Crustal motion in the zone of the 1960 Chile earthquake: Detangling earthquake‐cycle deformation and forearc‐sliver translation

Abstract

[1] Temporary deformation in great earthquake cycles and permanent shear deformation associated with oblique plate convergence both provide critical clues for understanding geodynamics and earthquake hazard at subduction zones. In the region affected by the M w 9.5 great Chile earthquake of 1960, we have obtained GPS observations that provide information on both types of deformation. Our velocity solutions for the first time span the entire latitudinal range of the 1960 earthquake. The new observations revealed a pattern of opposing (roughly arc-normal) motion of coastal and inland sites, consistent with what was reported earlier for the northern part of this region. This finding supports the model of prolonged postseismic deformation as a result of viscoelastic stress relaxation in the mantle. The new observations also provide the first geodetic evidence for the dextral motion of an intravolcanic arc fault system and the consequent northward translation of a forearc sliver. The sliver motion can be modeled using a rate of 6.5 mm/a, accommodating about 75% of the margin-parallel component of Nazca-South America relative plate motion, with the rate diminishing to the north. Furthermore, the new GPS observations show a southward decrease in margin-normal velocities of the coastal area. We prefer explaining the southward decrease in terms of changes in the width or frictional properties of the megathrust seismogenic zone. Because of the much younger age of the subducting plate and warmer thermal regime in the south, the currently locked portion of the plate interface may be narrower. Using a three-dimensional viscoelastic finite element model of postseismic and interseismic deformation following the 1960 earthquake, we demonstrate that this explanation, although not unique, is consistent with the GPS observations to the first order.

Seismicity distribution in the vicinity of the Chile Triple Junction, Aysén Region, southern Chile

“…Several consequences of the Chile Ridge subduction beneath the CTJ have been observed including: tectonic erosion, plutonism near the trench and uplift during the Upper Cretaceous in the Taitao Peninsula (Cande and Leslie, 1986), emplacement of an ophiolitic complex in the Taitao Peninsula (Forsythe et al, 1986) and the displacement of a fore-arc sliver (Forsythe and Nelson, 1985;Wang et al, 2007). Another important consequence is the occurrence of a 350 km long volcanic gap within the arc (Ramos and Kay, 1992), whilst in the back-arc zone widespread basaltic-plateau volcanism takes place (e.g.…”

“…This tectonic configuration corresponds to the only present-day active ridgetrench collision in which the overriding plate represents continental lithosphere. North of the CTJ, a young and buoyant portion of the Nazca plate is being subducted at a rate of 6.6 cm yr À1 , while to the south, the Antarctic plate is being subducted at 2 cm yr À1 below South America (Wang et al, 2007).…”

Reawakening of large earthquakes in south central Chile: The 2016 Mw 7.6 Chiloé event

“…The coastal area, on the other hand, has experienced landward motion of variable magnitude, consistent with interseismic plate locking (Figure 1). In addition, the fore-arc region is affected by rigid block translation in response to oblique plate convergence [Wang et al, 2007].…”

“…Monitoring surface deformation using regional GPS campaigns in south central Chile started in 1993 and continued periodically during the following years together with the installation of continuously recording stations since 2005 [Klotz et al, 2001;Wang et al, 2007;Moreno et al, 2011]. This instrumentation effort led to a detailed description of surface deformation in this region in response to different sources.…”

“…We combined all published GPS velocities [Klotz et al, 2001;Wang et al, 2007;Moreno et al, 2011] with new estimates from cGPS to obtain a decadal deformation field. The GPS time series were computed using …”