al, 2017;Lossada et al, 2017;Maksaev & Zentilli, 1999;Sanchez et al, 2017).…”