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 VP/VS 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.

Previous studies in the forearc of the northern Chilean subduction zone have identified important tectonic features in the upper crust. As a result of these works, the West Fissure Fault System (WFFS) has recently been imaged using microseismic events. The WFFS is the westward-dipping, sharp lower boundary of the northern Chilean forearc and is geometrically opposed to subduction of the Nazca plate. The present article builds on this previous work and is novel in that it characterizes this structure's stress distribution using focal mechanisms and stress tensor analysis. The results of the stress tensor analysis show that the state of stress in the WFFS is related to its strike-slip tectonic context and likely represents a manifestation of local forces associated with the highest areas in the Andes. Two seismic clusters have also been identified; these clusters may be associated with a blind branch of the WFFS. We studied these clusters in order to determine their sources and possible connection with fluid migration across the upper plate. We observed that the two clusters differ from one another in some regards. The central cluster has characteristics consistent with an earthquake swarm with two clearly identifiable phases. Conversely, the SW cluster has a clear main shock associated with it, and it can be separated into two subclusters (A and A'). In contrast, similarities among the two clusters suggest that the clusters may have a common origin. The b-values for both clusters are characteristic of tectonic plate boundaries. The spatial spreading, which is approximately confined to one plane, reflects progressive growth of the main fracture underlying the swarm and subcluster A. We also find that earthquakes themselves trigger aftershocks near the borders of their rupture areas. In addition, the spatio-temporal migration of hypocentres, as well as their spatial correlation with areas that are interpreted to be fluid migration zones, suggest that there is a close relationship between fluid movement and the earthquake sources associated with the swarm and subcluster A. These observations point to stick-slip behaviour of the rupture propagation, which can be explained by earthquake-induced stress transfer and fluid flow in a fluid-permeated, critically loaded fault zone.

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 20km. 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.

The coda of passive seismic recordings is often rich in arrivals that are coherent across several stations. If reflections can be extracted, then they may be used for seismic reflection subsurface imaging. With the objective to image the upper crust of the North Chilean Precordillera (Central Andes; approximate location 21 degrees S 69 degrees W), we developed a workflow to process passive seismic data into subsurface reflection images. We analysed the waveform recordings of several hundred microseismic events using signal processing and imaging techniques adapted from active (controlled source) seismic imaging as used in the oil industry. Key processing steps involved precise arrival time picking and hypocentre determination, removing signal amplitude variations due to varying source radiation patterns, identification and separation of reflections from coherent noise, and transformation of the processed waveforms into images of the subsurface reflectivity. When designing our microseismic reflection imaging workflow, we took advantage of the fact that the passive seismic recording geometry with the hypocentres located at depth and the receivers positioned at the surface resembles a reverse vertical-seismic profiling experiment. The resultant P- and S-wave reflection images reveal several reflective features, such as an approximate 15 degrees westward dipping reflector over the 5-25 km depth range that largely coincides with a distinct seismicity boundary. We interpret the imaged interface as the brittle-ductile transition zone boundary, possibly enhanced by a tectonic shear zone. For the area of the North Chilean Precordillera, the deduced microseismic reflection sections with horizontal extensions of about 50 km represent the first high-resolution images of the shallow crust, which could not be obtained from previous active-source seismic-reflection data.

The western Eger rift at the Czech–German border in central Europe is an important geodynamically active area within the European Cenzoic rift system (ECRS) in the forelands of the Alps. Along with two other active areas of the ECRS, the French Massif Central and the east and west Eifel volcanic fields, it is characterized by numerous CO2-rich fluid emission points and frequent micro-seismicity. Existence of a plume(s) is indicated in the upper mantle which may be responsible for these observations. Here we reprocess a pre-existing deep seismic reflection profile ‘9HR’ and interpret the subsurface structures as mapped by seismic reflectivity with previous findings, mainly from seismological and geochemical studies, to investigate the geodynamic activity in the subsurface. We find prominent hints of pathways which may allow magmatic fluids originating in the upper mantle to rise through the crust and cause the observed fluid emanations and earthquake activity.

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 21A degrees 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.

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.