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Seismic phase detection, identification and first-onset picking are basic but essential routines to analyse earthquake data. As both the number of seismic stations, globally and regionally, and the number of experiments greatly increase due to ever greater availability of instrumentation, automated data processing becomes more and more essential. For example for modern seismic experiments involving 100s to even 1000s instruments, conventional human analyst-based identification and picking of seismic phases is becoming unfeasible, and the introduction of automatic algorithms mandatory. In this paper, we introduce DeepPhasePick, an automatic two-stage method that detects and picks P and S seismic phases from local earthquakes. The method is entirely based on highly optimized deep neural networks, consisting of a first stage that detects the phases using a convolutional neural network, and a second stage that uses two recurrent neural networks to pick both phases. Detection is performed on three-component seismograms. P - and S -picking is then conducted on the vertical and the two-horizontal components, respectively. Systematic hyperparameter optimization was applied to select the best model architectures and to define both the filter applied to pre-process the seismic data as well as the characteristics of the window sample used to feed the models. We trained DeepPhasePick using seismic records extracted from two sets of manually picked event waveforms originating from northern Chile (∼39 000 records for detection and ∼36 000 records for picking). In different tectonic regimes, DeepPhasePick demonstrated the ability to both detect P and S phases from local earthquakes with high accuracy, as well as predict P - and S -phase time onsets with an analyst level of precision. DeepPhasePick additionally computes onset uncertainties based on the Monte Carlo Dropout technique as an approximation of Bayesian inference. This information can then further feed an associator algorithm in an earthquake location procedure. [ABSTRACT FROM AUTHOR]

The crust of the South American platform recorded imprints of dynamic processes related with the opening of the Central and South Atlantic but has not been well measured. Crustal structure can be retrieved from teleseismic receiver functions using H – κ stacking, but nearly parallel stripes of high stacking values existing in stacking images for seismic stations in sedimentary area cause difficulties in identifying solutions. We show that some seemingly spurious stripes that do not point to any layer solution are helpful in the identification of the solution position. With the aid of the auxiliary stripes, we retrieved thicknesses and Vp / Vs of sedimentary and crystalline crust for 65 permanent stations of the Brazilian Seismographic Network and six new portable seismic stations in Brazil and Uruguay. The resulted sedimentary thickness and Vp / Vs exhibit a good correlation with the Phanerozoic sediments in the South American basins. The crust of Paraná–Etendeka Large Igneous Province (LIP) had been expected to be more mafic since it had ever been penetrated by mantle magma in the Cretaceous related to the south Atlantic opening. However, we found very low Vp / Vs (1.67) in the crystalline crust beneath the LIP, implying a more felsic crust and that no significant mafic intruding/underplating has occurred in the region. The more felsic crust may be formed in a special evolution early than the magmatic event, or during the magmatic event by releasing crustal volatiles. The resulted sedimentary thickness and Vp / Vs ratios exhibit a good correlation with the Phanerozoic sediments in the South American basins, which implies that Triassic–Jurassic and Cretaceous magmatism did not cause significant metamorphism in sediments formed before the magmatic events. [ABSTRACT FROM AUTHOR]

This paper presents a new adaptive algorithm for topographic correction of gravity measurements based on a triangulated polyhedral representation of topographic surfaces. The adaptive grid resampling of topography is driven by the actual gravity effect on the station, which depends on the topographic mass and distance. This is a major improvement for accuracy and computational speed compared to some old algorithms that account only for distance. Today, global availability of gravity data allows the regional investigation (∼100 km anomaly wavelengths) of the lithosphere. The variety of different data sets (from terrestrial and aero surveys to satellite data), the size of possible investigation areas and the availability of high-resolution, digital terrain models ask for new approaches of topographic correction. Specifically, an algorithm for topographic correction should be useable on large-scale investigation areas, that is, correctly model the long wavelength signal of topography, work in spherical domain, be able to handle high-resolution topography data available and accurately represent topography. The new algorithm does consider sphericity of the Earth, calculates gravity gradients, deals with large data sets and uses an adaptive approach for resampling topography to save computation time significantly. The resampling uses an efficient quadtree representation of the topography grid. High resolution of the topography grid is only considered if it has a significant influence on the gravity effect at the station. This leads to an accurate representation of distant terrain and a massive speed up of computation time. The new algorithm is tested in an area of South America and Central Asia. The results are almost identical to calculations with constant grid resolutions, but need only 2 per cent of calculation time. This method enables to correct all gravity data sources (terrestrial, aero, satellite data) with the necessary topographic resolution with huge saving in computation time without presumptions by the user. [ABSTRACT FROM AUTHOR]

Crustal density models derived from seismic velocity models by means of velocity-density conversions typically reproduce the main features of the observed gravity anomaly over the area but often show significant misfits. Given the uncertainty in the relationship between velocity and density, seismically derived density models should be regarded as an initial estimate of the true subsurface density structure. In this paper, we present a method for estimating the adjustments necessary to a seismically derived density model to improve the fit to gravity data. The method combines the Genetic Algorithm paradigm with linear inversion as a way to approach the non-linear and linear aspects of the problem. The models are divided into three layers representing the sedimentary column, the crystalline crust and the lithospheric mantle; the depths of these layers are determined from the seismic velocity model. Each of the layers is divided into a number of provinces and a density adjustment (Δρ) value is found for each province so that the residual gravity (difference between the observed gravity anomaly and the anomaly calculated for the seismically derived model) is minimized while keeping Δρ between predefined bounds. The preferred position of the province boundaries is found through the artificial evolution of a population of solutions. Given the stochastic nature of the algorithm and the non-uniqueness of the problem, different realizations can yield different solutions. By performing multiple realizations we can analyse a set of solutions by taking their mean and standard deviation, providing not only an estimate of the Δρ distribution in the subsurface but also an estimate of the associated uncertainty. Synthetic tests prove the ability of the algorithm to accurately recover the location of province boundaries and the Δρ values for a known model when using noise-free synthetic data. When noise is added to the data, the algorithm broadly recovers the features that define the known model despite greater standard deviations of the solutions and the occurrence of artefacts in the mean solutions. The algorithm was applied to four profiles across the Caribbean-South America Plate boundary. Some general patterns in the distribution of Δρ were observed consistently in the profiles and are correlated with the interpretations of the velocity models. Positive Δρ values in the sedimentary layer, negative Δρ values for island arc and extended island arc crust with an abrupt change to positive values in South American crust, and positive values in the mantle under the continent and island arc with a transition to negative values under the Caribbean oceanic crust. [ABSTRACT FROM AUTHOR]

A large (M w 7.6) megathrust earthquake occurred on 2016 December 25 in Southern Chile, south of the Chiloé Island (74.2°W, 43.3°S) in the South America–Nazca subduction zone. This earthquake was the first large event in this seismotectonic segment since the M w 9.5 1960 Valdivia megathrust earthquake and broke a ~50-km-long segment of the southern part of its rupture zone. Source parameters are inferred from teleseismic broad-bands, strong motions, GPS, cGPS, InSAR and tide gauge data. We show that the joint inversion significantly improves the resolution of the slip distribution, taking advantage of each data set. Our slip models predict a single slip patch of 70 km × 60 km with a maximum slip of 3.2 ± 0.8 m and a moment magnitude of 7.64. The hypothesis from previous studies that the Chiloé earthquake released energy accumulated before the Valdivia earthquake is not supported by the ensemble of geodetic, seismological and tsunami data. Hence, the Chiloé earthquake most likely released all the strain accumulated in the rupture area since the 1960 earthquake. [ABSTRACT FROM AUTHOR]

The surface heat flow of southern South America was examined based on an updated database containing 1113 locations. Accordingly, this study presents the most accurate heat flow map of the southern portion of the continent (south of 16°30′S latitude), covering areas that previously presented limited information. The main anomalies show a strong spatial correlation with geothermal zones and with the most up-to-date lithospheric thickness maps. The blanketing effect produced by the sedimentary basins reduce the surface heat flow up to 27 mW m–2 over the thickest basins. The study region was separated into four large areas and their connection to tectonic processes analysed. The Central Andes present high heat flow zones related to a thick radiogenic crust, volcanic activity, and a hot asthenospheric wedge. In the Pampean flat-slab region, the low heat flow coincides mostly with the horizontal projection of the Juan Fernández aseismic ridge and not with a wide region as previously thought. Furthermore, a close relationship between the subduction of the ridges at different angles and a cold upper-plate lithosphere is suggested. Besides active regions of arc magmatism and a thin lithosphere, we propose that a hot upper-plate upwelling beneath the Patagonian Platform is also contributing to the high heat flow in the area. The foreland region exhibits a low heat flow coinciding with a thick cratonic lithosphere, and local high heat flow values in suture zones possibly triggered by ancient delamination beneath these regions. [ABSTRACT FROM AUTHOR]

Recent compilations of the Illawarra Reversals show remarkable differences in the onset age (265 Ma versus 269 Ma) and the magnetic polarity pattern, hampering the establishment of a reference geomagnetic timescale for the Middle–Late Permian. The Parana Basin hosts a 7-km-thick, discontinuous sedimentary succession spanning the Palaeozoic to the Mesozoic, including the Gondwana 1 supersequence which extends from Late Carboniferous to the Triassic or Jurassic. We provide the magnetostratigraphy of the middle and upper Permian part of this sequence of a ca. 300-m-long core, pertaining to the Passa Dois Group and the Piramboia Formation. Sample demagnetization of NRM revealed two magnetic overprints ascribed to the Brunhes chron and to the emplacement of Parana Large Igneous Province in the Early Cretaceous, respectively. Stable, dual polarity characteristic components (ChRM) were isolated at temperatures higher than 450 °C and interpreted as a primary detrital magnetic remanence (DRM), acquired during or soon after sediment deposition. Available U-Pb dating on volcanic zircons from the literature provided independent chronologic constraints for our magnetostratigraphy. A total of 8 reverse polarity intervals were identified, the lowermost of which (up to 110 m thick) correlates to the Kiaman Superchron. The overlying Illawarra is dominated by a reverse polarity magnetization with thin (4–5 m thick) normal polarity intervals. This pattern agrees with the Illawarra sequence from the Karoo Basin and other proposed timescales for the Permian. The onset of the Illawarra reversals is interpolated to ca. 270 Ma, close to the age of 269 Ma from the Karoo Basin. Combined magnetostratigraphy and geochronology yielded sediment accumulation rates of ca. 8 m Myr–1 for the Passa Dois Group in the northeastern (marginal) sector of the Parana Basin, indicating that Serra Alta and Teresina Formations span 279–274 Ma and 274–254 Ma, respectively. [ABSTRACT FROM AUTHOR]

The Andean Subduction Zone is one of the longest continuous subduction zones on Earth. The relative simplicity of the two-plate system has makes it an ideal natural laboratory to study the dynamics in subduction zones. We measure teleseismic S and SKS traveltime residuals at >1000 seismic stations that have been deployed across South America over the last 30 yr to produce a finite-frequency teleseismic S -wave tomography model of the mantle beneath the Andean Subduction Zone related to the Nazca Plate, spanning from ∼5°N to 45°S and from depths of ∼130 to 1200 km. Within our model, the subducted Nazca slab is imaged as a fast velocity seismic anomaly. The geometry and amplitude of the Nazca slab anomaly varies along the margin while the slab anomaly continues into the lower mantle along the entirety of the subduction margin. Beneath northern Brazil, the Nazca slab appears to stagnate at ∼1000 km depth and extend eastward subhorizontally for >2000 km. South of 25°S the slab anomaly in the lower mantle extends offshore of eastern Argentina, hence we do not image if a similar stagnation occurs. We image several distinct features surrounding the slab including two vertically oriented slow seismic velocity anomalies: one beneath the Peruvian flat slab and the other beneath the Paraná Basin of Brazil. The presence of the latter anomaly directly adjacent to the stagnant Nazca slab suggests that the plume, known as the Paraná Plume, may be a focused upwelling formed in response to slab stagnation in the lower mantle. Additionally, we image a high amplitude fast seismic velocity anomaly beneath the Chile trench at the latitude of the Sierras Pampeanas which extends from ∼400 to ∼1000 km depth. This anomaly may be the remnants of an older, detached slab, however its relationship with the Nazca–South America subduction zone remains enigmatic. [ABSTRACT FROM AUTHOR]

We investigate the impact of unmodelled 3-D structural heterogeneity on inverted W-phase source parameters. We generate a large data set of synthetic seismograms accounting for the Earths 3-D structure for 250 earthquakes globally distributed. The W-phase algorithm is then used to invert for earthquake CMT parameters, assuming a spherical Earth model. The impact of lateral heterogeneity is assessed by comparing inverted source parameters with those used to compute the 3-D synthetics. Results show that the 3-D structure mainly affects centroid location while the effect on the other source parameters remains small. Centroid mislocations present clear geographical patterns. In particular, W-phase solutions for earthquakes in South America are on average biased 17 km to the east of the actual centroid locations. This effect is significantly reduced using an azimuthally well balanced distribution of seismological stations. Source parameters are generally more impacted by mantle heterogeneity while the scalar moment of shallow earthquakes seems to be mainly impacted by the crustal structure. Shallow earthquakes present a variability of M r θ and M r ϕ moment tensor elements, resulting both from the small amplitude and a larger uncertainty of the associated Green's functions. [ABSTRACT FROM AUTHOR]

Subduction zones are the places on the Earth where the greatest earthquakes occur. It is now widely accepted that seismic asperities at the interface of subducting plates play a major role in whether a region of a subduction zone behaves seismically, creating strong earthquakes or exhibits aseismic slip. In the last decades, huge advances have been made to decipher the underlying processes; however, the physical parameters along the subduction zone interfaces are still not very well known due to a sparsity of high-resolution experiments and significant costs associated with amphibious seismic experiments. Therefore, synthetic tests are needed to investigate the potential of currently possible high density seismic deployments and to aid future experiment design. As standard local earthquake traveltime tomography in a subduction zone setting cannot resolve structures on a kilometre scale at depth, we explore the suitability of full-waveform inversion (FWI) to increase resolution by using amplitude and phase information in the recorded earthquake seismograms. We apply 2-D-elastic FWI to synthetic earthquake data, using vertical and horizontal receivers, and utilize a realistic model of the seismic velocities at the Ecuadorian margin. We add perturbations within the subducting plates of 4×4 km and 2×2 km in P - and S -wave velocities, respectively, such that potential crosstalk between the two models can be identified. Our results show that the location and amplitude of the perturbations can be reconstructed in high quality down to approximately 70 km depth. We find that the inversion of the S -wave velocity prior to the inversion of the P -wave velocity is necessary to guarantee a good reconstruction of both models; however, the spatial resolution of the S -wave model is superior to the P -wave model. We also show that frequencies up to 1 Hz are sufficient to achieve high resolution. Further tests demonstrate how results depend on the accuracy of the estimated source orientation. Resulting models do not suffer in quality as artefacts near the source positions compensate for the inaccuracy of source orientation. If sources are located within the subducted plate instead of beneath, resulting models are comparable and the convergence of the inversion scheme is sped up. The accuracy of the source position within the model compared to the true earthquake location is critical and implies that earthquake relocation during the inversion process is necessary, in a similar way as in local earthquake traveltime tomography. [ABSTRACT FROM AUTHOR]

Demarcating the continental–oceanic boundary (COB) is extremely important as it contributes to an understanding of the tectonic environments of marginal basins through plate reconstructions. In general, the COB is estimated by jointly interpreting the features of the basement geomorphology, bathymetry and geophysical datasets, including gravity, magnetic, seismic and geochemical data. We propose a novel methodology named the tilt-theta angles correlation (TTAC), a second-order filtering approach to estimate the COB using high-resolution satellite gravity data. This method computes the angular difference or correlation angle between the vectors computed from the gradients of the tilt and theta derivative-based first-order filters. Oceanic regions generally exhibit small correlation angles (∼0°), thereby indicating a direct correlation; in contrast, continental regions depict large correlation angles (∼180°) and thus display an inverse correlation. The TTAC method marks the COB at the spatial position where the correlation angles abruptly shift from inverse to direct. Tests using synthetic data and the larger spectral energy content of TTAC for long wavelengths over tilt, theta and minus of the sign of vertical derivative (-SiVD) filters attest the effectiveness of the new methodology over first-order derivative based filters. Additionally, tests utilizing real data over both the West African Transform Margin (WATM) and across the South Atlantic Conjugate Margin (SACM) provide favourable results compared with the previous methodologies used to demarcate the COB. The COB estimates derived using the new methodology are consistent with the COB estimates obtained during recent plate-reconstruction studies. We also compare our results with an earlier interpretation along a conjugate margin encompassing magma-poor and magma-rich regions. In the complex rifted environments of the SACM, in which volcanic rocks are dominant, the TTAC methodology substantially improves the previous COB estimates, thereby establishing it as an efficient edge marker by simultaneously reducing small-scale geological noise and enhancing regional-scale geological contributions. [ABSTRACT FROM AUTHOR]

Magnitude estimation is a central task in seismology needed for a wide spectrum of applications ranging from seismicity analysis to rapid assessment of earthquakes. However, magnitude estimates at individual stations show significant variability, mostly due to propagation effects, radiation pattern and ambient noise. To obtain reliable and precise magnitude estimates, measurements from multiple stations are therefore usually averaged. This strategy requires good data availability, which is not always given, for example for near real time applications or for small events. We developed a method to achieve precise magnitude estimations even in the presence of only few stations. We achieve this by reducing the variability between single station estimates through a combination of optimization and machine learning techniques on a large catalogue. We evaluate our method on the large scale IPOC catalogue with >100 000 events, covering seismicity in the northern Chile subduction zone between 2007 and 2014. Our aim is to create a method that provides low uncertainty magnitude estimates based on physically meaningful features. Therefore we combine physics based correction functions with boosting tree regression. In a first step, we extract 110 features from each waveform, including displacement, velocity, acceleration and cumulative energy features. We correct those features for source, station and path effects by imposing a linear relation between magnitude and the logarithm of the features. For the correction terms, we define a non-parametric correction function dependent on epicentral distance and event depth and a station specific, adaptive 3-D source and path correction function. In a final step, we use boosting tree regression to further reduce interstation variance by combining multiple features. Compared to a standard, non-parametric, 1-D correction function, our method reduces the standard deviation of single station estimates by up to |$57\, {\rm per\, cent}$|⁠ , of which |$17\, {\rm per\, cent}$| can be attributed to the improved correction functions, while boosting tree regression gives a further reduction of |$40\, {\rm per\, cent}$|⁠. We analyse the resulting magnitude estimates regarding their residuals and relation to each other. The definition of a physics-based correction function enables us to inspect the path corrections and compare them to structural features. By analysing feature importance, we show that envelope and P wave derived features are key parameters for reducing uncertainties. Nonetheless the variety of features is essential for the effectiveness of the boosting tree regression. To further elucidate the information extractable from a single station trace, we train another boosting tree on the uncorrected features. This regression yields magnitude estimates with uncertainties similar to the single features after correction, but without using the earthquake location as required for applying the correction terms. Finally, we use our results to provide high precision magnitudes and their uncertainties for the IPOC catalogue. [ABSTRACT FROM AUTHOR]

The 2015 Illapel earthquake (M w 8.3) occurred off central Chile on September 16, and generated a tsunami that propagated across the Pacific Ocean. The tsunami was recorded on tide gauges and Deep-ocean Assessment and Reporting of Tsunami (DART) tsunameters in east Pacific. Near-field and far-field tsunami forecasts were issued based on the estimation of seismic source parameters. In this study, we retroactively evaluate the potentiality of forecasting this tsunami in the far field based solely on tsunami data assimilation from DART tsunameters. Since there are limited number of DART buoys, virtual stations are assumed by interpolation to construct a more complete tsunami wavefront for data assimilation. The comparison between forecasted and observed tsunami waveforms suggests that our method accurately forecasts the tsunami amplitudes and arrival time in the east Pacific. This approach could be a complementary method of current tsunami warning systems based on seismic observations. [ABSTRACT FROM AUTHOR]

Recently an ambitious experiment combining deep seismic surveys from near-vertical and wide-angle acquisition methods was carried out in Brazil. The seismic lines are essentially coincident and crossed the Parnaíba Basin from west to east near latitude 5°S. Here, the wide-angle reflection and refraction (WARR) and deep seismic reflection (DSR) results, which were previously interpreted independently, are compared by directly correlating WARR interfaces converted to TWTT with the major reflective horizons identified in the zero-offset image and by considering coincident reflectivity patterns displayed in both data sets. This integrated WARR and DSR analysis allowed a spatial association of the apparently acoustically featureless crust imaged in the DSR profile to the high reflectivity observed in the WARR data. Numerical tests and elastic modelling show that variations of the elastic properties of the crust, particularly as they are characterized by low Vp and Vs contrasts with a possible increase of the Vp / Vs ratio, can only weakly explain the observed reflectivity patterns but that fine-scale lithological heterogeneity within the crust is capable of replicating the observed contrasting seismic responses. The segment of the Parnaíba Basin crust that is characterized by fine-scale lithological heterogeneity lies directly above a mafic crustal underplate defined by the WARR model and was named as the Grajaú domain on the basis of WARR-derived velocity model. The applied methodologies allow added value to be taken from the independent seismic data sets and provide new information about crustal structure that may have important implications for overlying intracontinental basin evolution. [ABSTRACT FROM AUTHOR]

From an inversion of ≈7000 crossings of 37 magnetic reversals between chrons C1n (0.78 Ma) and C13 (33.7 Ma) and numerous crossings of 43 transform faults and fracture zones in the central and southern Atlantic basin, we estimate finite rotations and stage angular velocities that describe South America plate motion relative to Africa (Nubia), and via plate circuit closures relative to the Antarctic and North America plates. Our newly estimated Nubia–South America rotations, which are spaced at ≈1-Myr intervals, reveal for the first time a transition from steady motion before 24–22 Ma to continuously slowing motion since 20 Ma, including a previously unknown, sudden spreading rate slowdown at 7–6 Ma, when similar declines in seafloor spreading rates also occurred in the northern Atlantic and Arctic basins and along the Southwest Indian Ridge. Geodetic measurements from Nubia and South America, which record the instantaneous plate motion, corroborate the slowdown and indicate it has continued to the present. Our newly determined North America–South America rotations and angular velocities indicate that the pole for this plate pair was stationary from 20 to 12 Ma, but has migrated steadily southward since ≈12 Ma, causing the plate motion direction to change by up to 90° anticlockwise along parts of the diffuse oceanic plate boundary. Since ≈14 Ma, the plate motion rates have averaged 3±0.5 mm yr−1 of N–S to NW–SE-directed divergence near the Mid-Atlantic Ridge and 1–2 mm yr−1 of obliquely convergent motion near the Lesser Antilles Trench, consistent with geodetic estimates. Our new Antarctic–South America rotations and angular velocities predict that American–Antarctic Ridge seafloor spreading rates and plate slip directions have slowed gradually by 60 per cent and rotated 5–7° clockwise since 20 Ma in response to the post-20 Ma slowdown in Nubia–South America seafloor spreading rates. Fracture zone flow lines that are predicted with the new Antarctic–South America rotations agree well with the trends of fracture zone valleys and ridges that flank the American–Antarctic Ridge, offering independent evidence for the accuracy of our new rotations. The Antarctic–South America rotations for chrons C5n.2 (11.1 Ma) and C6no (19.7 Ma) predict that 15–40 km less seafloor has accreted since these times than is indicated by identifications of these two reversals along the American–Antarctic Ridge. Slow westwards movement of the previously postulated Sur microplate with respect to the South America plate may explain the discrepancy—a strong test of this hypothesis awaits better magnetic anomaly data from this remote seafloor spreading centre. [ABSTRACT FROM AUTHOR]

Knowledge of the coastal ocean mass variations is important for understanding the ocean climate and sea level change. However, estimates of coastal ocean mass variations have been perplexed due to poor representativeness of previous Gravity Recovery and Climate Experiment (GRACE) data across the land/ocean boundary. We here use GRACE mascon solutions to investigate the coastal ocean mass variations (within 400 km band) at global and regional scales. GRACE mascon solutions are advanced by spatial constraints and leakage correction. For the global mean, it is found that mascons ocean mass variations are in rough agreement with those inferred from satellite altimeter and an ocean analysis; the agreement is better on seasonal scales (6.4 ± 0.5 mm annual amplitude for the mascons and 7.3 ± 0.7 mm annual amplitude for the inferred); on the other hand, large differences are shown for the linear trend (2.1 ± 0.1 mm yr−1 for the mascons and 3.2 ± 0.1 mm yr−1 for the inferred), indicating that it is far from closing the coastal sea level budget. At regional scales, high consistency between mascons and the inferred is observed over two shallow areas, that is, Europe coast and East China coast, whose annual amplitudes are 24 ± 5 and 42 ± 3 mm, respectively. Ocean mass variations are not well captured by mascons over other regions (e.g. Africa coast, Indian coast and North and South America coast). Possible explanations include (1) the steric component plays a more important role in sea level, consequently resulting in weak mass signal and (2) the performance of mascons varies with locations. We find that mascon solutions show better variability (especially seasonal cycles) than do the traditional spherical harmonic coefficients, suggesting that mascons applied with spatial constraints improve the coastal ocean mass variability. [ABSTRACT FROM AUTHOR]

Knowledge of deformation at plate boundaries has been improved greatly by the development of observational techniques in space geodesy. However, most theoretical and numerical models of coseismic deformation have remained very simple and do not include realistic Earth structure. 3-D material heterogeneity and topography are often neglected because simple models are assumed to be sufficient and available tools cannot easily accommodate complex heterogeneity. In this study, we demonstrate the importance of 3-D heterogeneity using a spectral element method that incorporates topography and 3-D material properties. Using a parabolic hill model and a topographic model of the 2010 Maule earthquake, we show that topographic features can alter the shape of observed surface deformation patterns. We also estimate the coseismic surface deformation due to a model of the slip distribution of the 2015 Gorkha earthquake using realistic topography and 3-D elastic structure, and find that the presence of topography causes changes in the shape of observed surface displacement patterns, while material heterogeneities primarily affect the magnitude of observed displacements. Our results show that the inclusion of topography in particular can affect predictions of coseismic deformation modelling. [ABSTRACT FROM AUTHOR]

The seismic gap hypothesis assumes that a characteristic earthquake is followed by a long period with a reduced occurrence probability for the next large event on the same fault segment, as a consequence of the induced stress shadow. The gap model is commonly accepted by geologists and is often used for time-dependent seismic hazard estimations. However, systematic and rigorous tests to verify the seismic gap model have often failed so far, which might be partially related to limited data and too tight model assumptions. In this study, we relax the assumption of a characteristic size and location of repeating earthquakes and analyse one of the best available data sets, namely the historical record of major earthquakes along a 3000 km long linear segment of the South American subduction zone. To test whether a stress shadow effect is observable, we compiled a comprehensive catalogue of mega-thrust earthquakes along this plate boundary from 1520 to 2015 containing 174 earthquakes with Mw > 6.5. In our new testing approach, we analyse the time span between an earthquake and the last event that ruptured the epicentre location, where we consider the impact of the uncertainties of epicentres and rupture extensions. Assuming uniform boundary conditions along the trench, we compare the distribution of these recurrence times with simple recurrence models. We find that the distribution is in all cases almost exponentially distributed corresponding to a random (Poissonian) process; despite some tendencies for clustering of the Mw = 7 events and a weak quasi-periodicity of the Mw = 8 earthquakes, respectively. To verify whether the absence of a clear stress shadow signal is related to physical assumptions or data uncertainties, we perform simulations of a physics-based stochastic earthquake model considering rate and state-dependent earthquake nucleation, which are adapted to the observations with regard to the number of events, spatial extend, size distribution and involved uncertainties. Our simulations show that the catalogue uncertainties lead to a significant blurring of the theoretically peaked distribution, but the distribution would be still distinguishable from the observed one forMw = 7 events. However, considering the stress transfer to adjacent fault segments and heterogeneous instead of constant stress drop within the rupture zone can explain the observed recurrence time distribution. We conclude that simplified recurrence models, ignoring the complexity of the underlying physical process, cannot be applied for forecasting the Mw = 7 earthquake occurrence at this plate boundary. [ABSTRACT FROM AUTHOR]

Measurements of seismic anisotropy are commonly used to constrain deformation in the upper mantle. Observations of anisotropy at mid-mantle depths are, however, relatively sparse. In this study we probe the anisotropic structure of the mid-mantle (transition zone and uppermost lower mantle) beneath the Japan, Izu-Bonin, and South America subduction systems. We present source-side shear wave splitting measurements for direct teleseismic S phases from earthquakes deeper than 300 km that have been corrected for the effects of upper mantle anisotropy beneath the receiver. In each region, we observe consistent splitting with delay times as large as 1 s, indicating the presence of anisotropy at mid-mantle depths. Clear splitting of phases originating from depths as great as ~600 km argues for a contribution from anisotropy in the uppermost lower mantle as well as the transition zone. Beneath Japan, fast splitting directions are perpendicular or oblique to the slab strike and do not appear to depend on the propagation direction of the waves. Beneath South America and Izu-Bonin, splitting directions vary from trench-parallel to trench-perpendicular and have an azimuthal dependence, indicating lateral heterogeneity. Our results provide evidence for the presence of laterally variable anisotropy and are indicative of variable deformation and dynamics at mid-mantle depths in the vicinity of subducting slabs. [ABSTRACT FROM AUTHOR]

On 2012 May 19, an mb = 4 earthquake shook the town of Montes Claros, Brazil in the middle of the Sãao Francisco Craton. Because of the scarce seismicity in the area, an event like this could provide valuable information to characterize the governing seismotectonics and stress field for the region. Here, we present the results of more than 1 yr of local seismic monitoring after the main shock. We found that the seismicity originated at approximately 1-km depth in an NNW-oriented blind reverse fault, dipping to the E. The magnitude of the main shock was 4mb, with aftershocks reaching up to 3.6mb. Focal mechanisms from first motion polarities and waveform moment tensor inversions indicate a reverse faulting in agreement with the orientation of the aftershock locations. In addition, we derived a new 1-D local velocity model using a simultaneous inversion of hypocentres and velocity layers. The results indicate P-wave velocities of 4.5 km s-1 for the upper layer of carbonate rocks and 5.23 and 5.69 km s-1 for the lower fractured and compact crystalline basement layers, respectively. Higher Vp/Vs ratios were obtained for the upper two layers compared to the lowermost layer, possibly indicating presence of rock fracturing and percolated water. The calculated stress drop for the main event is 0.33 MPa, which is a relatively low value for an intraplate earthquake but still within the observed range. The inversion of the main shock focal mechanism and previously published focal mechanisms suggests a compressional stress regime in the central part of the Sãao Francisco Craton, which is different from the strike-slip regime in the southern part, although both have an EW-oriented σ1. On the other hand, focal mechanisms of events located to the west of the craton indicate an NW-SE oriented σ1 for central Brazil. This variability highlights the importance of local sources of stresses (e.g. fkexural stresses) in mid-plate South America, unlike other mid-plate areas of the world, such as central and east North America, where a more uniform stress field is observed. [ABSTRACT FROM AUTHOR]

We estimate the influence of the geomagnetically induced currents (GIC) on the Uruguayan electrical distribution grid. The GIC are related to time variations in the geomagnetic field caused by the interaction of the solar wind with the Earth's magnetosphere. These currents may cause several problems in power grids, pipelines, among others. South America is close to the South Atlantic Magnetic Anomaly (SAA). Particle precipitation in the anomaly region could lead to pulsations in the horizontal component of the geomagnetic field, that may contribute to the GIC production. Scarce research has been done to assess the space weather impact on this area.Due to the lack of magnetic field data at the Uruguayan territory, spherical elementary currents system (SECS) was used to interpolate field values with data available from three magnetic observatories located at Vassouras, Trelew and São Martinho da Serra. The simple topology of the Uruguayan power grid provides some advantages in the calculation of GIC. To calculate these currents, a plane wave method combined with a ground conductivity model was used. Estimated GIC suggest peak currents ca. 15 A, at the earthing points of some substations in a 770 km power line during the Halloween Magnetic Storm on 2003 October 29. [ABSTRACT FROM AUTHOR]

SUMMARY We searched for earthquake swarms in South America between 1973 and 2009 using the global Preliminary Determination of Epicenters (PDE) catalogue. Seismicity rates vary greatly over the South American continent, so we employ a manual search approach that aims to be insensitive to spatial and temporal scales or to the number of earthquakes in a potential swarm. We identify 29 possible swarms involving 5-180 earthquakes each (with total swarm moment magnitudes between 4.7 and 6.9) within a range of tectonic and volcanic locations. Some of the earthquake swarms on the subduction megathrust occur as foreshocks and delineate the limits of main shock rupture propagation for large earthquakes, including the 2010 Mw 8.8 Maule, Chile and 2007 Mw 8.1 Pisco, Peru earthquakes. Also, subduction megathrust swarms commonly occur at the location of subduction of aseismic ridges, including areas of long-standing seismic gaps in Peru and Ecuador. The magnitude-frequency relationship of swarms we observe appears to agree with previously determined magnitude-frequency scaling for swarms in Japan. We examine geodetic data covering five of the swarms to search for an aseismic component. Only two of these swarms (at Copiapó, Chile, in 2006 and near Ticsani Volcano, Peru, in 2005) have suitable satellite-based Interferometric Synthetic Aperture Radar (InSAR) observations. We invert the InSAR geodetic signal and find that the ground deformation associated with these swarms does not require a significant component of aseismic fault slip or magmatic intrusion. Three swarms in the vicinity of the volcanic arc in southern Peru appear to be triggered by the Mw= 8.5 2001 Peru earthquake, but predicted static Coulomb stress changes due to the main shock were very small at the swarm locations, suggesting that dynamic triggering processes may have had a role in their occurrence. Although we identified few swarms in volcanic regions, we suggest that particularly large volcanic swarms (those that could be detected using the PDE catalogue) occur in areas of infrequent eruption and may be related to large regional fault zones. [ABSTRACT FROM AUTHOR]

Estimates of effective elastic thickness for the western portion of the South American Plate using, independently, forward flexural modelling and coherence analysis, suggest different thermomechanical properties for the same continental lithosphere. We present a review of these estimates and carry out a critical reappraisal using a common methodology of 3-D finite element method to solve a differential equation for the bending of a thin elastic plate. The finite element flexural model incorporates lateral variations of and the Andes topography as the load. Three maps for the entire Andes were analysed: Stewart & Watts (1997) , Tassara et al. (2007) and Pérez-Gussinyé et al. (2007) . The predicted flexural deformation obtained for each map was compared with the depth to the base of the foreland basin sequence. Likewise, the gravity effect of flexurally induced crust–mantle deformation was compared with the observed Bouguer gravity. estimates using forward flexural modelling by Stewart & Watts (1997) better predict the geological and gravity data for most of the Andean system, particularly in the Central Andes, where ranges from greater than in the sub-Andes to less than under the Andes Cordillera. The misfit between the calculated and observed foreland basin subsidence and the gravity anomaly for the Marañon basin in Peru and the Bermejo basin in Argentina, regardless of the assumed map, may be due to a dynamic topography component associated with the shallow subduction of the Nazca Plate beneath the Andes at these latitudes. [ABSTRACT FROM AUTHOR]

The salar de Uyuni in the Bolivian Andes is the largest salt flat on Earth, exhibiting less than 1 m of vertical relief over an area of 9000 km2. We report on a kinematic Global Positioning System (GPS) survey of a 45-by-54 km area in the eastern salar, conducted in September 2002 to provide ground truth for the Ice Cloud and land Elevation Satellite (ICESat) mission. GPS post-processing included corrections for long-period GPS noise that significantly improved survey accuracy. We fit corrected GPS trajectories with 2-D Fourier basis functions, from which we created a digital elevation model (DEM) of the surface whose absolute accuracy we estimate to be at least 2.2 cm RMSE. With over two magnitudes better vertical resolution than the Shuttle Radar Topography Mission data, this DEM reveals decimetre-level topography that is completely absent in other topographic data sets. Longer wavelengths in the DEM correlate well with mapped gravity, suggesting a connection between broad-scale salar topography and the geoid similar to that seen over the oceans. [ABSTRACT FROM AUTHOR]

Broad-band P- and S-waves from earthquakes in South America recorded at Californian network stations are analysed to image lateral variations of the D″-discontinuity beneath the Cocos plate. We apply two array processing methods to the data set: a simplified migration method to the P-wave data set and a double-array method to both the P- and S-wave data sets, allowing us to compare results from the two methods. The double-array method images a dipping reflector at a depth range from 2650 to 2700 km in the southern part of the study area. We observe a step-like topography of 100 km to a shallower reflector at about 2600 km depth to the north, as well as evidence for a second (deeper) reflector at a depth range from 2700 to 2750 km in the north. Results from the simplified migration agree well with those from the double-array method, similarly locating a large step in reflector depth in a similar location (about 2650 km depth in the south and about 2550 km in the north) as well as the additional deeper reflector at the depth of about 2750 km in the north. Waveform modelling of the reflected waves from both methods suggests a positive velocity contrast for S waves, but a negative velocity contrast for P waves for the upper reflector in agreement with predictions from mineral physical calculations for a post-perovskite phase transition. The data also show some evidence for the existence of another deeper reflector that could indicate a double intersection of the geotherm with the post-perovskite stability field, that is, the back-transformation of post-perovskite to perovskite close to the core–mantle boundary. [ABSTRACT FROM AUTHOR]

We model using analogue experiments the subduction of buoyant ridges and plateaus to study their effect on slab dynamics. Experiments show that simple local (1-D) isostatic considerations are not appropriate to predict slab behaviour during the subduction of a buoyant ridge perpendicular to the trench, because the rigidity of the plate forces the ridge to subduct with the dense oceanic lithosphere. Oceanic ridges parallel to the trench have a stronger effect on the process of subduction because they simultaneously affect a longer trench segment. Large buoyant slab segments sink more slowly into the asthenosphere, and their subduction result in a diminution of the velocity of subduction of the plate. We observe a steeping of the slab below those buoyant anomalies, resulting in smaller radius of curvature of the slab that augments the energy dissipated in folding the plate and further diminishes the velocity of subduction. When the 3-D geometry of a buoyant plateau is modelled, the dip of the slab above the plateau decreases, as a result of the larger velocity of subduction of the dense ‘normal’ oceanic plate on both sides of the plateau. Such a perturbation of the dip of the slab maintains long time after the plateau has been entirely incorporated into the subduction zone. We compare experiments with the present-day subduction zone below South America. Experiments suggest that a modest ridge perpendicular to the trench such as the present-day Juan Fernandez ridge is not buoyant enough to modify the slab geometry. Already subducted buoyant anomalies within the oceanic plate, in contrast, may be responsible for some aspects of the present-day geometry of the Nazca slab at depth. [ABSTRACT FROM AUTHOR]

A detailed passive seismic experiment was carried out across southwestern British Columbia and northwestern Washington to investigate the structure of the subducting Juan de Fuca plate and mantle wedge in Cascadia and its relation to intraslab seismicity. As part of thepolarisproject, 31 three-component broad-band stations were deployed in an approximately linear array spanning southern Vancouver Island, the Gulf and San Juan Islands, Watcom county and the British Columbia lower mainland.P-wave coda from 41 teleseismic events have been employed in formal inversions for fine-scale shear-velocity structure. Our results indicate a structure very similar to that identified across a comparable profile in central Oregon. The continental Moho is evident at the eastern end of the profile near 35-km depth but disappears towards Georgia Strait/Puget Sound. A prominent low-S-velocity zone is clearly evident below southern Vancouver Island dipping eastwards through Georgia Strait/Puget Sound, and coincides with the E-reflection zone originally identified inlithoprobestudies. Structure below the E-layer is much less prominent and varies intermittently along the array. Based on the observations and interpretations of similar structures beneath Oregon, Alaska and South America, and its projection to mantle depths, we suggest that the low-velocity E-layer represents the dehydrating oceanic crust of the subducting Juan de Fuca plate. This interpretation is consistent with recent seismicity studies that place shallow Wadati–Benioff events within the oceanic mantle, and implies that the oceanic crust is 6–8 km shallower beneath Vancouver Island than previously assumed. As in Oregon, we interpret the diminished signature of oceanic crust below a depth of 45 km to signal the presence of eclogitization, which in turn supplies water to serpentinize the overlying forearc mantle. [ABSTRACT FROM AUTHOR]

By definition, passive margins deform little or no further after continental break-up. Nevertheless, post-break-up deformation and associated uplift have been documented along many margins (for example Norway, Greenland, West Africa, South America, India and Australia). We have conducted a series of analogue experiments in which we subjected scaled multilayered models of the lithosphere (made of sand and viscous layers) to horizontal shortening. A multilayered lithosphere rested on an asthenosphere of low viscosity that allowed isostatic adjustments. Models with various strength profiles represented oceanic lithospheres of different ages. The continental lithosphere reacted to an applied boundary velocity according to the strength and density of oceanic lithosphere. Where oceanic lithosphere was stronger and heavier than continental lithosphere it subducted; where oceanic crust was stronger and lighter than continental crust it obducted. On this basis we suggest that the style of incipient shortening of a passive margin depends on the age of oceanic lithosphere. Where the ocean is young we expect obduction; where it is old we expect incipient subduction. In nature, ophiolites and shortened passive margins appear to conform to this simple rule. [ABSTRACT FROM AUTHOR]

Analyzes magnetotelluric data from the Central Andes in Northern Chile and Southern Bolivia using sensitivity analysis. Conductivity anomaly in the backarc of the South American subduction zone below the Bolivian Altiplano; Minimum depth of the Altiplano conductor; Linear sensitivity analysis.

Investigates the derivation of paleomagnetic poles from independently assessed inclination and declination data in South America. Exclusion of poles from regions having undergone vertical-axis rotation as a consequence of tectonics; Development of techniques for calculation of paleomagnetic poles; Observation of systematic biases in the data set.

SUMMARY On 2007 April 21, a Mw= 6.2 earthquake shook the Aysén Fjord, Southern Chile in an unprecedented episode for this region characterized by low seismicity. The area is intersected by the Liquiñe-Ofqui Fault System (LOFS), a +1000-km-long strike-slip fault that absorbs part of the oblique convergence motion between Nazca and South America plates. To study the aftershock sequence of this main event, we installed a seismic network of 15 stations in the area for a period of nearly 7 months. We characterized the seismogenic structure of the zone by calculating a minimum 1-D local velocity model and obtaining precise hypocentral coordinates and uncertainty estimates by using a non-linear probabilistic approach. We also obtained fault plane solutions based on first motion polarities and SV/ P amplitude ratios. The velocity model shows an average Vp/ Vs ratio of 1.76 for the area and low shear wave velocity values for the upper 3 km of crust. The aftershock seismicity was located mainly between 4 and 10 km depth and disposed in (1) an ∼N-S trending alignment that follows the trace of the LOFS and (2) an E-W alignment at the East of the main fault. Furthermore, we re-analysed the previously published foreshock and early aftershock activity of the sequence including four of its largest events, improving considerably previous location estimates. Selected focal mechanisms show a strong strike-slip component that coincides with the nature of the LOFS. Based on our new analysis we conclude that the 2007 Aysén seismic sequence had a tectonic origin related to activity on the southern end of the LOFS, however not discarding the presence and potential action of fluids on the aftershock activity. [ABSTRACT FROM AUTHOR]