Seismicity near the slip maximum of the 1960 Mw 9.5 Valdivia earthquake (Chile): Plate interface lock and reactivation of the subducted Valdivia Fracture Zone

Abstract

Understanding the processes behind subduction‐related hazards is an important responsibility and major challenge for the Earth sciences. Few areas demonstrate this as clearly as south‐central Chile, where some of the largest earthquakes in human history have occurred. We present the first observation of local seismicity in the Villarrica region (39°–40°S), based on a temporary local network of 55 stations installed from the Chilean coast into the Argentinian back‐arc for one year. While consistent with the Chilean national catalog (SSN), our results allow us to observe smaller magnitudes with a completeness of about 2.0 and image the geometry of the Wadati‐Benioff Zone from the Chile Trench down to 200 km. Offshore, a gap in interplate seismicity is observed in the region of the 1960 Valdivia earthquake slip. Above the interface, two offshore seismicity clusters possibly indicate ongoing stress relaxation. In the subducting Nazca Plate, we find a prominent seismicity cluster along the extrapolated trace of the oceanic Valdivia Fracture Zone (VFZ). The seismicity cluster is observed between 70 and 130 km depth and comprises mainly strike‐slip events. It indicates weakening and reactivation of the major VFZ by dehydration of oceanic crust and mantle. Interpreting the subducted VFZ section as a localized reservoir of potential fluid release offers an explanation for the Villarrica volcanic complex that is located above the reactivated VFZ and shows the highest volcanic activity in South America. Crustal seismicity is observed near Puyehue volcano, which recently started to erupt (June 2011).

Contrasting records from mantle to surface of Holocene lavas of two nearby arc volcanic complexes: Caburgua-Huelemolle Small Eruptive Centers and Villarrica Volcano, Southern Chile

“…lower crust -upper mantle conditions) were obtained for the CHSEC olivine-augite antecryst pairs (Table 3). This wide pressure range includes the estimated pressure of ~ 10 kb (~35-40 km) for the mantlecrust boundary beneath the Andes at this latitude according to different techniques such as gravimetry (Folguera et al, 2007), earthquake traveltime tomography (Haberland et al, 2009), nearby receiver function profile (Dzierma et al, 2012a), and location of seismic clusters (Dzierma et al, 2012b). Although lower crust and upper mantle reservoirs cannot be ruled out, we favor a reservoir at the mantle-crust boundary because it constitutes a rheological barrier that facilitates mantle-derived magma storage (Hildreth and Moorbath, 1988).…”

Alaska megathrust 1: Seismicity 43 years after the great 1964 Alaska megathrust earthquake

“…[26] Most subduction zones exhibit interplate earthquakes on the megathrust faults [e.g., Ekström et al, 2012;Pacheco and Sykes, 1992;Ruff and Kanamori, 1980]. In a few regions currently in the interseismic interval of giant earthquakes (Southern Chile [Dzierma et al, 2012], Southwest Japan [Obana et al, 2004], Cascadia [Wada et al, 2010;McCrory et al, 2012]), even small thrust earthquakes are absent. In these places, clusters of small earthquakes frequently occur adjacent to the seismogenic zone but within the downgoing plate, with focal mechanisms similar to deeper WBZ seismicity.…”

“…Even during the intervals between Mw > 8 earthquakes, abundant small-to-medium seismicity occurs [e.g., Byrne et al, 1988;Tichelaar and Ruff, 1993]. Although most subduction zones show small to moderate interplate thrust earthquakes [e.g., Igarashi et al, 2003;Pacheco et al, 1993;Zhang and Schwartz, 1992], a few show only seismicity within the downgoing slab, not on the plate interface, even at megathrust depths [e.g., Taber and Smith, 1985;Dzierma et al, 2012]. The different patterns of seismic behavior could reflect different levels of material heterogeneity on megathrust, manifest as long-lived asperities [Lay et al, 1982;Lay and Bilek, 2007;Thatcher, 1990].…”

Structure and tectonics of the central Chilean margin (31°–33°S): implications for subduction erosion and shallow crustal seismicity

“…1). However, shallow crustal seismicity observed near the coast has also largely been observed off southern Chile (Haberland et al 2006;Lange et al 2007;Dzierma et al 2012) in regions where subducting seamounts are absent. Alternatively, it may be attributed to the presence of crustal faulting defining the contact between a paleoaccretionary prism against the continental metamorphic basement (paleo-backstop) (Contreras-Reyes et al 2013).…”