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Highlights • First high-resolution data from a creepmeter array collocated with seismometers. • Monitored faults are highly susceptible to dynamically triggered shallow slip. • Far-field triggering relates to surface waves, near-field triggering to body waves. • Large earthquakes can trigger shallow slip events over distances of 103–105 km. • Triggered shallow slip scales with seismic energy density. Abstract The M w = 8.8 Maule Earthquake of February 27, 2010, remotely triggered minor surface displacement along the Atacama Fault System in Northern Chile located 1500–1800 km from the epicenter. Here we present evidence that surface displacement events recorded by the IPOC (Integrated Plate Boundary Observatory in N-Chile) Creepmeter Array are related to earthquakes on the underlying subduction zone interface or to large earthquakes around the world. Our observations document dynamic triggering of shallow slip on faults in a forearc setting. Continuous monitoring revealed that up to 30 displacement events per year occur on the monitored fault segments, >90% of them triggered by earthquakes in the near or far field synchronous to the passage of the seismic wave. We can clearly attribute far field triggering to the passage of the surface wave and near field triggering to the passage of the body wave. The most distant triggered displacement event monitored occurred after the 2011, M w = 9.0 Tohoku-Oki Earthquake located 16 468 km away following the passage of large amplitude SS waves. Comparing the triggered displacement events with other triggered phenomena like seismicity, volcanic eruptions, hydrological changes as well as with data from the California Creepmeter Array we find that the seismic energy density appropriately describes a magnitude–distance scaling relationship of slip triggering. Finally, the observed surface displacement is found to not occur by continuous creep, but rather through nearly exclusively discrete displacement events. [ABSTRACT FROM AUTHOR]

Abstract: A systematic set of analogue models has been carried out to study the structural evolution of a sedimentary pile consisting of superposed brittle and ductile lithologies in a regional context of passive margins. The models are composed of silicone representing a ductile basal layer of evaporites or overpressured shales, and sand progressively sedimented on top to simulate brittle overburden sediments. Channels with various shapes are embedded in the silicone. Deformation is triggered by gravitational instability of the growing sedimentary pile and basal slope angles ranging from 0 to 5°. The main results follow. [•] A polygonal fault pattern developed, due to the 3D boudinage of the overburden sand above the silicone layer during multidirectional extension. [•] If a sandy channel was embedded in the ductile base, it was immediately cut by faults, which were perpendicular to the channel boundaries, no matter what the angle between channel and slope direction. [•] The boundaries of the channel preferentially localised faults, which propagated through the sedimentary pile above. Ongoing deformation along these faults therefore strongly influenced further accumulation of sediments. [Copyright &y& Elsevier]

Quaternary deformation in the northern Chile forearc is controlled by trench parallel shortening along reactivated Mesozoic faults. Dextral strikes-slip is expressed in NW–SE striking faults of the Atacama Fault System, and reverse displacement dominates in E–W faults. This deformation results of the convergence in a concave-seaward continental margin. On September 11th, 2020, a Mw 6.3 earthquake and its subsequent aftershocks took place in the coastal region of northern Chile, revealing the reactivation of the deepest segment of a WNW–ESE striking upper plate fault. The reactivation of this fault occurred after the Mw 8.1 Iquique earthquake, and it seems to be connected to a N–S interplate locking segmentation of the plate margin, which is clearly shown by the locking pattern before the Iquique earthquake. This poses the question of how heterogeneous locking influences upper plate seismicity and how it relates to trench-parallel shortening. [ABSTRACT FROM AUTHOR]

Surface rupturing earthquakes occur in a wealth of plate tectonic settings and pose important hazard to nearby cities and infrastructure. Nevertheless mechanical models to describe the slip behavior and displacement accumulation pattern of the shallow fault zone are contradictory. Recent observations suggest that, despite low confining pressure and phyllosilicate rich fault gouges at shallow crustal levels, faults can exhibit velocity weakening behavior all the way to the surface. We use continuous time series from the IPOC Creepmeter Array (www.ipoc-network.org/observatory/creepmeter/creepmeter-at-ipoc/) in N-Chile, monitoring 4 segments of the Atacama Fault System, to investigate the slip mode of the different fault segments.This instrumental dataset covers a time span of 8 years at 11 monitoring sites and comprises both triggered shallow slip events and creep signals. The recorded shallow slip events range from μm to mm scale in fault parallel displacement. Slip velocities for these events range between 1 -2000 mm/year therefore representing slow slip events. In fact, the current sampling rate of 2 samples/min does not allow for the detection of seismic velocities for the size of the so far recorded slip events.Decomposing the time series at the different monitored fault segments we are able to differentiate between slow creeping fault segments not accumulating displacement by triggered shallow slip events and others which are accumulating displacement solely by triggered shallow slip events. Comparing these observations with the fault gouge thickness at the monitored sites, we find a strong correlation of segments exhibiting 10 cm thick fault gouge at the monitoring site and creeping fault behavior. Whereas these fault segments, accumulating displacement only via triggered shallow slip events, do not have a fault gouge developed. Instead those fault contacts are developed in a strongly fragmented basement/basement contact zone or show a submillimeter thin sharp contact in alluvial fan sediment. These first results demonstrate the ability to extract fault frictional behavior from time series collected with the IPOC Creep Observatory, that match fault gouge properties studied in the field directly at the monitoring sites. [ABSTRACT FROM AUTHOR]

The Northern Chilean subduction zone is characterized by long-term subduction erosion with very little sediment input at the trench and the lack of an accretionary prism. Here, multichannel seismic reflection (MCS) data were acquired as part of the CINCA (Crustal Investigations off- and onshore Nazca Plate/Central Andes) project in 1995. These lines cover among others the central part of the M W 8.1 Iquique earthquake rupture zone before the earthquake occurred on 1 April 2014. We have re-processed one of the lines crossing the updip parts of this earthquake at 19°40′S, close to its hypocentre. After careful data processing and data enhancement, we applied a coherency-based pre-stack depth migration algorithm, yielding a detailed depth image. The resulting depth image shows the subduction interface prior to the Iquique megathrust earthquake down to a depth of approximately 16 km and gives detailed insight into the characteristics of the seismogenic coupling zone. We found significantly varying interplate reflectivity along the plate interface which we interpret to be caused by the comparably strong reflectivity of subducted fluid-rich sediments within the grabens and half-grabens that are predominant in this area due to the subduction-related bending of the oceanic plate. No evidence was found for a subducted seamount associated to the Iquique Ridge along the slab interface at this latitude as interpreted earlier from the same data set. By comparing relocated fore- and aftershock seismicity of the Iquique earthquake with the resulting depth image, we can divide the continental wedge into two domains. First, a frontal unit beneath the lower slope with several eastward dipping back-rotated splay faults but no seismicity in the upper plate as well as along the plate interface. Secondly, a landward unit beneath the middle slope with differing reflectivity that shows significant seismicity in the upper plate as well as along the plate interface. Both units are separated by a large eastward dipping mega splay fault, the root zone of which shows diffuse seismicity, both in the upper plate and at the interface. The identification of a well-defined nearly aseismic frontal unit sheds new light on the interplate locking beneath the lower continental slope and its controls. [ABSTRACT FROM AUTHOR]

Along the Northern Chilean active continental margin, the subducting Nazca plate is characterized by a rough sea floor topography that has been suggested to control the rupture behaviour of megathrust earthquakes. However, there is still debate of what structures exactly controlled the extent of the rupture of the Mw 8.12014 April 1st Iquique earthquake and why it only broke 1/3 of a large seismic gap that last ruptured completely in 1877. To better understand the seismotectonic segmentation of the northern Chilean convergent margin, we use datasets from different geophysical and geodetic studies in this area to produce a 3D model. We combine depth migrated images of the two northernmost multi-channel seismic reflection CINCA'95 (Crustal Investigations off- and onshore Nazca Plate/Central Andes) lines, bathymetry data, coseismic slip models, geodetic coupling, seismic b values, relocated seismic events and the morphology of the subduction interface from gravity modelling. The interface morphology shows a prominent surface relief that spacially correlates with the rupture process of the mainshock on April 1st and also for the largest aftershock on April 3rd. The main slip area exhibits a strong correlation with a large elongated topographic depression of the subducting slab. An elongated topographic high on the subducting plate to the south of that depression correlates with low pre-seismic locking and very likely acted as a barrier for rupture propagation for the main shock, as well as for the largest after shock. A subducted circular topographic high of 25 km in diameter located updip of the rupture area, possibly prevented coseismic slip to rupture all the way up to the trench axis. Thus, our observations support that subducting sea floor morphology plays an important role controlling rupture processes. • We present the pre-stack depth migrated image of CINCA'95 line25. • The subducting slab is clearly imaged until a depth of 16 km. • More large scale structures are clearly imaged within the continental crust • A surface from a 3D density model yields topographic structures on the subducted oceanic crust. • A topographic depression on the subducting oceanic crust correlates with the location of a seismic asperity. [ABSTRACT FROM AUTHOR]

Fault-related uplift is one of the possible drivers for the abandonment of ancient and, due tothe desert’s hyper-arid climate, pristinely preserved fluvial networks in the northern ChileanAtacama Desert. In this study, we examine fluvial features like paleochannels, fans andclaypans which were blocked or offset by trench-parallel faults in the Coastal Cordillera. Wefocus on two fault strands within the Cordillera, the Chomache (CFZ), and the Salar GrandeFault Zones (SGFZ) 10 km north. By gaining a better understanding of the morphology andsedimentological properties of the features, we aim to quantify the vertical and horizontaldisplacement of these faults. Therefore, we conducted Ground Penetrating Radar (GPR)measurements with 100 MHz and 270 MHz antennas, terrestrial light detectionand ranging (LiDAR) scans, differential global positioning system (dGPS) as wellas drone-based structure-from-motion (SfM) measurements. For visualizing thetectonic framework, we also compared these data sets to findings from remote sensingmethods. The NW-SE striking dextral reverse faults of the CFZ offset Pleistocene surfaces andtherefore indicate Holocene activity (Gonzalez et al., 2003). At the western segments of thePunta de Lobos fault, which is one of the CFZ’s branches, the interpretation of radargramsindicates sediment layers of paleofans which warp up to the fault’s footwall. Furthermore, bycomparing the relative location of reflector fields linked to paleochannel infills, we couldderive horizontal displacements ranging from a few metres up to 100 m. Drone elevationmodels visualize knickpoints also in the most recently blocked and deflected drainages in thisarea. By measuring the displaced channels’ horizontal and vertical offsets as well as therelative height of knickpoints along the fault segments, we determined approximateslip vectors for displacement on the faults. Furthermore, the drone images givevaluable information about the spatial extent and orientation of the fault-related crackfields. We also apply the GPR and drone measurement techniques on parts of the seismicallyactive SGFZ, a network of NW-SE oriented strike-slip faults, several subparallel normalfaults and EW striking reverse faults (Gonzalez et al., 2003). Radargrams measured on theSalar perpendicular to the main strand of the SGFZ indicate a vertical offset of up to 4 m.Further south, radargrams of fault-parallel and -perpendicular measurements showblocked fan sediments which provide information about the fans’ shape, sedimentlayering, relative timing of sedimentary deposits, as well as paleochannel gradients andshape. Reference:Gonzalez, G., J. Cembrano, D. Carrizo, A. Macci, and H. Schneider (2003): The linkbetween forearc tectonics and Pliocene-Quaternary deformation of the CoastalCordillera, northern Chile, Journal of South American Earth Sciences, 16, pp. 321-342. [ABSTRACT FROM AUTHOR]

The prevalence of ancient abandoned fluvial landforms in the northern Chilean Atacama Desert is a testament to both the long-term hyperaridity of this setting and its sensitivity to change. Ascertaining what has driven drainage adaptations has the potential to reveal past environmental changes and whether they were localized or regional in extent. However, it is typically unclear whether processes related to climate, tectonics, or both, are the cause of drainage reorganization. Alternatively, an often overlooked driver of topographic change is the breaching of geomorphic thresholds, whereby abrupt erosional or depositional episodes occur without the need for external causes. Instances of geomorphic thresholds include periods of degradation and aggradation experienced by ephemeral streams, or stream piracy events. In this paper we will discuss the incision history of the Tiliviche River (Quebrada Tiliviche) in northern Chile and unravel the contributions made by climate, tectonics and geomorphic thresholds to the evolution of this drainage.Like most of the rivers in northern Chile that reach the Pacific Ocean, the Tiliviche River has its source in the Andes Mountains (Precordillera) and takes a westward route through the hyperarid Central Depression and Coastal Cordillera. Using a cosmogenic nuclide derived chronology for the formation of a flight of river terraces located in the Central Depression we infer a dramatic increase of the incision rate of the Tiliviche River at around 350 ka ago. The fluvial terraces lie downstream of a subtle, fault-related, upward warping of the surrounding pediment surface for tens of km north and south of our sample site. Explanations for a more rapid rate of river incision over the last ca. 350 ka include: an increase in the amount of water available for discharge, possibly related to more precipitation in the Andean headwaters of the basin, or brought about by a stream capture event increasing the upstream catchment area; base-level drop in response to local fault driven uplift; or a base-level fall resulting from the draining of a paleolake downstream or connection of the Tiliviche River to the sea. The likelihood of each of these explanations is considered in greater detail with reference to characteristics of the local faults, the timing of abandonment of adjacent paleochannels and mapping of other Tiliviche River terraces, the dating of which is currently underway. [ABSTRACT FROM AUTHOR]

On 1 April 2014, Northern Chile was struck by a magnitude 8.1 earthquake following a protracted series of foreshocks. The Integrated Plate Boundary Observatory Chile monitored the entire sequence of events, providing unprecedented resolution of the build-up to the main event and its rupture evolution. Here we show that the Iquique earthquake broke a central fraction of the so-called northern Chile seismic gap, the last major segment of the South American plate boundary that had not ruptured in the past century. Since July 2013 three seismic clusters, each lasting a few weeks, hit this part of the plate boundary with earthquakes of increasing peak magnitudes. Starting with the second cluster, geodetic observations show surface displacements that can be associated with slip on the plate interface. These seismic clusters and their slip transients occupied a part of the plate interface that was transitional between a fully locked and a creeping portion. Leading up to this earthquake, the b value of the foreshocks gradually decreased during the years before the earthquake, reversing its trend a few days before the Iquique earthquake. The mainshock finally nucleated at the northern end of the foreshock area, which skirted a locked patch, and ruptured mainly downdip towards higher locking. Peak slip was attained immediately downdip of the foreshock region and at the margin of the locked patch. We conclude that gradual weakening of the central part of the seismic gap accentuated by the foreshock activity in a zone of intermediate seismic coupling was instrumental in causing final failure, distinguishing the Iquique earthquake from most great earthquakes. Finally, only one-third of the gap was broken and the remaining locked segments now pose a significant, increased seismic hazard with the potential to host an earthquake with a magnitude of >8.5. [ABSTRACT FROM AUTHOR]

The Coppenbrügge subaqueous ice-contact fan complex of early Saalian age is about 10 km long and up to 10 km wide and is composed of offset-stacked fan clinothems that are transgressive-regressive sequences formed during an overall lake level rise. The individual fan bodies consist of coarse gravel in the ice-proximal part, passing distally into sandy facies and showing large-scale foreset bedding. The iceberg scour recognized in an open-pit outcrop is up to 1.5 m deep, up to 2 m wide and cut in undisturbed mid-fan deposits. The scour-fill can be traced laterally for about 15 m and consists of sheared sand and, in the frontal zone, of downbent overlying strata surrounded by a zone of deformed sediments. The deformed sediment produced by the iceberg keel's shearing of the trough walls is a sand mass containing angular soft-sediment clasts that show internal folds and fractures. The basal surface of the deformed sediment is a nearly horizontal shear plane, steepening up laterally as a discrete thrust and showing a flat-ramp-flat geometry. The scour was formed by the iceberg keel's ploughing the substrate and pushing the sediment sideways and frontally, forming a ridge of deformed sediments at the trough end. This ridge was concurrently eroded by an accompanying meltwater underflow which apparently developed a horseshoe system of scouring vortices around the grounded iceberg. The current's scour was filled with massive, nonstratified sand deposited rapidly from turbulent suspension. The iceberg eventually broke up and its keel part was buried. As these ice fragments gradually melted, the space was closed by normal faulting and downbending of overlying strata. The collapsing scour-fill became partly liquified, and the resulting water-escape structures cut the normal faults and the overlying deposits. Though produced chiefly by tangential shear strain, iceberg-ploughing features are readily distinguishable from other glaciotectonic deformations. They can serve... [ABSTRACT FROM AUTHOR]