We deployed a dense geodetic and seismological network in the Atacama seismic gap in Chile. We derive a microseismicity catalog of >30,000 events, time series from 70 GNSS stations, and apply a transdimensional Bayesian inversion to estimate interplate locking degree. We identify two highly locked regions of different sizes whose geometries appear to control seismicity patterns. Interface seismicity concentrates beneath the coastline just downdip of the highest locking. A region of lower interplate locking around 27.5ºS coincides with higher seismicity levels, a high number of repeating earthquakes and events extending further towards the trench. Having shown numerous signs of aseismic deformation (slow-slip events and earthquake swarms), this area is situated where the Copiapó Ridge is subducted. While these findings suggest that the structure of the downgoing oceanic plate prescribes patterns of interplate locking and seismicity, we note that the Taltal Ridge further north lacks a similar signature.

On August 12, 2021 a > 220 s lasting complex earthquake with Mw > 8.2 hit the central and southern South Sandwich trench. Due to its remote location and short interevent times, reported earthquake parameters varied significantly between different international agencies. We studied the complex rupture by combining different seismic source characterization techniques sensitive to different frequency ranges based on teleseismic broadband recordings from 0.001–2 Hz, including point and finite fault inversions and the back-projection of high-frequency signals. We also determined moment tensor solutions for 88 aftershocks. The rupture initiated simultaneously with a Mw 7.6 thrust earthquake in the deep part of the seismogenic zone in the central subduction interface and a shallow megathrust rupture which propagated unilaterally to the south with a very slow rupture velocity of 1.2 km/s and varying strike following the curvature of the trench. The slow rupture covered nearly two thirds of the entire subduction zone length, and with Mw 8.2 released the bulk of the total moment of the earthquake. Tsunami modelling indicates the inferred shallow rupture can explain the tsunami records. The southern segment of the shallow rupture overlaps with another activation of the deeper part of the megathrust equivalent to a Mw 7.6. The aftershock distribution confirms the extent and curvature of the rupture. Some mechanisms are consistent with the mainshocks, but many indicate also activation of secondary faults. Rupture velocities and radiated frequencies varied strongly between different stages of the rupture, which might explain the variability of published source parameters.

Teleseismic back-projection has emerged as a widely-used tool for understanding the rupture histories of large earthquakes. However, its application often suffers from artifacts related to the receiver array geometry, notably the ‘swimming’ artifact. We present a teleseismic back-projection method with multiple arrays and combined P and pP waveforms. The method is suitable for defining arrays ad-hoc in order to achieve a good azimuthal distribution for most earthquakes. We present a catalog of short-period rupture histories (0.5-2.0 Hz) including all 54 earthquakes from 2010 to 2021 with M_w ≥ 7.5 and depth less than 200 km. The method provides semi-automatic estimates of rupture length, directivity, speed, and aspect ratio, which are related to the complexity of large ruptures. We determined short-period rupture length scaling relations that are in good agreement with previously published relations based on estimates of total slip. Rupture speeds were consistently in the sub-Rayleigh regime for thrust and normal earthquakes, whereas a tenth of strike-slip events propagated in the unstable supershear range. Many of the rupture histories exhibited complex behaviors such as rupture on conjugate faults, bilateral ruptures, and dynamic triggering by a P wave. For megathrust earthquakes, ruptures encircling asperities were frequently observed, with down-dip, up-dip, double encircling, and segmented patterns. Although there is a preference for short-period emissions to emanate from central and down-dip parts of the megathrust, emissions up-dip of the main asperities are more frequent than suggested by earlier results.

A new seismic velocity model for the south Central Andes is derived from full waveform inversion, covering the Pampean flat and adjacent Payenia steep subduction segments. Strong focused crustal low-velocity anomalies indicate partial melts in the Payenia segment along the volcanic arc, whereas weaker low-velocity anomalies covering a wide zone in Pampean possibly indicates remnant melts in the past. Thinning and tearing of the flat Nazca slab below the Pampean is inferred by gaps in the high-velocity slab along the inland projection of the Juan-Fernandez-Ridge. A high-velocity anomaly in the upper mantle below the flat slab is interpreted as a relic Nazca slab segment, which indicates an earlier slab break-off during the flattening process, triggered by the buoyancy of the Juan-Fernandez-Ridge. In Payenia, large-scale low-velocity anomalies atop and below the re-steepened Nazca slab are associated with the re-opening of the mantle wedge and sub-slab asthenospheric flow, respectively.

Recent research showed that machine learning, in particular deep learning, can be applied with great success to a multitude of seismological tasks, e.g. phase picking and earthquake localization. One reason is that neural networks can be used as feature extractors, generating generally applicable representations of complex data. We employ a convolutional network to condense earthquake waveforms from a varying set of stations into a high dimensional vector, which we call event embedding. For each event the embedding is calculated from instrument-corrected waveforms beginning at the first P pick and updated continuously with incoming data. We employ event embeddings for real time magnitude estimation, earthquake localization and ground motion prediction, which are central tasks for early warning and for guiding rapid disaster response. We evaluate our model on the IPOC catalog for Northern Chile, containing ∼100,000 events with low uncertainty hypocenters and magnitude estimates. We split the catalog sequentially into a training and a test set, with the 2014 Iquique event (Mw 8.1) and its fore- and aftershocks contained in the test set. Following preliminary results the system achieves a test RMSE of 0.28 magnitude units (m.u.) and 35 km hypocentral distance 1 s after the first P arrival at the nearest station, which improves to 0.17 m.u. and 22 km after 5 s and 0.11 m.u. and 15 km after 25 s. As applications in the hazard domain require proper uncertainty estimates, we propose a probabilistic model using Gaussian mixture density networks. By analyzing the predictions in terms of their calibration, we show that the model exhibits overconfidence i.e. overly optimistic confidence intervals. We show that deep ensembles substantially improve calibration. To assess the limitations of our model and elucidate the pitfalls of machine learning for early warning in general, we conduct an error analysis and discuss mitigation strategies. Despite the size of our catalog, we observe issues with two kinds of data sparsity. First, we observe increased residuals for the source parameters of the largest events, as training data for these events is scarce. Second, similar inaccuracies occur in areas without events of a certain size in the training catalog. We investigate the impact of these limitations on the Iquique fore- and aftershocks.

We present a new seismic tomography model for the crust and upper-mantle beneath the Central Andes based on multi-scale full seismic waveform inversion, proceeding from long periods (40–80~s) over several steps down to 12–60~s. The spatial resolution and trade-offs among inversion parameters are estimated through the multi-parameter point-spread functions. P and S wave velocity structures with a spatial resolution of 30–40 km for the upper mantle and 20 km for the crust could be resolved in the central study region. In our study, the subducting Nazca slab is clearly imaged in the upper mantle, with dip-angle variations from the north to the south. Bands of low velocities in the crust and mantle wedge indicate intense crustal partial melting and hydration of the mantle wedge beneath the frontal volcanic arc, respectively and they are linked to the vigorous dehydration from the subducting Nazca plate and intermediate depth seismicity within the slab. These low velocity bands are interrupted at 19.8º–21°S, both in the crust and uppermost mantle, hinting at the lower extent of crustal partial melting and hydration of the mantle wedge. The variation of lithospheic high velocity anomalies below the backarc from North to South allows insight into the evolutionary foundering stages of the Central Andean margin. A high velocity layer beneath the southern Altiplano suggests underthrusting of the leading edge of the Brazilian Shield. In contrast, a steeply westward dipping high velocity block and low velocity lithospheric uppermost mantle beneath the southern Puna plateau hints at the ongoing lithospheric delamination.