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 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.
Highlights
Chile is located on the tectonic convergent contact between the Nazca and South American plates, where large tsunamigenic subduction earthquakes occur, like the Mw 8.8 Maule earthquake in 2010 (Madariaga et al 2010; Lay et al 2010; Vigny et al 2011), and the largest event recorded is Mw 9.5 Valdivia earthquake in 1960 (Plafker and Savage 1970; Astiz and Kanamori 1986; Cifuentes 1989; Barrientos et al 1992; Vita-Finzi and Mann 1994; Lomnitz 2004)
On the lights of the results obtained from waveform modeling, one can conclude that the seismic events associated with SJCH and FAR stations are located at an average depth of 9–10 km, essentially having the same focal mechanism, characterized by an N–S reverse fault with a dip between 30° and 40°, a rake of about 100°–120° and an S–P travel time of 1.2 s with little dispersion (Fig. 6)
We have documented seismological and tectonic evidences related to the San Ramon Fault System that have strong implications for any seismic hazard assessment and risk study for the Santiago Metropolitan area
Summary
Chile is located on the tectonic convergent contact between the Nazca and South American plates, where large tsunamigenic subduction earthquakes occur, like the Mw 8.8 Maule earthquake in 2010 (Madariaga et al 2010; Lay et al 2010; Vigny et al 2011), and the largest event recorded is Mw 9.5 Valdivia earthquake in 1960 (Plafker and Savage 1970; Astiz and Kanamori 1986; Cifuentes 1989; Barrientos et al 1992; Vita-Finzi and Mann 1994; Lomnitz 2004). First-order predicting ground-motion parameters given different earthquake scenarios can be achieved through the development of empirical relationships that relate a specific characteristic of the ground motion with few parameters, such as magnitude and distance to the seismic source (e.g., Sabetta and Pugliese 1987; Ambraseys et al 1996; Abrahamson and Silva 1997; Boore et al 1997; Kanno et al 2006). To obtain these empirical relationships is difficult in zones characterized by moderate seismicity rates and limited seismological records. We statistically analyzed and compared ground-motion parameters predicted by empirical attenuation laws (Kanno et al 2006), with the ones obtained numerically in this work for different earthquake rupture scenarios
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