Efrain Rivera

The plate interface beneath the Mejillones Peninsula in Northern Chile is characterized by anomalous seismogenic behaviors, with seismic and aseismic slip, and low coupling values. We analyze this zone through the seismicity pattern and a 3‐D tomography model. We identify high V_P/V_S values within the oceanic crust and in the lower continental crust, which we interpret as hydrated zones rich in fluids. These zones are correlated with the Mejillones fracture zone and with highly permeable lithologies of the lower continental crust, which allow a greater accumulation of fluids at the plate interface beneath the Mejillones Peninsula. Additionally, these areas exhibit a high rate of seismicity and concentrated swarms and repeaters. We propose that the presence of fluids controls the anomalous seismogenic behavior along the plate interface beneath the Mejillones Peninsula.

On 16 September 2015, the M w 8.3 Illapel, Chile, earthquake broke a large area of the Coquimbo region of north‐central Chile. This area was well surveyed by more than 15 high‐rate Global Positioning System (GPS) instruments, installed starting in 2004, and by the new national seismological network deployed in Chile. Previous studies had shown that the Coquimbo region near Illapel was coupled to about 60%. After the M w 8.8 Maule megathrust earthquake of 27 February 2010, we observed a large‐scale postseismic deformation, which resulted in a strain rate increase of about 15% in the region of Illapel. This observation agrees with our modeling of viscous relaxation after the Maule earthquake. The area where upper‐plate GPS velocity increased coincides very well with the slip distribution of the Illapel earthquake inverted from GPS measurements of coseismic displacement. The mainshock started with a small‐amplitude nucleation phase that lasted 20 s. Backprojection of seismograms recorded in North America confirms the extent of the rupture, determined from local observations, and indicates a strong directivity from deeper to shallower rupture areas. The coseismic displacement shows an elliptical slip distribution of about 200 km×100 km with a localized zone where the rupture is deeper near 31.3° S. This distribution is consistent with the uplift observed in some GPS sites and inferred from field observations of bleached coralline algae in the Illapel coastal area. Most of aftershocks relocated in this study were interplate events, although some of the events deeper than 50 km occurred inside the Nazca plate and had tension (slab‐pull) mechanisms. The majority of the aftershocks were located outside the 5 m contour line of the inferred slip distribution of the mainshock.

Intermediate depth earthquakes often occur along subducting lithosphere, but despite their ubiquity the physical mechanism responsible for promoting brittle or brittle‐like failure is not well constrained. Large concentrations of intermediate depth earthquakes have been found to be related to slab break‐off, slab drip, and slab tears. The intermediate depth Hindu Kush nest is one of the most seismically active regions in the world and shows the correlation of a weak region associated with ongoing slab detachment process. Here we study relocated seismicity in the nest to constraint the geometry of the shear zone at the top of the detached slab. The analysis of the rupture process of the M_w 7.5 Afghanistan 2015 earthquake and other several well‐recorded events over the past 25 years shows an initially slow, highly dissipative rupture, followed by a dramatic dynamic frictional stress reduction and corresponding large energy radiation. These properties are typical of thermal driven rupture processes. We infer that thermal shear instabilities are a leading mechanism for the generation of intermediated‐depth earthquakes especially in presence of weak zone subjected to large strain accumulation, due to ongoing detachment process.