ALBERT

Description: We obtained high-precision locations for 5250 earthquakes in the Iquique segment of the northern Chilean subduction zone from two temporary local seismic networks around 21°S. A double seismic zone in the downgoing Nazca slab can be clearly identified. One band of seismicity is located at the plate interface and a second one 20–25 km deeper in the oceanic mantle. It can be traced updip to uncommonly shallow levels of 50 km. A combined interpretation of seismicity and reflectivity along the seismic ANCORP’96 experiment suggests the prevalence of fluid processes in the subducted oceanic crust as well as in the uppermost 20 km of the mantle. Crustal seismicity is pervasive below the Coastal Cordillera. Beneath the Precordillera, the lower bound of crustal seismicity delineates a sharp west-dipping boundary down to 20 km depth, consistent with earlier findings indicating a rheological boundary.

Description: The focus of this study is the high- resolution localization of more than 800 earthquakes in the Northern Chilean Salar Grande region at about 21S within the Andean Costal Cordillera. The events have been recorded by a temporary local network in 2010. We find, that seismicity is not only related to the Nazca slab but also occurs widely scattered within the overlying continental crust. Our highly resolved locations with typical uncertainties below 200 meters image two distinct seismogenic zones at the top and deeper within the mantle of the Nazca slab, as well as the prominent Atacama Fault Zone. The latter could eventually penetrate the entire crust, possibly joining the subduction interface at a depth of about 40 kilometers. In our further investigation, we have applied a waveform cross-correlation approach by which we were able to identify clusters of similar events with respect to location and source mechanism. Within these clusters we took advantage of waveform similarity to further decrease location uncertainties. Most of the crustal seismicity clusters locate on a subvertical planar structure beneath the surface traces of the Atacama Fault Zone, which extends from close to the surface down to the slab. This could indicate that seismicity in the forearc is not only caused by subduction- related deformation, but also by fluid processes. The irregular spatial distribution of the Nazca slab related clusters may be a consequence of topographic variations within the downgoing slab.

Description: The ratio of seismic P- to S-wave velocities (the Vp/Vs ratio) of a given rock volume is a sensitive proxy for the detection of fluids and melts. In subduction regimes it has often been inferred from seismic tomography and been used, e.g., to detect pathways of ascending melt above the seismogenic zone, where tomographic methods have their highest resolution. We present Vp/Vs ratios that were computed using only seismic arrival time observations following the approach of Lin and Shearer (2007). This approach has its highest sensitivity in the source volume of a set of nearby seismic events and is hence particularly well suited to directly probe the plate interface. We present data from a temporary local network of short period seismometers that was in operation in the forearc of the Central Andean subduction zone at 21 ◦ S between 2005 and 2012. From this database we were able to localize 3253 seismic events (Ml ∼ 0.5–4) with high precision, yielding a detailed image of the seismicity distribution in this region. Seismicity is pervasive within the entire crust of the South American continental plate and exhibits three distinct bands in the subducting slab, the lowermost one being located in the lithospheric mantle of the subducting plate. The highest concentration of seismic events is found in the contact zone between the continental and the oceanic lithosphere at depths between 30 and 50 km. We group seismic events into approximately 100 subsets of nearby events that origin from the same geo- logical structure. For about half of these subsets we are able to extract a reliable local Vp/Vs ratio. In the middle continental crust, Vp/Vs ratios show slightly enhanced values ( ∼ 1.75). In the lower continental crust towards the plate interface they tend to increase from this value updip and decrease downdip. At the plate interface itself, we observe higher Vp/Vs ratios (〉1.8) at shallower depths (between 20 and 40 km). Downdip (40–60 km depth) Vp/Vs ratios decrease to rather typical values ( ∼ 1.75). The same trend is observed in the lowermost band of mantle seismicity in the subducting slab. Below 80 km depth, where mineral transitions toward the eclogite facies are expected to occur, Vp/Vs ratios tend to be low (〈1.75). The consistently high Vp/Vs ratios in the shallow part of the subducting slab hint at the presence of fluids in the porespace of the subducting lithosphere there. In the deeper part, downdip variations of Vp/Vs may be attributed to mineral phase transitions due to the changing P-T-conditions along the subduction pathway.

Description: We present a dataset of seismicity from a temporary local network that was installed in the Iquique segment of the northern Chilean subduction zone. The segment has experienced its last activation 135 years ago and is hence expected to be in the late interseismic phase of the earthquake cycle. The dataset exhibits great details and fine structures of the fore-arc subduction system in general and the plate-interface in detail. We performed a state-of-the-art relocation procedure that features a waveform-based correction of arrivaltime pick uncertainties, the incorporation of an independently obtained velocity model, and the application of source-specific station terms to reduce effects of inconsistencies in the latter. This yielded locations of nearly 5,500 events with a mean RMS-misfits as low as 30ms. We find a high downdip-variability in seismic activity along the plate interface. This includes well-defined, platy shaped patches of enhanced seismicity at depths around 35 and 45km, respectively, and a sudden downdip end of seismicity near the tip of the continental mantle wedge. Seismicity at the plate interface correlates tightly with the previously obtained reflectivity image of the down-going slab from the ANCORP’96 experiment. More details are revealed in the direction perpendicular to the slab. Whilst seismicity is highest within a few kilometer thick layer directly at the plate contact, an overlying region of reduced seismicity separates this from the also abundant seismicity within the deforming continental crust of the overriding plate. Apart from seismicity at the plate interface, we also find a second band deeper inside the slab near the oceanic Moho and a well defined third band approximately 15km below the oceanic Moho inside the oceanic lithospheric mantle that streches from a depth of only 40km offshore to 90km near 69.0°W.

Description: We obtained high-precision locations for 5250 earthquakes in the Iquique segment of the northern Chilean subduction zone from two temporary local seismic networks around 21°S. A double seismic zone in the downgoing Nazca slab can be clearly identified. One band of seismicity is located at the plate interface and a second one 20–25 km deeper in the oceanic mantle. It can be traced updip to uncommonly shallow levels of 50 km. A combined interpretation of seismicity and reflectivity along the seismic ANCORP’96 experiment suggests the prevalence of fluid processes in the subducted oceanic crust as well as in the uppermost 20 km of the mantle. Crustal seismicity is pervasive below the Coastal Cordillera. Beneath the Precordillera, the lower bound of crustal seismicity delineates a sharp west-dipping boundary down to 20 km depth, consistent with earlier findings indicating a rheological boundary.

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.