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During the large-N MAUPASACQ passive seismic experiment in the foreland of the western Pyrenees (southwest France), an unusual swarm-type seismic sequence was serendipitously recorded in a normally quiet area. Thanks to the density of the deployment and the proximity of all events, it was possible to relocate the hypocenters with a very good relative accuracy through template matching, cross-correlation phase picks, and double-difference algorithm. The four-month seismic activity consists of more than 600 events with local magnitudes ranging between -1.4 and 2.1, clustered in an extremely small volume, and rooted at 4 km depth. The sequence can be divided in two phases of similar durations and event occurrence rates, but of different magnitude-frequency distributions. The presence of an asperity is suggested by the relative abundance of stronger events during the second phase. Fine mapping suggests a small but clear geographic offset of a few tens of meters between the events of the two phases and a very slow migration suggesting a process involving fluids. Changes in the correlation matrices and waveforms of late arrivals at a specific station are also observed, arguing either (and again) for migration of the hypocenters or for changes in the propagation medium between the two phases. The geographical coincidence with the repeated observation of hydrogen leaks on the surface, almost above the swarm, suggests a connection through channels that could carry fluids. [ABSTRACT FROM AUTHOR]

We assess the role of inherited structures on the Meso‐Cenozoic tectonic evolution of the main Andean Cordillera in central Chile (33°30′S‐34°S). Based on extensive field mapping, U‐Pb geochronology and palinspastic restorations along the Yeso and Volcán river valleys, we propose a tectono‐stratigraphic model for the evolution of a hybrid fold‐and‐thrust belt, originated from the inversion of Mesozoic extensional basins. With these results, we highlight the structural graben configuration of the Yeguas Muertas and Nieves Negras depocenters, as evidenced by synextensional deposition of the Río Damas and Lo Valdés Formations, controlled by normal faulting. The uppermost Cretaceous evolution can be approached through the analysis of the Las Coloradas Unit (ULC), which overlies the volcanic rocks of the upper Colimapu Formation and can be correlated, north of 32°S, with the Juncal Formation, and south of 35°S, with the Plan Los Yeuques Formation. The contractional Neogene‐Quaternary deformation in the fold‐and‐thrust belt domain studied in this work, accommodated 27–28 km of minimum crustal shortening. The Neogene‐Quaternary deformation that generated the final uplift of the Andes, has a close relationship with preexisting inverted Mesozoic structures. These structures deform the volcano‐sedimentary Abanico Formation, deposited since the late Eocene, based on new U‐Pb detrital zircon data. We propose that the active seismicity observed in the eastern border of the Principal Cordillera, located to the east of Santiago, can be associated with major crustal faults, as the Estero de Yeguas Muertas‐Baños Colina fault system and the Chacayes‐Yesillo fault. Key Points: The geochronological new data confirm the Andean Jurassic arc as a sediment source for the Río Damas FormationThe geometry of the western AFTB is controlled by heterogeneities inherited from the Neuquén BasinThe Chacayes‐Yesillo fault and the root of the Estero de Yeguas Muertas‐Baños Colina fault system concentrate the recent seismic activity [ABSTRACT FROM AUTHOR]

The southern Central Andes recorded retroarc shortening, basin evolution, and magmatic arc migration during Neogene changes in subduction. At 31–33°S, above the modern flat‐slab segment, spatial and temporal linkages between thin‐ and thick‐skinned foreland shortening, basement‐involved exhumation of the main Cordillera, and lower‐crustal hinterland thickening remain poorly resolved. We integrate new geochronological and thermochronological data for thrust sheets and Neogene foreland basin fill with structural, sedimentological, and passive seismic results to reconstruct the exhumation history and evaluate potential geometric linkages across structural domains. 40Ar/39Ar ages for volcanic horizons and zircon U‐Pb ages for synorogenic clastic deposits in the Manantiales Basin constrain the minimum duration of synorogenic sedimentation to ~22–14 Ma. Detrital zircon age distributions record sequential unroofing of hinterland thrust sheets until ~15 Ma, followed by eastward (cratonward) advance of the deformation front, shutoff of western sediment sources, and a shift from fluvial to alluvial fan deposition at ~14 Ma. Apatite (U‐Th)/He cooling ages confirm rapid exhumation of basement‐involved structural blocks and basin partitioning by ~14–5 Ma, consistent with the timing of the Manantiales facies and provenance shifts and a coeval (~12–9 Ma) pulse of thin‐skinned shortening and exhumation previously identified in the eastern foreland. Late Miocene–Pliocene (~8–2 Ma) cooling ages along the Chile‐Argentina border point to hinterland uplift during the latest stage of Andean orogenesis. Finally, geophysical constraints on crustal architecture and low‐temperature thermochronometry results are compatible with a hybrid thin‐ and thick‐skinned décollement spanning retroarc domains. Key Points: 40Ar/39Ar and zircon U‐Pb ages constrain the minimum duration of synorogenic accumulation in the Neogene Manantiales Basin to ~22–14 MaForeland basin strata record sequential unroofing of hinterland thrust sheets, then regionally synchronous shortening by ~14–12 MaGeo/thermochronological data and thermo‐kinematic models are compatible with hybrid thin‐ and thick‐skinned retroarc deformation [ABSTRACT FROM AUTHOR]

Most of the recorded seismicity in central Chile can be linked to the subduction of the Nazca plate. To the east, a much smaller fraction is observed at 0-30 km depths beneath the western Andean thrust. Paleoseismic studies evidenced the occurrence of at least two major earthquakes (M>7) over the past 17 ka, associated with the San Ramón fault (SRF): an important tectonic feature characterizing the west Andean thrust, close the Santiago metropolitan area. To better constrain the crustal seismicity in this area, the Chilean Seismological Center (CSN) extended its permanent seismic network with seven new broadband seismometers deployed around the scarp of the SRF and farther east. The improved azimuthal distribution and reduced station spacing allowed to complete the CSN catalog with more than 900 smaller magnitude earthquakes (ML<2.5) detected and located within the study region. The use of a 3D velocity model derived from P- and S-wave travel-time tomography considerably lowered the uncertainties associated with hypocentral locations. Our results show an important seismicity beneath the Principal Cordillera located at a depth of ~10 km, and a deeper seismicity (~15 km) aligned with the main Andean thrust more to the west, parallel to the scarp of the SRF. Regional stress inversion results suggest that the seismicity of the west Andean thrust is accommodating northeast-southwest compressional stress, consistent with the convergence of the Nazca plate. Based on our improved crustal seismicity, combined with observations from previous studies, we have been able to refine the scenario of an Mw 7.5 earthquake rupturing the SRF. Ground-motion prediction results show peak ground accelerations of ~0.8g close to the fault scarp. [ABSTRACT FROM AUTHOR]

Most of the recorded seismicity in central Chile can be linked to the subduction of the Nazca plate. To the east, a much smaller fraction is observed at 0-30 km depths beneath the western Andean thrust. Paleoseismic studies evidenced the occurrence of at least two major earthquakes (M>7) over the past 17 ka, associated with the San Ramón fault (SRF): an important tectonic feature characterizing the west Andean thrust, close the Santiago metropolitan area. To better constrain the crustal seismicity in this area, the Chilean Seismological Center (CSN) extended its permanent seismic network with seven new broadband seismometers deployed around the scarp of the SRF and farther east. The improved azimuthal distribution and reduced station spacing allowed to complete the CSN catalog with more than 900 smaller magnitude earthquakes (ML<2.5) detected and located within the study region. The use of a 3D velocity model derived from P- and S-wave travel-time tomography considerably lowered the uncertainties associated with hypocentral locations. Our results show an important seismicity beneath the Principal Cordillera located at a depth of ~10 km, and a deeper seismicity (~15 km) aligned with the main Andean thrust more to the west, parallel to the scarp of the SRF. Regional stress inversion results suggest that the seismicity of the west Andean thrust is accommodating northeast-southwest compressional stress, consistent with the convergence of the Nazca plate. Based on our improved crustal seismicity, combined with observations from previous studies, we have been able to refine the scenario of an Mw 7.5 earthquake rupturing the SRF. Ground-motion prediction results show peak ground accelerations of ~0.8g close to the fault scarp. [ABSTRACT FROM AUTHOR]

In the central Andes, the Nazca plate displays large along strike variations in dip with a near horizontal subduction angle between 28 and 32°S referred to the Pampean flat slab segment. The upper plate above the Pampean flat slab has high rates of crustal seismicity and active basement cored uplifts. The SIEMBRA experiment, a 43-broad-band-seismic-station array was deployed to better characterize the Pampean flat slab region around 31°S. In this study, we explore the lithospheric structure above the flat slab as a whole and its relation to seismicity. We use the SIEMBRA data to perform a joint inversion of teleseismic receiver functions and Rayleigh wave phase velocity dispersion to constrain the shear wave velocity variations in the lithosphere. Our joint inversion results show: (1) the presence of several upper-plate mid-crustal discontinuities and their lateral extent that are probably related to the terrane accretion history; (2) zones of high shear wave velocity in the upper-plate lower crust associated with a weak Moho signal consistent with the hypothesis of partial eclogitization in the lower crust; (3) the presence of low shear-wave velocities at ~100 km depth interpreted as the subducting oceanic crust. Finally, in order to investigate the relation of the lithospheric structure to seismicity, we determine an optimal velocity-depth model based on the joint inversion results and use it to perform regional moment tensor inversions (SMTI) of crustal and slab earthquakes. The SMTI for 18 earthquakes that occurred between 2007 and 2009 in the flat slab region below Argentina, indicates systematically shallower focal depths for slab earthquakes (compared with inversions using previous velocity models). This suggests that the slab seismicity is concentrated mostly between 90 and 110 km depths within the subducting Nazca plate's oceanic crust and likely related to dehydration. In addition, the slab earthquakes exhibit extensional focal mechanisms suggesting new faulting at the edges of the flat portion of the slab. SMTI solutions for upper-plate crustal earthquakes match well the geological observations of reactivated structures and agree with crustal shortening. Our new constraints on flat slab structure can aid earthquake characterization for regional seismic hazard assessment and efforts to help understand the mechanisms for slab flattening in the central Andes. [ABSTRACT FROM AUTHOR]

The Magallanes-Fagnano fault (at ~51-56°S) is a continental NW-SE transform fault zone that marks the boundary between the South American Plate (north) and the Scotia Plate (South). In 1949, two crustal seismic events of estimated magnitude M > 7 were located in this area using teleseismic records. Although, this region is scarcely populated, such shallow seismicity could represent a considerable threat to the remote urban centers in this sector. The characterization of the local seismicity always lacked good quality records and permanent instrumentation. In the recent past years, permanent seismic stations (CSN and INPRES-CTBTO) were deployed in the Tierra del Fuego Island and Southern Patagonia, providing with continuous time, broadband records of waveform data. In order to obtain better insights about the local seismicity associated with the Magallanes-Fagnano fault, we jointly inverted teleseismic receiver functions with Rayleigh-wave dispersion data to build a calibrated velocity model for the study region. In parallel, we analyzed the continuous waveforms to automatically detect local earthquakes and locate them using our regional velocity model. Our results show: 1) Shallow seismic events distributed along the Magallanes-Fagnano fault. 2) Diffuse seismicity characterizing the South American crust, north of 53°S. Our observations suggest two mechanisms of crustal deformation. In the Tierra del Fuego Island the seismicity is mostly associated with the Magallanes-Fagnano fault system. In the Southern Patagonia, along the Patagonian Andes the seismicity would be mostly due to isostatic rebound following the retreat of the Patagonian Ice sheet. • A new regional velocity model for the Southern Patagonia and Tierra del Fuego. • New hypocenter locations from local seismicity records. • The Magallanes-Fagnano fault is the main mechanism of crustal deformation in Tierra del Fuego. • Isostatic rebound may be the main mechanism of crustal deformation in the Southern Andes. [ABSTRACT FROM AUTHOR]