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Based on gravity and bathymetric data and using a novel two-dimensional joint flexural-density modeling approach, this work studies the physical properties of the oceanic Nazca plate around the Taltal, Copiapó, and Iquique hotspot ridges offshore northern Chile. The area is located westward of the Chilean Trench where the Taltal and Copiapó Ridges collide with the continental margin. The results show that the variability in density structure at different scales is a key factor in explaining the observed gravity signal, playing an important role in the lithospheric flexure and hence the elastic properties of the Nazca plate in this setting. The results can be interpreted as evidence of spatial and temporal heterogeneities in the plate-weakening process at the hotspots, magmatic underplating, and crustal and upper mantle fracturing and/or hydration. These processes might be relevant for the ascent of magma pathways of later (secondary) volcanism and influence the mechanical segmentation of the oceanic plate. The latter is critical in explaining the active seismogenic contact between the oceanic Nazca and overriding South America plates. [ABSTRACT FROM AUTHOR]

Until now, few offshore seismic studies have acquired simultaneously P‐ and S‐ wave data to derive in detail the seismic structure of the oceanic crust. We present 2‐D Vp and Vs models using wide‐angle seismic data at the Indian basin adjacent to the NinetyEast Ridge. Here, an outcrop basement located at the middle of the seismic line presents uppermost crustal Poisson's ratios (ν) of 0.28–0.29 (Vp ∼ 4.2 km/s and Vs ∼ 2.3 km/s). At the flanks of the outcrop basement, the sediment cover is 200–300 m thick and ν values are similar (0.28–0.3), but Vp and Vs values are higher (4.5–4.8 and 2.4–2.6 km/s, respectively). We interpret the relatively lower Vp and Vs around the basement outcrop in terms of hydrothermal alteration, while at the flanks of the basement outcrop, hydrothermal alteration has most likely ceased by sedimentation and compaction processes. Across the seismic layer 2, the Vp–Vs trend is linear and follows a ν value of 0.28–0.29, however, at the seismic layer 2/3 transition, the Vp–Vs trend abruptly changes following a ν value of 0.25–0.26. These reduced observed ν values at the layer 2/3 transition are lower than those reported by laboratory measurements for gabbro (ν ∼ 0.293) and are interpreted in terms of epidotization at the dike‐gabbro contact and/or crack‐change properties around the lower part of the intrusive sheeted dike section. Plain Language Summary: The Ninety(90°)East(E) Ridge (NER) is a hotspot track formed by the Kerguelen hotspot mantle plume onto the Indo‐Australian plate more than 60 Myr ago. The length and width of the NER is about 5,000 and 200 km, respectively, and it extends from the Bengal bay to the southeast Indian mid ocean ridge. West and east of the NER are sited the Indian and Wharton basin, respectively. Here, we study the structure of the Indian basin at the western flank of the NER where the seafloor is highlighted by the presence of prominent oceanic basement outcrops. Our seismic results suggest that mineral alteration processes occur both in the upper and lower crust, respectively, under a basement outcrop due to the contact of seawater with crustal rocks. Probably, crustal accretion in a setting characterized by the interaction of a hotspot mantle plume with an active spreading center affect the porosity and permeability of the ocean crust around the NER region at large‐scale. Key Points: We obtain 2‐D Vp and Vs models from active seismic data for the Indian oceanic crustThe seismic models suggest hydrothermal alteration near a basement outcropPoisson's ratios change at the layer 2/3 transition from 0.28–0.29 to 0.25–0.26 [ABSTRACT FROM AUTHOR]

[1] Based on a compilation of published and new seismic refraction and multichannel seismic reflection data along the south central Chile margin (33[degrees]-46[degrees]S), we study the processes of sediment accretion and subduction and their implications on megathrust seismicity. In terms of the frontal accretionary prism (FAP) size, the marine south central Chile fore arc can be divided in two main segments: (1) the Maule segment (south of the Juan Fernandez Ridge and north of the Mocha block) characterized by a relative large FAP (20-40 km wide) and (2) the Chilo6 segment (south of the Mocha block and north of the Nazca-Antarctic-South America plates junction) characterized by a small FAP ([less than or equal to]10 km wide). In addition, the Maule and Chiloe segments correlate with a thin (

We investigate the variations of the seismic source properties and aftershock activity using kinematic inversions and template-matching for six large magnitude intermediate-depth earthquakes occurred in northern Chile. Results show similar rupture geometry and stress drop values between 7 and 30 MPa. Conversely, aftershock productivity systematically decreases for the deeper events within the slab. Particularly, there is a dramatic decrease in aftershock activity below the 400–450 °C isotherm depth, which separates high- and low-hydrated zones. The events exhibit tensional focal mechanisms at unexpected depths within the slab, suggesting a deepening of the neutral plane, where the extensional regimen reaches the 700–800 °C isotherm depth. We interpret the reduction of aftershocks in the lower part of the extensional regime as the absence of a hydrated-slab at those depths. Our finding highlights the role of the thermal structure and fluids in the subducting plate in controlling the intermediated-depth seismic activity and shed new light in their causative mechanism. [ABSTRACT FROM AUTHOR]

The aftershock distribution of the 2014 Mw 8.1 Iquique earthquake offshore northern Chile, identified from a long‐term deployment of ocean bottom seismometers installed eight months after the mainshock, in conjunction with seismic reflection imaging, provides insights into the processes regulating the updip limit of coseismic rupture propagation. Aftershocks updip of the mainshock hypocenter frequently occur in the upper plate and are associated with normal faults identified from seismic reflection data. We propose that aftershock seismicity near the plate boundary documents subduction erosion that removes mass from the base of the wedge and results in normal faulting in the upper plate. The combination of very little or no sediment accretion and subduction erosion over millions of years has resulted in a very weak and aseismic frontal wedge. Our observations thus link the shallow subduction zone seismicity to subduction erosion processes that control the evolution of the overriding plate. Plain Language Summary: To better understand the controls on shallow seismicity and subduction erosion following large subduction earthquakes, we use marine recordings of the Mw 8.1 2014 Iquique earthquake aftershocks and long‐offset multi‐channel seismic data. By comparing the aftershock locations and seismic imaging, we observe that most aftershocks occurred in the upper continental plate and abruptly stopped in the frontal forearc. The amplitude characteristics of upper‐crust reflections indicate a fractured and fluid‐filled outer forearc, which is associated with the absence of aftershocks. Large‐scale faulting, as evidenced by disrupted reflections in the seismic image, can be correlated to upper plate seismicity. We propose that the aftershocks updip of the main earthquake area reflect active subduction erosion processes. Key Points: We investigate structure and seismicity at the updip end of the 2014 Iquique earthquake rupture using amphibious seismic dataSeismicity updip of the 2014 Iquique earthquake occurs over a broad range likely interpreted to be related to the basal erosion processesCoseismic stress changes and aftershocks activate extensional faulting of the upper plate and subduction erosion [ABSTRACT FROM AUTHOR]

Abstract Hotspots are generated by partial melting due to hot plumes rising within the Earth's mantle, and when tectonic plates move relative to the plume source, hotspot tracks form. Off South America, the oceanic Nazca Plate hosts a large population of hotspot tracks. Examples include seamounts formed far from the Pacific-Nazca spreading center (" off-ridge " seamounts), such as the Juan Fernández Ridge (Juan Fernández hotspot), the Taltal Ridge (San Félix hotspot), and the Copiapó Ridge (Caldera hotspot). These hotspot tracks are characterized by a rough and discontinuous topography. Other examples include seamounts formed near the East Pacific Rise (EPR) (" on-ridge " seamounts), such as the Nazca Ridge (Salas y Gómez hotspot) and Easter Seamount Chain (Easter hotspot), and the Iquique Ridge (Foundation hotspot). These oceanic ridges developed a relatively smooth and broad morphology. Here, we present a plate reconstruction of these six oceanic hotspot tracks since the Paleocene, providing a kinematic model of ridge-continental margin collision. For the " off-ridge " seamount group, the plate kinematic reconstruction indicates that the collision point remained quasi-stationary from 40 to 30–25 Ma. Eventually, the southward migration of the collision point of this seamount group accelerated from 23 to 15 Ma (reaching a maxima speed of 300 km/Ma along the trench). From 15 Ma to present the collision point has remained quasi-stationary. The predicted location of the subducted portion of the Taltal, Copiapó and Juan Fernández Ridges coincides with the southward migrating (relative to South America) flat slab segment. For the " on-ridge " seamount group, the kinematic plate reconstruction indicates a continuous southward migration of the collision point from ~23 Ma, which is related to the fragmentation of the Farallon Plate. The southward migration accelerated until 15 Ma, reaching approximately 150 km/Ma. From 15 Ma to present, the southward migration has been decelerating except an increment of the migration velocity during the Chron 4 due to an increase of the convergence velocity. The migration velocity differences between the on-ridge and off-ridge hotspot tracks are mainly result from the hotspot track azimuth and the margin azimuth on the collision point. Convergence velocity varies along the trench, but it is a minor factor comparing different hotspot tracks migration velocity. Due to the EPR-plume interactions, our reconstruction suggests that the eastern Tuamotu Island Plateau formation occurred 48–27 Ma on the Easter Hotspot, which was located near to the EPR segment between the Marquesas and Austral Fracture Zones. Our model also predicts that the Iquique Ridge seamounts track is consistent with the position of the Foundation hotspot. The Foundation hotspot jumped to the Challenger (Resolution) Fracture Zone from the Farallon plate to the Pacific plate. This process triggered the cessation of the Iquique Ridge volcanic formation, and initiated volcanism at Foundation Chain in the Pacific Plate at ~25 Ma. Graphical abstract Unlabelled Image Highlights • We present a plate reconstruction model for the Hotspot tracks in the Nazca Plate. • We show a paleo-bathymetric reconstruction from 60 Ma to present off South America. • We provide a kinematic model of ridge-trench collision off South America. [ABSTRACT FROM AUTHOR]

Recent studies have proposed that nucleation zones, seismic barriers and high-slip patches of megathrust earthquakes correlate with physical heterogeneities, both in the oceanic/subducting plate and in the continental wedge. We present a density-depth model along the Nazca-South America subduction margin, from 18°S to 23°30′S, where a partial segment of this zone was ruptured by the M w 8.2 Iquique earthquake on 2014 April 1. The density modelling and interpretation were constrained with seismic reflection profiles, published Vp -depth tomographic models and relocated seismicity. These results were used to estimate the variability of normal stress on the seismogenic contact. Our results show a heterogeneous structure for the Northern Chile marine forearc. In particular, we observed a latitudinal and longitudinal segmentation of continental wedge properties, where changes in density can be explained by changes in fracturing degree, which could have an important control on the Iquique earthquake rupture process. This study provides new insights into the analysis of large earthquakes and seismic/tsunami hazard in this active segment of the Chilean margin. [ABSTRACT FROM AUTHOR]

New multibeam bathymetry allows an unprecedented view of the tectonic regime and its along‐strike heterogeneity of the North Chilean marine forearc and the oceanic Nazca Plate between 19 and 22.75°S. Combining bathymetric and backscatter information from the multibeam data with subbottom profiler and published and previously unpublished legacy seismic reflection lines, we derive a tectonic map. The new map reveals a middle and upper slope configuration dominated by pervasive extensional faulting, with some faults outlining a >500‐km‐long ridge that may represent the remnants of a Jurassic or pre‐Jurassic magmatic arc. Lower slope deformation is more variable and includes slope‐failures, normal faulting, re‐entrant embayments, and NW‐SE trending anticlines and synclines. This complex pattern likely results from the combination of subducting lower‐plate topography, gravitational forearc collapse, and the accumulation of permanent deformation over multiple earthquake cycles. We find little evidence for widespread fluid seepage despite a highly faulted upper‐plate. An explanation could be a lack of fluid sources due to the sediment starved nature of the trench and most of the upper‐plate in vicinity of the hyperarid Atacama Desert. Changes in forearc architecture partly correlate to structural variations of the oceanic Nazca Plate, which is dominated by the spreading‐related abyssal hill fabric and is regionally overprinted by the Iquique Ridge. The ridge collides with the forearc around 20–21°S. South of the ridge‐forearc intersection, bending‐related horst‐and‐grabens result in vertical seafloor offsets of hundreds of meters. To the north, plate‐bending is accommodated by reactivation of the paleo‐spreading fabric and new horst‐and‐grabens do not develop. Key Points: Multibeam bathymetric and seismic reflection data image the structure of the North Chilean marine forearc and the oceanic Nazca plateThe structural character and tectonic configuration of the offshore forearc and the oceanic plate change significantly along the marginThe derived pattern of permanent deformation may hold information for studying seismicity or other types of short term deformation [ABSTRACT FROM AUTHOR]

The central Chilean margin (32°–33°S) is characterized by the subduction of the Juan Fernández Ridge (JFR) beneath the continental South American plate. The JFR corresponds to a hotspot track composed by seamounts typically 3–3.5 km high above the surrounding seafloor, with a ridge-trench collision zone underlying the prominent Valparaiso Forearc Basin (VFB). This region has been affected by several large and mega earthquakes, where the last event corresponds to a complex seismic sequence that took place at the southern edge of VFB in April 2017. The spatio/temporal distribution of the seismic events is characterized by a predominant southeast migration of the seismicity. An M w 6.9 earthquake triggered two days after the sequence started and occurred at the northern end of the rupture area of the 1985 M w 8.0 Valparaiso earthquake. We compute the kinematic rupture process of the 2017 M w 6.9 Valparaiso earthquake from the joint inversion of teleseismic body waves and near-field data. The Akaike’s Bayesian Information Criterion was used to objectively estimate both, the relative weighting between datasets and the weighting of spatial and temporal constraints used as a priori information. The coseismic slip is distributed over an area of dimensions ∼35 × 10 km 2 , with a maximum slip of 1.5 m. The rupture propagated unilaterally downdip. The source duration from the moment-rate solution is ∼20 s, with a total seismic moment of 3.05 × 10 19  Nm (M w 6.9). The analysis of the seismicity shows that most of the events occurred along the plate interface, foreshock clustered northern from the mainshock epicenter and the aftershocks occurred to the southeast, at a deeper location. The inverted regional moment tensors show similar faulting mechanism than the mainshock. The seismic sequence started two days before the mainshock and lasted for about two weeks, and a migration pattern of the seismicity was observed. The rupture of the 2017 M w 6.9 earthquake nucleated where the San Antonio seamount (belonging to the JFR) is subducting, and propagated downwards along a zone that presents high interseismic coupling. The complex seismic sequence might be explained by an aseismic slip transient in the zone and the influence of the downdip migration of fluids from the accretionary prism along the subduction channel. The erosive and tunneling effect left by the sudden slip of the subducting seamount might provide the cavity for downdip migration of fluids and subsequent swarm seismicity. [ABSTRACT FROM AUTHOR]

The outer rise is a topographic bulge seaward of the trench at a subduction zone that is caused by bending and flexure of the oceanic lithosphere as subduction commences. The classic model of the flexure of oceanic lithosphere w ( x ) is a hydrostatic restoring force acting upon an elastic plate at the trench axis. The governing parameters are elastic thickness T e , shear force V 0 , and bending moment M 0 . V 0 and M 0 are unknown variables that are typically replaced by other quantities such as the height of the fore-bulge, w b , and the half-width of the fore-bulge, ( x b − x o ). However, this method is difficult to implement with the presence of excessive topographic noise around the bulge of the outer rise. Here, we present an alternative method to the classic model, in which lithospheric flexure w ( x ) is a function of the flexure at the trench axis w 0 , the initial dip angle of subduction β 0 , and the elastic thickness T e . In this investigation, we apply a sensitivity analysis to both methods in order to determine the impact of the differing parameters on the solution, w ( x ) . The parametric sensitivity analysis suggests that stable solutions for the alternative approach requires relatively low β 0 values (<15°), which are consistent with the initial dip angles observed in seismic velocity-depth models across convergent margins worldwide. The predicted flexure for both methods are compared with observed bathymetric profiles across the Izu-Mariana trench, where the old and cold Pacific plate is characterized by a pronounced outer rise bulge. The alternative method is a more suitable approach, assuming that accurate geometric information at the trench axis (i.e., w 0 and β 0 ) is available. [ABSTRACT FROM AUTHOR]

Subduction zones provide direct insight into plate boundary deformation and by studying these areas we better understand tectonic processes and variability over time. We studied the structure of the offshore subduction zone system of the Pampean flat-slab segment ( ca. 29-33°S) of the Chilean margin using seismic and bathymetric constraints. Here, we related and analysed the structural styles of the offshore and onshore western fore-arc. Overlying the acoustic top of the continental basement, two syn-extensional seismic sequences were recognised and correlated with onshore geological units and the Valparaíso Forearc Basin seismic sequences: ( SII) Pliocene-Pleistocene and ( SI) Miocene-Pliocene (Late Cretaceous (?) to Miocene-Pliocene) syn-extensional sequences. These sequences are separated by an unconformity (i.e. Valparaíso Unconformity). Seismic reflection data reveal that the eastward dipping extensional system ( EI) recognised at the upper slope can be extended to the middle slope and controlled the accumulation of the older seismic package ( SI). The westward dipping extensional system ( EII) is essentially restricted to the middle slope. Here, EII cuts the eastward dipping extensional system ( EI), preferentially parallel to the inclination of the older sequences ( SI), and controlled a series of middle slope basins which are filled by the Pliocene-Pleistocene seismic sequence ( SII). At the upper slope and in the western Coastal Cordillera, the SII sequence is controlled by eastward dipping faults ( EII) which are the local reactivation of older extensional faults ( EI). The tectonic boundary between the middle (eastern outermost forearc block) and upper continental slope (western coastal block) is a prominent system of trenchward dipping normal fault scarps ( ca. 1 km offset) that resemble a major trenchward dipping extensional fault system. This prominent structural feature can be readily detected along the Chilean erosive margin as well as the two extensional sets ( EI and EII). Evidence of slumping, thrusting, reactivated faults and mass transport deposits, were recognised in the slope domain and locally restricted to some eastern dipping faults. These features could be related to gravitational effects or slope deformation due to coseismic deformation. The regional inclination of the pre-Pliocene sequences favoured the gravitational collapse of the outermost forearc block. We propose that the structural configuration of the study area is dominantly controlled by tectonic erosion as well as the uplift of the Coastal Cordillera, which is partially controlled by pre-Pliocene architecture. [ABSTRACT FROM AUTHOR]

Subduction megathrust earthquakes show complex rupture behaviour and large lateral variations of slip. However, the factors controlling seismic slip are still under debate. Here, we present 2-D velocity-depth tomographic models across four trench-perpendicular wide angle seismic profiles complemented with high resolution bathymetric data in the area of maximum coseismic slip of the M w 8.8 Maule 2010 megathrust earthquake (central Chile, 34°–36°S). Results show an abrupt lateral velocity gradient in the trench-perpendicular direction (from 5.0 to 6.0 km/s) interpreted as the contact between the accretionary prism and continental framework rock whose superficial expression spatially correlates with the slope-shelf break. The accretionary prism is composed of two bodies: (1) an outer accretionary wedge (5–10 km wide) characterized by low seismic velocities of 1.8–3.0 km/s interpreted as an outer frontal prism of poorly compacted and hydrated sediment, and (2) the middle wedge (∼50 km wide) with velocities of 3.0–5.0 km/s interpreted as a middle prism composed by compacted and lithified sediment. In addition, the maximum average coseismic slip of the 2010 megathrust event is fairly coincident with the region where the accretionary prism and continental slope are widest (50–60 km wide), and the continental slope angle is low (<5°). We observe a similar relation along the rupture area of the largest instrumentally recorded Valdivia 1960 M w 9.5 megathrust earthquake. For the case of the Maule event, published differential multibeam bathymetric data confirms that coseismic slip must have propagated up to ∼6 km landwards of the deformation front and hence practically the entire base of the middle prism. Sediment dewatering and compaction processes might explain the competent rheology of the middle prism allowing shallow earthquake rupture. In contrast, the outer frontal prism made of poorly consolidated sediment has impeded the rupture up to the deformation front as high resolution seismic reflection and multibeam bathymetric data have not showed evidence for new deformation in the trench region. [ABSTRACT FROM AUTHOR]

Reloca Slide is the relict of an ~24-km3 submarine slope collapse at the base of the convergent continental margin of central Chile. Bathymetric and seismic data show that directly to the north and south of the slide the lower continental slope is steep (~10°), the deformation front is shifted landwards by 10-15 km, and the frontal accretionary prism is uplifted. In contrast, ~80 km to the north the lower continental margin presents a lower slope angle of about 4° and a wide frontal accretionary prism. We propose that high effective basal friction conditions at the base of the accretionary prism favoured basal accretion of sediment and over-steepening of the continental slope, producing massive submarine mass wasting in the Reloca region. This area also spatially correlates with a zone of low coseismic slip of the 2010 Maule megathrust earthquake, which is consistent with high basal frictional coefficients. [ABSTRACT FROM AUTHOR]

Powerful subduction zone earthquakes rupture thousands of square kilometres along continental margins but at certain locations earthquake rupture terminates. To date, detailed knowledge of the parameters that govern seismic rupture and aftershocks is still incomplete. On 2015 September 16, the Mw 8.3 Illapel earthquake ruptured a 200 km long stretch of the Central Chilean subduction zone, triggering a tsunami and causing significant damage. Here, we analyse the temporal and spatial pattern of the coseismic rupture and aftershocks in relation to the tectonic setting in the earthquake area. Aftershocks cluster around the area of maximum coseismic slip, in particular in lateral and downdip direction. During the first 24 hr after the main shock, aftershocks migrated in both lateral directions with velocities of approximately 2.5 and 5 kmhr-1. At the southern rupture boundary, aftershocks cluster around individual subducted seamounts that are related to the downthrusting Juan Fernández Ridge. In the northern part of the rupture area, aftershocks separate into an upper cluster (above 25 km depth) and a lower cluster (below 35 km depth). This dual seismic-aseismic transition in downdip direction is also observed in the interseismic period suggesting that it may represent a persistent feature for the Central Chilean subduction zone. [ABSTRACT FROM AUTHOR]

The pre- and current collision of the Juan Fernández Ridge with the central Chilean margin at 31°-33°S is characterized by large-scale crustal thinning and long-term subsidence of the submarine forearc caused by subduction erosion processes. Here, we study the structure of the central Chilean margin in the ridge-trench collision zone by using wide-angle and multichannel seismic data. The transition from the upper to middle continental slope is defined by a trenchward dipping normal scarp with variable offsets of 500-2000 m height. Beneath the scarp, the 2-D velocity-depth models show a prominent lateral velocity contrast of >1 s-1 that propagates deep into the continental crust defining a major lateral seismic discontinuity. The discontinuity is interpreted as the lithological contact between the subsided/collapsed outermost forearc (composed of eroded and highly fractured volcanic rocks) and the seaward part of the uplifted Coastal Cordillera (made of less fractured metamorphic/igneous rocks). Extensional faults are abundant in the collapsed outermost forearc, however, landward of the continental slope scarp, both extensional and compressional structures are observed along the uplifted continental shelf that forms part of the Coastal Cordillera. Particularly, at the landward flank of the Valparaíso Forearc Basin (32°-33.5°S), shallow crustal seismicity has been recorded in 2008-2009 forming a dense cluster of thrust events of Mw 4-5. The estimated hypocentres spatially correlate with the location of the fault scarp, and they highlight the upper part of the seismic crustal discontinuity. [ABSTRACT FROM AUTHOR]

The Maule 2010 megathrust earthquake M w 8.8 has been characterized by two coseismic high-slip patches (asperities) north and south of the epicenter, separated by a region of lower slip. Here, we invert full broadband waveforms to obtain regional moment tensors, yielding precise centroid depth and source parameters of outer rise events (M w > 4.5), including a large M w 7.4 event that occurred just 1.5 h after the Maule mainshock. Outer rise seismicity occurred mainly in two clusters: (1) a large number of outer rise events in the subducting plate located just seaward of the northern asperity of the Maule earthquake, and (2) a second cluster with fewer events seaward of the southern edge of the Maule rupture area. Thus, the outer rise seismicity is correlated with the coseismic rupture of the Maule earthquake, reflecting the stress state of the interplate coupled zone. The moment tensor results indicate similar extensional focal mechanisms for all outer rise events in the northern zone. In the southern region, most of the outer rise events are also extensional, except for one strike slip event located near the oceanic Mocha Fracture Zone. The centroid depths vary from 5 to 20 km depth, and present similar magnitudes. Many of the outer rise events nucleated near the Mocha Fracture Zone, including the M w 7.4 event and one strike–slip event. The calculated yield strength envelope for the oceanic Nazca lithosphere suggests that the centroid depths of intraplate tensional events span almost the entire upper-brittle part of the oceanic lithosphere. [ABSTRACT FROM AUTHOR]

When describing the mechanical behaviour of the lithosphere modelled as a thin plate, the most important parameter corresponds to its flexural rigidity, which is commonly expressed through the effective elastic thickness, Te. This parameter is a measure of the stiffness of the plate and defines the maximum magnitude and wavelength of those surface loads that can be supported without suffering unelastic deformation. Realistic 3-D models of the flexural response of the lithosphere near the trench are scarce because of the mathematical and computational complexity. We present a method for determining the flexure of the lithosphere caused by the combined effect of 3-D seamount loading and bending of the lithosphere near the trench. Our method consists on solving numerically the flexure equations of the Reissner–Mindlin thin plate theory, including variable thickness, using the finite element method with mesh adaptation. The method was applied to study the flexure of the oceanic Nazca lithosphere beneath the O'Higgins seamount group which lies ∼70 km seaward of the Chile trench. The results show that an elastic thickness Te of ∼5 km under the seamounts, a Te of ∼15 km far from the trench and a Te of ∼13 km near the trench can explain both, the down deflection of the oceanic Moho and bending of the oceanic lithosphere observed in seismic and gravity profiles. In order to study the impact of high trench curvature on the morphology of the outer rise, we apply the same methodology to study and model the flexure of the lithosphere in the Arica Bend region (14°S–23°S). Results indicate that the Te values are overestimated if the 3-D trench curvature is not included in the modelling. [ABSTRACT FROM PUBLISHER]

Abstract: Non erosive margins are characterized by heavily sedimented trenches which obscure the morphological expression of the outer rise; a forebulge formed by the bending of the subducting oceanic lithosphere seaward of the trench. Depending on the flexural rigidity (D) of the oceanic lithosphere and the thickness of the trench sedimentary fill, sediment loading can affect the lithospheric downward deflection in the vicinity of the trench and hence the amount of sediment subducted. We used seismic and bathymetric data acquired off south central Chile, from which representative flexural rigidities are estimated and the downward deflection of the oceanic Nazca plate is studied. By flexural modeling we found that efficient sediment subduction preferentially occurs in weak oceanic lithosphere (low D), whereas wide accretionary prisms are usually formed in rigid oceanic lithosphere (high D). In addition, well developed forebulges in strong oceanic plates behaves as barrier to seaward transportation of turbidites, whereas the absence of a forebulge in weak oceanic plates facilitates seaward turbidite transportation for distances >200km. [Copyright &y& Elsevier]

Abstract: The Arauco Basin is located on the continental shelf near the Chilean subduction zone (~38°S). Main deformational stages that affected this setting show a good correlation with Andean constructional phases. We studied the kinematic evolution of the Arauco Basin using high-resolution seismic reflection data across the southern Arauco forearc basin. Three structural domains are identified: (1) Inversion, (2) Extension and (3) Accretion. In addition, seven syn-kinematic sequences have been recognized overlying the Late Carboniferous to Triassic basement, and their relationships with four traditional tectonostratigraphic sequences of the Arauco Basin: (S1) Late Cretaceous syn-extension, (S2) Eocene syn-extension, (S3) and (S4) Eocene syn-inversion, (S5–S6) Miocene syn-inversion with middle extensional structures, and (S7) Pliocene-Quaternary post-inversion and syn-compression. Shortening began in the southern Arauco Basin coeval with a major readjustment of the plate convergence rate (~34Ma) that is represented by inversion structures and kinematic syn-inversion sequences. A marked erosional unconformity (34–23Ma) represents a subsequent event of erosion/uplift during the Oligocene. A contractional Miocene deformation phase reinforced the inversion tectonic and generated Miocene extensional structures. Since the Pliocene the rapid exhumation of the Nahuelbuta Range allows the emergence of the Arauco Basin. Within the basin, Pliocene and Quaternary sequences were affected by contractional structures including the inversion of the Miocene extensional faults. The contraction along the Arauco Basin can be related to the shift to accretionary conditions of the margin and the oblique collision of the Mocha Fracture Zone (~3.6Ma) at ~38°S. The mechanics of regional subsidence which affected the basin during the Miocene cannot be attributed to major scale extensional structures (>1.0km). [Copyright &y& Elsevier]

Abstract: The Chile Triple Junction (CTJ) is the place where the Chile Ridge (Nazca–Antarctic spreading center) is subducting beneath the continental South American plate. Sediment accretion is active to the south of the CTJ in the area where the northward migrating Chile Ridge has collided with the continent since 14Ma. At the CTJ, tectonic erosion of the overriding plate narrows and steepens the continental slope. We present here a detailed tomographic image of the upper lithospheric Antarctic–South America subduction zone where the Chile Ridge collided with the continent 3–6Ma off Golfo de Penas. Results reveal that a large portion of trench sediment has been scraped off and frontally accreted to the forearc forming a 70–80km wide accretionary prism. The velocity–depth model shows a discontinuity at 30–40km landward of the deformation front, which is interpreted as the contact between the frontal (poorly consolidated sedimentary unit) and middle (more compacted sedimentary unit) accretionary prism. The formation of this discontinuity could be related to a short term episode of reduced trench sedimentation. In addition, we model the shape of the continental slope using a Newtonian fluid rheology to study the convergence rate at which the accretionary prism was formed. Results are consistent with an accretionary prism formed after the collision of the Chile Ridge under slow convergence rate similar to those observed at present between Antarctic and South America (∼2.0cm/a). Based on the kinematics of the Chile Ridge subduction during the last 13Ma, we propose that the accretionary prism off Golfo de Penas was formed recently (∼5Ma) after the collision of the Chile Ridge with South America. [Copyright &y& Elsevier]

Abstract: We discuss the earthquake rupture behavior along the Chile–Peru subduction zone in terms of the buoyancy of the subducting high oceanic features (HOF''s), and the effect of the interplay between HOF and subduction channel thickness on the degree of interplate coupling. We show a strong relation between subduction of HOF''s and earthquake rupture segments along the Chile–Peru margin, elucidating how these subducting features play a key role in seismic segmentation. Within this context, the extra increase of normal stress at the subduction interface is strongly controlled by the buoyancy of HOF''s which is likely caused by crustal thickening and mantle serpentinization beneath hotspot ridges and fracture zones, respectively. Buoyancy of HOF''s provide an increase in normal stress estimated to be as high as 10–50MPa. This significant increase of normal stress will enhance seismic coupling across the subduction interface and hence will affect the seismicity. In particular, several large earthquakes (M w ≥7.5) have occurred in regions characterized by subduction of HOF''s including fracture zones (e.g., Nazca, Challenger and Mocha), hotspot ridges (e.g., Nazca, Iquique, and Juan Fernández) and the active Nazca-Antarctic spreading center. For instance, the giant 1960 earthquake (M w =9.5) is coincident with the linear projections of the Mocha Fracture Zone and the buoyant Chile Rise, while the active seismic gap of north Chile spatially correlates with the subduction of the Iquique Ridge. Further comparison of rupture characteristics of large underthrusting earthquakes and the locations of subducting features provide evidence that HOF''s control earthquake rupture acting as both asperities and barriers. This dual behavior can be partially controlled by the subduction channel thickness. A thick subduction channel smooths the degree of coupling caused by the subducted HOF which allows lateral earthquake rupture propagation. This may explain why the 1960 rupture propagates through six major fracture zones, and ceased near the Mocha Fracture Zone in the north and at the Chile Rise in the south (regions characterized by a thin subduction channel). In addition, the thin subduction channel (north of the Juan Fernández Ridge) reflects a heterogeneous frictional behavior of the subduction interface which appears to be mainly controlled by the subduction of HOF''s. [Copyright &y& Elsevier]

The Chilean subduction zone presents a unique opportunity to study trench outer rise deformation of the subducting oceanic lithosphere at different thermal ages. The shape of the outer rise for plate ages ranging from 0 to 50 Ma is predicted by using an elastic plate model with variable elastic thickness as a function of the distance measured from the trench axis. In addition, the uncertainties of our results are estimated by performing a Monte Carlo-type analysis and we considered explicitly the sediment loading effect on the lithospheric flexure in regions where the trench is heavily sedimented. The results show a systematic reduction in of up to 50 per cent (or reduction in the flexural rigidity D of up to ∼90 per cent) from the peak of the outer rise to the trench axis. The reduction in and D observed in most of the bathymetric profiles is coincident with (i) high plate curvatures ( m−1), (ii) strong bending moments ( N m), (iii) pervasive fracturing and faulting of the oceanic basement (as imaged by high-resolution bathymetry data) and (iv) reduction in crustal and mantle seismic velocities. The reduction in flexural rigidity towards the trench suggests a weakening of the oceanic lithosphere and is interpreted to be caused partially by fracturing and a likely increase in fluid-pore pressure. In general, our estimates do not show consistent increases in elastic thickness as a function of plate age. This result suggests that either is independent of plate age or depends strongly on other factors. These factors could include lithospheric weakening due to hydro-fracturing and the loading history of the plate prior to subduction. [ABSTRACT FROM AUTHOR]

Kinematic paleo-reconstruction plate models show that the portion of the Iquique Ridge (IR) located next to the northern Chilean trench was formed 45–50 Ma onto the former oceanic Farallon (currently Nazca) plate near the intersection of the Foundation hotspot and the ancient Farallon-Pacific spreading center. The IR started to collide with South America >40 Ma off northern Peru, and the collision point started to migrate progressively southwards until present becoming northern Chile (20°-22°S) the current collision zone. In order to better understand the mechanical properties of the former oceanic Farallon plate during the IR formation, we used bathymetric and gravimetric data to characterize the hotspot swell and crustal structure of the IR. Results show an anomalous thick crust (10–15 km total thickness) that is capable of producing most of the swell topography (~200 km wide and 500–1000 m high) under nearly isostatic conditions (elastic thickness T e < 5 km, which is consistent with a hotspot track formed onto young oceanic lithosphere < 5 Ma). The swell and anomalous thick crust of the IR is heterogeneously distributed suggesting discontinuous magmatic pulses from the Foundation hotspot to the overlying oceanic lithosphere. As the 45–50 Ma oceanic Nazca plate approaches to the northern Chilean trench, a well pronounced fore-bulge (>200 km wide and >500 m in amplitude) develops accompanied by tensional faulting related to plate bending. By simultaneously modelling, the shape of the outer rise and the hotspot swell topography adjacent to the IR in the ridge-trench collision zone, we find a decrease in T e towards the trench axis (10–50%). This trenchward T e reduction is interpreted in terms of plate weakening caused by fracturing and hydration of the oceanic lithosphere as its yield strength is exceeded by plate bending under conditions of high plate curvatures (>10−7 m−1). • The Iquique Ridge overlies an anomalous thick crust that was formed under isostatic conditions (T e < 5 km). • The anomalous thick crust beneath the Iquique Ridge is heterogeneously distributed. • We report a trenchward decrease in T e (10–50%) off northern Chile. [ABSTRACT FROM AUTHOR]

Several trench-outer rise settings in subduction zones worldwide are characterized by a high degree of alteration, fracturing and hydration. These processes are induced by bending-related faulting in the upper part of the oceanic plate prior to its subduction. Mapping of P- and S-wave velocity structures in this complex tectonic setting provides crucial information for understanding the evolution of the incoming oceanic lithosphere, and serves as a baseline for comparison with seismic measurements elsewhere. Active source seismic investigations at the outer rise off Southern Central Chile (∼43°S) were carried out in order to study the seismic structure of the oceanic Nazca Plate. Seismic wide-angle data were used to derive 2-D velocity models of two seismic profiles located seaward of the trench axis on 14.5 Ma old crust; P01a approximately parallel to the direction of spreading and P03 approximately parallel to the spreading ridge and trench axes. We determined P- and S-velocity models using 2-D traveltime tomography. We found that the Poisson's ratio in the upper crust (layer 2) ranges between ∼0.33 at the top of the crust to ∼0.28 at the layer 2/3 interface, while in the lowermost crust and uppermost mantle it reaches values of ∼0.26 and ∼0.29, respectively. These features can be explained by an oceanic crust significantly weathered, altered and fractured. Relative high Poisson's ratios in the uppermost mantle may be likely related to partially hydrated mantle and hence serpentinization. Thus, the seismic structure of the oceanic lithosphere at the Southern Central Chile outer rise exhibits notable differences from the classic ophiolite seismic model (‘normal’ oceanic crust). These differences are primarily attributed to fracturing and hydration of the entire ocean crust, which are direct consequences of strong bending-related faulting at the outer rise. On the other hand, the comparison of the uppermost mantle P-wave velocities at the crossing point between the perpendicular profiles (∼90 km oceanward from the trench axis) reveals a low degree of Pn anisotropy (<2 per cent). [ABSTRACT FROM AUTHOR]

Complexities in the rupture process during a megathrust earthquake can be attributed to the combined effect of inhomogeneous distribution of stress accumulated during the interseismic period and inhomogeneous rheology of the seismogenic contact. We modeled the free-air gravity field of the southern Central Chile convergent margin along five 2-D profiles that cross the patch of highest slip during the Chilean 2010 megathrust earthquake in order to analyze variability in the density and shape of the continental wedge and its relationship with seismotectonics. We also analyzed the bathymetry to derive the long-term interplate friction coefficient. The results show that the high slip patch during the Maule earthquake corresponds to a segment of the margin characterized by (1) low densities in the continental wedge, (2) low vertical loading over the inter-plate contact, (3) a well-developed shelf basin and, (4) low taper angles consistent with a low effective basal friction coefficient. We interpret the correlation between these parameters in terms of the total potential energy change during the earthquake and conclude that if the normal stress or frictional coefficient are low, then a large slip does not necessarily imply a large amount of coseismic work. Heterogeneities in density of the continental basement can therefore be related to complexities in the pattern of coseismic slip and in the aftershock distribution. Locally, a subducted seamount or seaward spur of high-density continental crust may be present near the high slip patch. [ABSTRACT FROM AUTHOR]

Abstract: The 27 February, 2010 Maule earthquake (Mw=8.8) ruptured ~400km of the Nazca-South America plate boundary and caused hundreds of fatalities and billions of dollars in material losses. Here we present constraints on the fore-arc structure and subduction zone of the rupture area derived from seismic refraction and wide-angle data. The results show a wedge shaped body ~40km wide with typical sedimentary velocities interpreted as a frontal accretionary prism (FAP). Landward of the imaged FAP, the velocity model shows an abrupt velocity-contrast, suggesting a lithological change which is interpreted as the contact between the FAP and the paleo accretionary prism (backstop). The backstop location is coincident with the seaward limit of the aftershocks, defining the updip limit of the co-seismic rupture and seismogenic zone. Furthermore, the seaward limit of the aftershocks coincides with the location of the shelf break in the entire earthquake rupture area (33°S–38.5°S), which is interpreted as the location of the backstop along the margin. Published seismic profiles at the northern and southern limit of the rupture area also show the presence of a strong horizontal velocity gradient seismic backstop at a distance of ~30km from the deformation front. The seismic wide-angle reflections from the top of the subducting oceanic crust constrain the location of the plate boundary offshore, dipping at ~10°. The projection of the epicenter of the Maule earthquake onto our derived interplate boundary yielded a hypocenter around 20km depth, this implies that this earthquake nucleated somewhere in the middle of the seismogenic zone, neither at its updip nor at its downdip limit. [Copyright &y& Elsevier]

A grid of closely spaced, high-resolution multichannel seismic (MCS) reflection profiles was acquired in May 2012 over the outer accretionary prism up dip from the patch of greatest slip during the 2010 Mw 8.8 Maule earthquake (offshore Chile) to complement a natural-source seismic experiment designed to monitor the post-earthquake response of the outer accretionary prism. We describe the MCS data and discuss the implications for the response of the accretionary prism during the earthquake and for the long-term evolution of the margin. The most notable observation from the seismic reflection survey is a rapid north-to-south shift over a short distance from nearly total frontal accretion of the trench sediments to nearly total underthrusting of undeformed trench sediments that occurs near the northern edge of slip in the 2010 earthquake. Integrating our structural observations with other geological and geophysical observations, we conclude that sediment subduction beneath a shallow décollement is associated with propagation of slip to the trench during great earthquakes in this region. The lack of resolvable compressive deformation in the trench sediment along this segment of the margin indicates that the plate boundary here is very weak, which allowed the outer prism to shift seaward during the earthquake, driven by large slip down dip. The abrupt shift from sediment subduction to frontal accretion indicates a stepdown in the plate boundary fault, similar to the stepovers that commonly arrest slip propagation in strikeslip faults. We do not detect any variation along strike in the thickness or reflective character of the trench sediments adjacent to the change in deformation front structure. This change, however, is correlated with variations in the morphology and structure of the accretionary prism that extend as far as 40 km landward of the deformation front. We speculate that forearc structural heterogeneity is the result of subduction of an anomalously shallow or rough portion of plate that interacted with and deformed the overlying plate and is now deeply buried. This study highlights need for three-dimensional structural images to understand the interaction between geology and slip during subduction zone earthquakes. [ABSTRACT FROM AUTHOR]

Splay faults, large thrust faults emerging from the plate boundary to the seafloor in subduction zones, are considered to enhance tsunami generation by transferring slip from the very shallow dip of the megathrust onto steeper faults, thus increasing vertical displacement of the seafloor. These structures are predominantly found offshore, and are therefore difficult to detect in seismicity studies, as most seismometer stations are located onshore. The Mw (moment magnitude) 8.8 Maule earthquake on 27 February 2010 affected ~500 km of the central Chilean margin. In response to this event, a network of 30 ocean-bottom seismometers was deployed for a 3 month period north of the main shock where the highest coseismic slip rates were detected, and combined with land station data providing onshore as well as offshore coverage of the northern part of the rupture area. The aftershock seismicity in the northern part of the survey area reveals, for the first time, a well-resolved seismically active splay fault in the submarine forearc. Application of critical taper theory analysis suggests that in the northernmost part of the rupture zone, coseismic slip likely propagated along the splay fault and not the subduction thrust fault, while in the southern part it propagated along the subduction thrust fault and not the splay fault. The possibility of splay faults being activated in some segments of the rupture zone but not others should be considered when modeling slip distributions. [ABSTRACT FROM AUTHOR]

Abstract: On 27 February 2010 the M w 8.8 Maule earthquake in Central Chile ruptured a seismic gap where significant strain had accumulated since 1835. Shortly after the mainshock a dense network of temporary seismic stations was installed along the whole rupture zone in order to capture the aftershock activity. Here, we present the aftershock distribution and first motion polarity focal mechanisms based on automatic detection algorithms and picking engines. By processing the seismic data between 15 March and 30 September 2010 from stations from IRIS, IPGP, GFZ and University of Liverpool we determined 20,205 hypocentres with magnitudes M w between 1 and 5.5. Seismic activity occurs in six groups: 1.) Normal faulting outer rise events 2.) A shallow group of plate interface seismicity apparent at 25–35km depth and 50–120km distance to the trench with some variations between profiles. Along strike, the aftershocks occur largely within the zone of coseismic slip but extend ~50km further north, and with predominantly shallowly dipping thrust mechanisms. Along dip, the events are either within the zone of coseismic slip, or downdip from it, depending on the coseismic slip model used. 3.) A third band of seismicity is observed further downdip at 40–50km depth and further inland at 150–160km trench perpendicular distance, with mostly shallow dipping (~28°) thrust focal mechanisms indicating rupture of the plate interface significantly downdip of the coseismic rupture, and presumably above the intersection of the continental Moho with the plate interface. 4.) A deep group of intermediate depth events between 80 and 120km depth is present north of 36°S. Within the Maule segment, a large portion of events during the inter-seismic phase originated from this depth range. 5.) The magmatic arc exhibits a small amount of crustal seismicity but does not appear to show significantly enhanced activity after the M w 8.8 Maule 2010 earthquake. 6.) Pronounced crustal aftershock activity with mainly normal faulting mechanisms is found in the region of Pichilemu (~34.5°S). These crustal events occur in a ~30km wide region with sharp inclined boundaries and oriented oblique to the trench. The best-located events describe a plane dipping to the southwest, consistent with one of the focal planes of the large normal-faulting aftershock (M w =6.9) on 11 March 2010. [Copyright &y& Elsevier]

• New seismic images off south-central Chile acquired with a 15-km-long receiver array. • The decollement is unusually shallow at the deformation front along S. central Chile. • Shallow sediment subduction along south-central Chile is among the world's thickest. • P-wave velocities imply S. central Chile trench sediments are well-drained and strong. • Thick, strong sediment subduction may enhance interplate coupling and earthquake size. The south-central Chile margin experienced the largest and sixth largest earthquakes ever recorded - the 1960 Mw 9.5 Valdivia and 2010 Mw 8.8 Maule megathrust earthquakes, respectively. In early 2017, we conducted a seismic survey along 1,000 km of south-central Chile to image these rupture zones using a 15.15-km-long multi-channel seismic streamer. We processed these data using pre-stack depth migration, which provides the best look at the shallow part of the south-central Chile margin to date. Relative to other sediment-dominated subduction zones, where sediment is typically accreted at the toe, an unusually large percentage of the thick trench sediments are consistently subducted beneath the slope with little thrust faulting or deformation. Analysis of the sediment P-wave velocities and structure in the trench and outer wedge leads us to conclude that most of south-central Chile contains well-drained, strong sediments. An exception in the vicinity of the subducting Mocha Fracture Zone (MFZ) has trench sediments that appear to experience localized delayed compaction, thus lowering their strength and allowing the development of protothrusts, similar to what is seen in other accretionary subduction zones. The very shallow décollement along the south-central Chile allows more sediment to pass beneath the lower slope than almost all other subduction zone, many of which have much thicker trench sections. We conclude that subduction of the strong, well-drained, thick sediment layer beneath the lower slope is typical for nearly all of the south-central Chile. Comparison to other thick-sedimented subduction zones worldwide reveals that the subduction of such a large fraction of the trench sediment is particularly unusual. This strong, thick, subducting sediment is likely a determining factor for developing a smooth plate interface located well above the subducted crust topography that ultimately becomes a broad megathrust with high, homogeneous frictional properties, and generates particularly large earthquakes along the south central Chile margin. [ABSTRACT FROM AUTHOR]