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International audience; We study the 2020 ${{\rm{M}}_{\rm{W}}}$ 6.8 Calama earthquake sequence that occurred within the subducting oceanic Nazca plate. The mainshock is modeled via waveform inversion using a dynamic rupture model, while detection and location techniques are used to better characterize its aftershock sequence. We analyze the local seismotectonic and thermal context of the subducting Nazca plate to understand the trigger mechanism of this earthquake and how it compares with other significant earthquakes in the vicinity. The stress drop and the related dynamic rupture parameters of the Calama mainshock are similar to those of the nearby 2007 ${{\rm{M}}_{\rm{W}}}$ 6.8 Michilla and 2015 ${{\rm{M}}_{\rm{W}}}$ 6.7 Jujuy intraslab earthquakes, which occurred to the west (trenchwards) and to the east (under the back-arc) of the Calama earthquake, respectively. The sequences of these three events were located using a 3-D tomographic velocity model. While the Michilla earthquake sequence occurred within the oceanic crust at temperatures of ~250°C, the Calama sequence occurred within the upper lithospheric mantle at ~350°C and exhibited a smaller aftershock productivity than Michilla. Additionally, the 3-D tomographic model shows intermediate ${{\rm{V}}_{\rm{P}}}/{{\rm{V}}_{\rm{S}}}$ ratios (1.72-1.76) in the region of the Calama earthquake. This indicates a less hydrated environment that could be responsible for the smaller aftershock productivity of the Calama earthquake.

The subduction of seamounts and basement ridges affects the structure, morphology, and physical state of a convergent margin. To evaluate their impact on the seismo-tectonic setting of the subduction zone and the tectonic development of the lower subducting and upper overriding plate, it is essential to know the precise location of subducted topographic features under the marine forearc. Offshore Northern Chile, the Iquique Ridge represents a broad zone of complex and heterogeneous structure of variable width on the oceanic Nazca Plate, which complicates attempts to project it beneath the forearc of the Chilean subduction zone. Here we use a state-of-the-art seismic reflection data processing approach to map structures related to ridge subduction under the marine forearc with unprecedented accuracy and resolution and evaluate their impact on the deformation of both the plate boundary and the upper plate. We show that significant ridge-related topography is currently subducting south of 20.5 °S and that the combined effect of horst and graben subduction with subduction of Iquique ridge-related thickened and elevated crust causes an upward bulging of the entire upper plate from the plate interface up to the seafloor as well as the presence of kilometer-scale anticlinal structures observed in multibeam bathymetric data that are approximately aligned with horsts seaward of the trench. In the area affected by the subducting ridge, a frontal prism is absent, which may relate to frontal subduction erosion caused by the excess lower plate topography. In contrast farther towards the north, where only isolated seamounts subduct, a small frontal prism and a slope/apron sediment cover down to 3000 m water depth are found.

We study the 2020 MW 6.8 Calama earthquake sequence that occurred within the subducting oceanic Nazca plate. The mainshock is modeled via waveform inversion using a dynamic rupture model, while detection and location techniques are used to better characterize its aftershock sequence. We analyze the local seismotectonic and thermal context of the subducting Nazca plate to understand the trigger mechanism of this earthquake and how it compares with other significant earthquakes in the vicinity. The stress drop and the related dynamic rupture parameters of the Calama mainshock are similar to those of the nearby 2007 MW 6.8 Michilla and 2015 MW 6.7 Jujuy inslab earthquakes, which occurred to the west (trenchwards) and to the east (under the back-arc) of the Calama earthquake, respectively. The sequences of these three events were located using a 3-D tomographic velocity model. While the Michilla earthquake sequence occurred within the oceanic crust at temperatures of ~250°C, the Calama sequence occurred within the upper oceanic mantle at ~350°C and exhibited a smaller aftershock productivity than Michilla. Additionally, the 3-D tomographic model shows intermediate Vp/Vs ratios in the region of the Calama earthquake. This indicates a less hydrated environment that would be responsible for the smaller aftershock productivity of the Calama earthquake.

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 strike-slip 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.

Dehydration of the oceanic subducting slab promotes the formation of magmatic arcs, intra-slab intermediate-depth seismicity, and hydration of the overlying mantle wedge. However, the complex permeability structure of the overriding plate controls the magma and fluid migration and their accumulation at shallower depths. In this regard, mapping the inner structure of the overriding crust and mantle is crucial to understand the magmatic and hydrological processes in subduction zones. We integrate 3-D P-wave, $$V_p/V_s$$ V p / V s , and electrical resistivity tomographic models of the northern Chilean subduction zone to map the magmatic and fluids derived from the subducting oceanic Nazca plate. Results show a continental crust relatively thick (50–65 km) characterized by a lower zone of high $$V_p$$ V p values (7.2–7.6 km/s), which is interpreted as the presence of plutonic rocks. The mantle lithospheric wedge is weakly hydrated ($$V_p/V_s$$ V p / V s = 1.75–1.8) while the forearc continental crust is traversed by regions of reduced electrical resistivity values ($$< 10^2$$ < 10 2 $$\Omega m$$ Ω m ) interpreted as zones of relatively high permeability/fracturing and fluid content. These regions spatially correlate with upper plate trans-lithospheric deformation zones. Ascending melts accumulate preferentially in the back-arc, whereas hydrothermal systems form trenchward of the volcanic arc. The results highlight the complex permeability structure of the upper South American plate.

The Chile Triple Junction, where the hot active spreading centre of the Chile Rise system subducts beneath the South American plate, offers a unique opportunity to understand the influence of the anomalous thermal regime on an otherwise cold continental margin. Integrated analysis of various geophysical and geological datasets, such as bathymetry, heat flow measured directly by thermal probes and calculated from gas hydrate distribution limits, thermal conductivities, and piston cores, have improved the knowledge about the hydrogeological system. In addition, rock dredging has evidenced the volcanism associated with ridge subduction. Here, we argue that the localized high heat flow over the toe of the accretionary prism results from fluid advection promoted by pressure-driven discharge (i.e., dewatering/discharge caused by horizontal compression of accreted sediments) as reported previously. However, by computing the new heat flow values with legacy data in the study area, we raise the assumption that these anomalous heat flow values are also promoted by the eastern flank of the currently subducting Chile Rise. Part of the rift axis is located just below the toe of the wedge, where active deformation and vigorous fluid advection are most intense, enhanced by the proximity of the young volcanic chain. Our results provide valuable information to current and future studies related to hydrothermal circulation, seismicity, volcanism, gas hydrate stability, and fluid venting in this natural laboratory.

SUMMARY 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.

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 up‐dip limit of coseismic rupture propagation. Aftershocks up‐dip 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. Key Points: - We investigate structure and seismicity at the up-dip end of the 2014 Iquique earthquake rupture using amphibious seismic data. - Seismicity up-dip of the 2014 Iquique earthquake occurs over a broad range likely interpreted to be related to the basal erosion processes. - Coseismic stress changes and aftershocks activate extensional faulting of the upper plate and subduction erosion.

We study the structure and tectonics of the collision zone between the Nazca Ridge (NR) and the Peruvian margin constrained by seismic, gravimetric, bathymetric, and natural seismological data. The NR was formed in an on-ridge setting, and it is characterized by a smooth and broad shallow seafloor (swell) with an estimated buoyancy flux of ~7 Mg/s. The seismic results show that the NR hosts an oceanic lower crust 10–14 km thick with velocities of 7.2–7.5 km/s suggesting intrusion of magmatic material from the hot spot plume to the oceanic plate. Our results show evidence for subduction erosion in the frontal part of the margin likely enhanced by the collision of the NR. The ridge-trench collision zone correlates with the presence of a prominent normal scarp, a narrow continental slope, and (uplifted) shelf. In contrast, adjacent of the collision zone, the slope does not present a topographic scarp and the continental slope and shelf become wider and deeper. Geophysical and geodetic evidence indicate that the collision zone is characterized by low seismic coupling at the plate interface. This is consistent with vigorous subduction erosion enhanced by the subducting NR causing abrasion and increase of fluid pore pressure at the interplate contact. Furthermore, the NR has behaved as a barrier for rupture propagation of megathrust earthquakes (e.g., 1746 Mw 8.6 and 1942 Mw 8.1 events). In contrast, for moderate earthquakes (e.g., 1996 Mw 7.7 and 2011 Mw 6.9 events), the NR has behaved as a seismic asperity nucleating at depths >20 km.

The September 16, 2015, Mw 8.4 Illapel, Chile earthquake is the first large event occurring in north-central Chile after the 1943 earthquake, filling a known seismic gap in the region. The earthquake took place in a complex tectonic region, nearby an area where transition from erosive to accretionary margin occurs due to the collision of Juan Fernandez Ridge (JFR) along the Chilean margin. We inverted the kinematic rupture process of the 2015 Mw 8.4 Illapel earthquake from the joint inversion of teleseismic body waves and near-field data. The relative weighting between datasets and the weighting of spatial/temporal constraints are objectively estimated by applying the Akaike's Bayesian Information Criterion. The coseismic slip model yields a total seismic moment of 4.92 × 1021 Nm occurred over ~120 s. The rupture shows both downdip and updip propagation with slip extending along the thrust interface from ~50 km depth to shallow near-trench depths (

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 Fernandez Ridge (Juan Fernandez hotspot), the Taltal Ridge (San Felix hotspot), and the Copiapo 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 Gomez 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, Copiapo and Juan Fernandez 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.

National Science Foundation (NSF) OCE-1559293 OCE-1558867 Comision Nacional de Investigacion Cientifica y Tecnologica (CONICYT) ACT172002

We investigate the differences of the seismic source and aftershock activity using kinematic inversions and template matching respectively, for the six largest intraslab intermediate-depth earthquakes occurred in northern Chile (Mw ~6.3) since 2010 at depths between 90 and 130 km and recorded by dense strong-motion and broad-band seismic networks. In addition, we developed a thermal model using the finite element method in the study region with the aim of analyze the impact of temperature on seismic behavior as the oceanic plate subducts. Our results show that geometries of rupture zones are similar, with semi-axis for an elliptical patch approach about 5 km, and stress drop values between 7 and 30 MPa. On the other hand, the number of aftershocks exhibits clear differences, and their amount decreases with increasing the depth within the slab bounded by the 450 ºC isotherm, which represents a limit between a high-hydrated and a dry or low-hydrated region. Furthermore, mainshocks occur at distances from the top of the slab from 7 to 40 km, and all of them exhibit normal focal mechanisms suggesting that the extensional regimen deepens within the slab to the 700-750 ºC isotherm-depth. We suggest that in northern Chile the abrupt decrease of aftershocks in the lower part of the extensional regimen is caused by the absence of a hydrated slab at those depths.

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 degrees S to 23 degrees 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.

The central Chilean margin (32°–33°S) is characterized by the subduction of the Juan Fernandez 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 Mw 6.9 earthquake triggered two days after the sequence started and occurred at the northern end of the rupture area of the 1985 Mw 8.0 Valparaiso earthquake. We compute the kinematic rupture process of the 2017 Mw 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 km2, 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 × 1019 Nm (Mw 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 Mw 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.

Fundamental processes of the seismic cycle in subduction zones, including those controlling the recurrence and size of great earthquakes, are still poorly understood. Here, by studying the 2016 earthquake in southern Chile—the first large event within the rupture zone of the 1960 earthquake (moment magnitude (Mw) = 9.5)—we show that the frictional zonation of the plate interface fault at depth mechanically controls the timing of more frequent, moderate-size deep events (Mw 8.5). We model the evolution of stress build-up for a seismogenic zone with heterogeneous friction to examine the link between the 2016 and 1960 earthquakes. Our results suggest that the deeper segments of the seismogenic megathrust are weaker and interseismically loaded by a more strongly coupled, shallower asperity. Deeper segments fail earlier (~60 yr recurrence), producing moderate-size events that precede the failure of the shallower region, which fails in a great earthquake (recurrence >110 yr). We interpret the contrasting frictional strength and lag time between deeper and shallower earthquakes to be controlled by variations in pore fluid pressure. Our integrated analysis strengthens understanding of the mechanics and timing of great megathrust earthquakes, and therefore could aid in the seismic hazard assessment of other subduction zones. The recurrence time of megathrust earthquakes in Chile may be controlled by frictional contrasts at depth, according to analyses of stress build-up and release related to the December 2016 southern Chile earthquake.

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 Te, shear force V0, and bending moment M0. V0 and M0 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, (xb − xo). 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 β0, and the elastic thickness Te. 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 β0 values ( w 0 and β0) is available.

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 ( 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.

We predict the shape of the outer rise along the Peruvian Trench (6°-21°S) using flexural and gravity modelling in order to study the elastic properties of the subducting/oceanic Nazca Plate (NP). We include in our modelling the hotspot swell topography of the Nazca Ridge (NR) in the ridge-trench collision zone (~15°S). Results show an anomalous overthickening of the oceanic crust beneath the NR (from ~6 to 20 km), which is capable to produce most of the swell topography (500–700 km wide and up to 2.2 km high). The swell was formed likely under isostatic conditions (Te ~ 0 km) by the interaction of the NP with a hotspot-spreading center system. Despite the high buoyancy forces of the NR (0.5–4.0 × 1013 N/m) associated to the anomalous thick crust, the 45–50 Ma oceanic NP approaching the Peruvian Trench presents a well pronounced fore-bulge morphology with similar elastic thicknesses (Te = 35 ± 5 km) to those found for cold and old oceanic plate in the western Pacific. Consistently, our results do not show evidence for plate weakening (reduced Te values) in the NR-trench collision zone. We obtain similar results (Te = 40 ± 10 km) north of the NR implying that the oceanic NP is strong prior to subduction along most of the Peruvian convergent margin. This is consistent with the lack of great outer rise events (Mw ≥ 6) and the absence of reduced uppermost mantle velocities offshore Peru suggesting the presence of a poorly hydrated and rigid lithospheric mantle.

The Juan Fernandez Ridge (JFR) is a hotspot track that is currently subducting off central Chile (32°–33°S). In this region, a fundamental change in trench curvature (in plan view) and sedimentation occurs near the collision zone between the JFR and the Chile Trench. Subduction erosion and sediment accretion have been active north and south of this collision zone. Here, I show a compilation of wide-angle seismic profiles in the area surrounding the collision zone in order to compare the seismic structure of the contact between the accretionary prism and continental basement. The seismic results north of the collision zone show that the toe of continental basement is ~ 5–10 km landward of the trench, whereas the contact between the accretionary prism and continental basement south of the collision zone is situated 50–60 km landward of the deformation front. The plan-view curvature of the continental basement toe is constrained by seismic results to have a value of ~ 0.793 × 10− 3 km− 1. This value, which accounts for only ~ 30% of the overall trench curvature, is related to the mechanical behavior of continental basement and is predominantly controlled by regional-scale processes such as deep subduction of the JFR and slab rollback. The remaining trench curvature can be explained by subduction erosion and sediment accretion.

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 Valpara iso 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. Valpara iso 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.

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 km hr−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.

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 favored 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.

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 decollement 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.

The crustal-scale west-vergent San Ramón thrust fault system, which lies at the foot of the main Andean Cordillera in central Chile, is a geologically active structure with manifestations of late Quaternary complex surface rupture on fault segments along the eastern border of the city of Santiago. From the comparison of geophysical and geological observations, we assessed the subsurface structural pattern that affects the sedimentary cover and rock-substratum topography across fault scarps, which is critical for evaluating structural models and associated seismic hazard along the related faults. We performed seismic profiles with an average length of 250 m, using an array of 24 geophones (Geode), with 25 shots per profile, to produce high-resolution seismic tomography to aid in interpreting impedance changes associated with the deformed sedimentary cover. The recorded travel-time refractions and reflections were jointly inverted by using a 2-D tomographic approach, which resulted in variations across the scarp axis in both the velocities and the reflections that are interpreted as the sedimentary cover-rock substratum topography. Seismic anisotropy observed from tomographic profiles is consistent with sediment deformation triggered by west-vergent thrust tectonics along the fault. Electrical soundings crossing two fault scarps were used to construct subsurface resistivity tomographic profiles, which reveal systematic differences between lower resistivity values in the hanging wall with respect to the footwall of the geological structure, and clearly show well-defined east-dipping resistivity boundaries. These boundaries can be interpreted in terms of structurally driven fluid content change between the hanging wall and the footwall of the San Ramón fault. The overall results are consistent with a west-vergent thrust structure dipping ~55° E in the subsurface beneath the piedmont sediments, with local complexities likely associated with variations in fault surface rupture propagation, fault splays and fault segment transfer zones.

18 pages, 9 figures.-- Access to the field data: Bathymetric data from German research cruises (e.g., R/V SONNE SO104, SO244) can be requested through the German Bundesamt für Seeschifffahrt und Hydrographie (BSH; http://www.bsh.de). The bathymetric data collected during R/V Marcus G. Langseth cruise MGL1610 are available from the Rolling Deck to Repository (R2R) Web site (http://www.rvdata.us/). Seismic reflection data from R/V SONNE cruise SO104 are stored at the Bundesanstalt für Geowissenschaften und Rohstoffe (BGR) and can be requested through the Geo‐Seas data portal (http://www.geo‐seas.eu/). Access to processed data: The following data used in this manuscript are available from the PANGAEA data archive (https://doi.pangaea.de/10.1594/PANGAEA.893034): (I) A gridded DEM (100 m) that combines the bathymetric data from R/V SONNE cruises SO104, SO244, and R/V Marcus Langseth cruise MGL1610. (II) A GeoTIFF that contains the backscatter information from the multibeam data collected during R/V SONNE cruise SO244, 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, R/V SONNE Cruises SO104 (grant 03G104A) and SO244 (grant 03G0244A) were supported by the German Bundesministerium für Bildung und Forschung (BMBF). A. M. T. thanks the U.S. National Science Foundation (NSF) for support through grant OCE1459368. We also thank the crew and technical staff of the R/V Marcus Langseth, which is funded by the NSF and operated by the Lamont‐Doherty Earth Observatory. […] Jacob Geersen was partly funded by a grant (CP1404) of the Cluster of Excellence 80 “The Future Ocean,” funded within the framework of the Excellence Initiative by Deutsche Forschungsgemeinschaft (DFG) on behalf of the German federal and state governments. C. R. R. was supported by the Grup de Recerca 2014SGR940 de la Generalitat de Catalunya and Spanish Ministry of Science grant PCIN‐2015‐053. E. C.‐R. gratefully acknowledges the support of the Chilean National Science Foundation (FONDECYT) grant 1170009 and the Programa de Investigación Asociativa: ANILLOS DE INVESTIGACIÓN EN CIENCIA Y TECNOLOGÍA, CONICYT, grant ACT172002, project “The interplay between subduction processes and natural disasters in Chile.”

Based on a compilation of published 2-D velocity-depth models along the Chilean margin (22°–48° S), I review the structure and tectonic processes that govern this convergent margin in terms of subduction erosion and sediment accretion/subduction. North of the collision point between the Juan Fernandez Ridge with the overriding continental South American plate (Chile at ~32.5° S), subduction erosion has been active since Jurassic resulting in large-scale crustal thinning and long-term subsidence of the outermost forearc. Published 2-D velocity–depth models show a prominent lateral velocity contrast that propagates deep into the continental crust defining a major lateral seismic discontinuity (interpreted as the volcanic-continental basement contact of the submerged Coastal Cordillera characterized by a gravitational collapse of the outermost fore arc). Between the Juan Fernandez Ridge and the Chile Triple Junction (CTJ) of the Nazca-Antarctic-South American plates (Chile at ~46.5° S), an accretionary prism 5–50 km wide has been formed due to an increase of trench sedimentation triggered by denudation processes of the Andes after the last Pleistocene Glaciation. However, the relatively small size of the accretionary prism is not compatible with an efficient history of sediment accretion since the Pleistocene, and sediment subduction is a dominant process especially south of the oceanic Mocha Fracture Zone (Chile at ~38° S) and north of the CTJ. In the overriding plate, seismic studies reveal two prominent velocity transition zones characterized by steep lateral velocity gradients, resulting in a seismic segmentation of the marine fore arc. The southern central Chilean margin is composed of three main domains: (1) a frontal prism at the toe of the continental slope, (2) a paleoaccretionary complex, and (3) the seaward edge of the Paleozoic continental framework that forms part of the Coastal Cordillera. Near the CTJ, where the Nazca-Antarctic spreading center (Chile Rise) collides with the margin, subduction erosion is active, and rapid uplift followed by subsidence of the forearc area, normal faulting and intensive sedimentary mass wasting are documented. South of the CTJ, the convergence between the oceanic Antarctic and continental South American plate is slow allowing more time sediment accumulation at the trench enhancing the formation of relatively large accretionary prisms (width of 70–90 km).

The Maule 2010 megathrust earthquake Mw 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 (Mw > 4.5), including a large Mw 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 Mw 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.

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.

The Mw 8.8 megathrust earthquake that occurred on 27 February 2010 offshore the Maule region of central Chile triggered a destructive tsunami. Whether the earthquake rupture extended to the shallow part of the plate boundary near the trench remains controversial. The up-dip limit of rupture during large subduction zone earthquakes has important implications for tsunami generation and for the rheological behavior of the sedimentary prism in accretionary margins. However, in general, the slip models derived from tsunami wave modeling and seismological data are poorly constrained by direct seafloor geodetic observations. We difference swath bathymetric data acquired across the trench in 2008, 2011 and 2012 and find ~3–5 m of uplift of the seafloor landward of the deformation front, at the eastern edge of the trench. Modeling suggests this is compatible with slip extending seaward, at least, to within ~6 km of the deformation front. After the Mw 9.0 Tohoku-oki earthquake, this result for the Maule earthquake represents only the second time that repeated bathymetric data has been used to detect the deformation following megathrust earthquakes, providing methodological guidelines for this relatively inexpensive way of obtaining seafloor geodetic data across subduction zone.

The Chile Trench of the convergent continental margin of Central Chile is a sediment-filled basin that stretches over 1500 km in a north–south direction. The sediment fill reflects latitudinal variations in climate as well as in the morphology and geology of Chile, but also of sediment transport processes to the trench and within the trench. We try to untangle these signals by calculating the total volume and the latitudinal volume distribution of trench sediments and by relating this distribution to a number of factors that affect this pattern. The volume calculation is based on a model geometry of the top of the subducting oceanic plate that is buried beneath trench sediments and the sea floor as measured by swath bathymetry. We obtain the model geometry of the subducting plate by interpolating between depth-converted seismic reflection profiles that cross the trench. The total volume of the trench fill between 32 and 46°S is calculated to be 46000 ± 500 km 3 . The resulting latitudinal volume distribution is best explained by a sedimentation model that alternates between (1) glacial phases of high sediment flux from Southern Chile combined with active latitudinal sediment transport within the trench and (2) interglacial phases over which sediment input is dominated by local factors. Supplementary material: Top of the oceanic basement (TOB) grid is available as ascii raw data files (xyz) at www.geolsoc.org.uk/SUP18664 .

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 (~ 34 Ma) that is represented by inversion structures and kinematic syn-inversion sequences. A marked erosional unconformity (34–23 Ma) 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 (~ 3.6 Ma) at ~ 38°S. The mechanics of regional subsidence which affected the basin during the Miocene cannot be attributed to major scale extensional structures (> 1.0 km).

[1] Seismic refraction studies of deep-crustal and upper mantle structure beneath some oceanic hotspot provinces reveal the presence of ultramafic bodies with P-wave velocities of Vp ~ 7.4–8.0 km/s lying at or above the Moho, e.g., Hawaii, the Marquesas, and La Reunion. However, at other hotspot provinces such as the Galapagos, Nazca Ridge, and Louisville the lower crust is intruded by large volumes of gabbroic (mafic) rocks (Vp ~ 6.8–7.5 km/s). Ultramafic primary melts formed beneath mature oceanic lithosphere at pressures of ~2–3 GPa (60–90 km depth), and ponded at the Moho due to their relatively high density, can explain the observed ultramafic deep-crustal bodies. By contrast, plume melts formed at depths of ~15–30 km beneath thin lithosphere crystallize assemblages that are more gabbroic. The velocity and density gradient is particularly strong in the pressure range 0.6–1.5 GPa due to the replacement of plagioclase by olivine as melts become more MgO-rich with increasing pressure (and degree) of melting. This anomalous density gradient suggests a possible filtering effect whereby plume melts equilibrated at relatively shallow depths beneath very young and thin oceanic lithosphere may be expected to be of nearly gabbroic (mafic) composition (~6–10% MgO), whereas ultramafic melts (MgO ~ 12–20%) formed beneath older, thicker oceanic lithosphere must pond and undergo extensive olivine and clinopyroxene fractionation before evolving residual magmas of basaltic composition sufficiently buoyant to be erupted at the surface. A survey of well-studied hotspot provinces of highly-varying lithospheric age at the time of emplacement shows that deep-crustal and upper mantle seismic refraction data are consistent with this hypothesis. These results highlight the importance of large-volume intrusive processes in the evolution of hotspot magmas, with intrusive volumes being significantly larger than those of the erupted lavas in most cases. Pyrolite melting can account, to first order, for the total crustal column of magmatic products, whereas alternative models such as selective melting of pyroxenite blobs probably cannot.

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 14 Ma. 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 3–6 Ma 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 70–80 km wide accretionary prism. The velocity–depth model shows a discontinuity at 30–40 km 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 (∼2.0 cm/a). Based on the kinematics of the Chile Ridge subduction during the last 13 Ma, we propose that the accretionary prism off Golfo de Penas was formed recently (∼5 Ma) after the collision of the Chile Ridge with South America.

The pre- and current collision of the Juan Fernandez 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 Valparaiso 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.

The 27 February, 2010 Maule earthquake (Mw=8.8) ruptured ~400 km 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 ~40 km 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 ~30 km 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 20 km depth, this implies that this earthquake nucleated somewhere in the middle of the seismogenic zone, neither at its updip nor at its downdip limit.

The Mw 8.8 Maule earthquake on February 27, 2010 affected the central-southern Chilean forearc of the Central Andes. Here we show the results of field investigations of surface deformation associated with this major earthquake. Observations were carried out within three weeks after the seismic event, mostly in the central and northern part of the forearc overlying the rupture zone. We provide a detailed field record of co-seismic surface deformation and examine its implications on active Andean tectonics. Surface rupture consisted primarily of extensional cracks, push-up structures, fissures with minor lateral displacements and a few but impressive extensional geometries similar to those observed in analogical modeling of rift systems. A major group of NW-WNW striking fractures representing co-seismic extensional deformation is found at all localities. These appear to be spatially correlated to long-lived basement fault zones. The NW-striking normal focal mechanism of the Mw 6.9 aftershock occurred on March 11 demonstrates that the basement faults were reactivated by the Mw 8.8 Maule earthquake. The co-seismic surface ruptures show patterns of distributed deformation similar to those observed in mapped basement-involved structures. We propose that co-seismic reactivation of basement structures play a fundamental role in stress release in the upper plate during large subduction earthquakes. The fundamental mechanism that promotes stress relaxation is largely driven by elastic rebound of the upper plate located right above the main rupture zone.

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 10–50 MPa. 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 (Mw ≥ 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 Fernandez) and the active Nazca-Antarctic spreading center. For instance, the giant 1960 earthquake (Mw = 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 Fernandez Ridge) reflects a heterogeneous frictional behavior of the subduction interface which appears to be mainly controlled by the subduction of HOF's.

We report here the first detailed 2D tomographic image of the crust and upper mantle structure of a Cretaceous seamount that formed during the interaction of the Pacific plate and the Louisville hotspot. Results show that at ∼ 1.5 km beneath the seamount summit, the core of the volcanic edifice appears to be dominantly intrusive, with velocities faster than 6.5 km/s. The edifice overlies both high lower crustal (> 7.2-7.6 km/s) and upper mantle (> 8.3 km/s) velocities, suggesting that ultramafic rocks have been intruded as sills rather than underplated beneath the crust. The results suggest that the ratio between the volume of intra-crustal magmatic intrusion and extrusive volcanism is as high as ∼ 4.5. In addition, the inversion of Moho reflections shows that the Pacific oceanic crust has been flexed downward by up to ∼ 2.5 km beneath the seamount. The flexure can be explained by an elastic plate model in which the seamount emplaced upon oceanic lithosphere that was ∼ 10 Myr at the time of loading. Intra-crustal magmatic intrusion may be a feature of hotspot volcanism at young, hot, oceanic lithosphere, whereas, magmatic underplating below a pre-existing Moho may be more likely to occur where a hotspot interacts with oceanic lithosphere that is several tens of millions of years old. © 2009 Elsevier B.V. All rights reserved.