ALBERT

Description: In this paper, we present a compilation of modern seismic and seismological methods applied to image the subduction process in North Chile, South America. We use data from active and passive seismic experiments that were acquired within the framework of the German Collaborative Research Center SFB267 ‘Deformation Processes in the Andes’. The investigation area is located between 20° and 25°S and extends from the trench down to 100 km depth. In the depth range between the sea bottom and 15 km, we process an offshore seismic reflection profile using a recently developed velocity-model-independent stacking procedure. We find that the upper part of the subducting oceanic lithosphere in this depth range is characterized by a horst-and-graben structure. This structure supports an approximately 3 km thick coupling zone between the plates. In the depth range between 15 and 45 km, we analyse the spatial distribution of aftershocks of the Antofagasta earthquake (1995). The aftershock hypocenters are concentrated in an approximately 3 km thick layer. Finally, in the depth range between 45 and 100 km, we apply Kirchhoff prestack depth migration to the onshore ANCORP profile. A double reflection zone is observed between 45 and 60 km depth, which may represent the upper and lower boundary of the subducted oceanic crust. Over the whole range down to more than 80–90 km depth, we obtain an image of the subducting slab. At that depth, the hypocenters of local earthquakes deviate significantly in the direction perpendicular to the slab face from the reflective parts of the slab. Consequently, our results yield a complete seismic image of the downgoing plate and the associated seismic coupling zone.

Description: Recently, the 2014 MW=8.1 Pisagua earthquake ruptured a substantial part of the Northern Chile seismic gap north of 21°S. We present data from a temporary local seismic network that was in operation in this region between 2005 and 2012, in the phase of late interseismic locking. We localized over 5,300 seismic events (ML ∼0.5–4) with high precision using hand-picked seismic arrival times. Seismicity is pervasive within the entire crust of the South American continental plate. In the subducting slab the seismicity exhibits three distinct bands of activity. The highest concentration is found in a sharp band at the contact zone between the continental and the oceanic lithosphere. Here it constraints the transitionally locked zone between 30 and 50 km depth. A second band is visible near the oceanic Moho and a third one in the lithospheric mantle of the subducting plate, 30km below the plate contact. Seismicity tightly correlates with the reflectivity image acquired in the ANCORP '96 experiment. We complement our observations with the determination of local seismic P- to S-wave velocity ratios (Vp/Vs) that we determined in full independence of the localization procedure following the approach of Lin and Shearer (2007). Along the plate contact we observe a clear down-dip decrease of Vp/Vs from high values (〉1.80) in the shallow part (~30km depth) to moderate values (~1.70) in the deeper part of the system (~90km). Locally very high or very low Vp/Vs occur (〉1.90 or ~1.60, respectively). The continental crust exhibits rather typical Vp/Vs (~1.75). Towards the trench we find a continuation of high Vp/Vs from the subducting slab into the lower continental crust. Arc-ward of the down-dip end of interplate seismicity, below the continental Moho as constrained by receiver functions, we find very low Vp/Vs (~1.60). In the oceanic mantle we observe high Vp/Vs (~1.90) at a depth between 50 and 60km. Fluids and serpentine are prominent candidates to explain anomalously high Vp/Vs. Values as found at the plate contact between 30 and 40km depth may therefore witness the presence of free fluids there. A migration of these fluids into the overriding continental plate would be consistent with our data. In case of the oceanic mantle, the degree of serpentinization of peridotite may be estimated, constraining the water budget of the down-going plate.

Description: Prominent trench-parallel fault systems in the arc and fore-arc of the Chilean subduction zone can be traced for several thousand kilometers in north–south direction. These fault systems possibly crosscut the entire crust above the subduction megathrust and are expected to have a close relationship to transient processes of the subduction earthquake cycles. With the motivation to image and characterize the structural inventory and the processes that occur in the vicinity of these large-scale fault zones, we re-processed the ANCORP'96 controlled-source seismic data set to provide images of the faults at depth and to allow linking geological information at the surface to subsurface structures. The correlation of the imaging results with observed hypocenter locations around these fault systems reveals the origin and the nature of the seismicity bound to these fault systems. Active and passive seismic data together yield a picture of a megathrust splay fault beneath the Longitudinal Valley at mid-crustal level, which can be observed from the top of the subduction plate interface and which seems to be connected to the Precordilleran Fault System (PFS) known at the surface. This result supports a previously proposed tectonic model where a megathrust splay fault defines the Western Altiplano as a crustal-scale fault-bend-fold. Furthermore, we clearly imaged two branches of the Uyuni-Kenayani Fault (UKF) in a depth range between 0 and 20 km. In summary, imaging of these faults is important for a profound understanding of the tectonic evaluation and characterization of the subduction zone environment, for which the results of this study provide a reliable basis.