The offshore Castor Underground Gas Storage (UGS) project had to be halted after gas injection triggered three M4 earthquakes, each larger than any ever induced by UGS. The mechanisms that induced seismicity in the crystalline basement at 5–10 km depth after gas injection at 1.7 km depth remain unknown. Here, we propose a combination of mechanisms to explain the observed seismicity. First, the critically stressed Amposta fault, bounding the storage formation, crept by the superposition of well‐known overpressure effects and buoyancy of the relatively light injected gas. This aseismic slip brought an unmapped critically stressed fault in the hydraulically disconnected crystalline basement to failure. We attribute the delay between induced earthquakes to the pressure drop associated to expansion of areas where earthquakes slips cause further instabilities. Earthquakes occur only after these pressure drops have dissipated. Understanding triggering mechanisms is key to forecast induced seismicity and successfully design deep underground operations. ; The authors would like to acknowledge Álvaro González for sharing the catalogs that were used in Cesca et al. (2014). Funding: Víctor Vilarrasa acknowledges funding from the European Research Council (ERC) under the European Union's Horizon 2020 Research and Innovation Program through the Starting Grant GEoREST (www.georest.eu) (grant agreement No. 801809). Antonio Villaseñor acknowledges funding from Spanish Ministry of Science and Innovation grant CGL2017‐88864‐R. IDAEA‐CSIC is a Center of Excellence Severo Ochoa (Spanish Ministry of Science and Innovation, Project CEX2018‐000794‐S). ICM‐CSIC is a Center of Excellence Severo Ochoa (Spanish Ministry of Science and Innovation, Project CEX2019‐000928‐S). ; Peer reviewed
Horizontal-to-Vertical Spectral Ratios (HVSR) and Rayleigh group velocity dispersion curves (DC) can be used to estimate the shallow S-wave velocity (VS) structure. Knowing the VS structure is important for geophysical data interpretation either in order to better constrain data inversions for P-wave velocity (VP) structures such as travel time tomography or full waveform inversions or to directly study the VS structure for geo-engineering purposes (e.g., ground motion prediction). The joint inversion of HVSR and dispersion data for 1D VS structure allows characterising the uppermost crust and near surface, where the HVSR data (0.03 to 10s) are most sensitive while the dispersion data (1 to 30s) constrain the deeper model which would, otherwise, add complexity to the HVSR data inversion and adversely affect its convergence. During a large-scale experiment, 197 three-component short-period stations, 41 broad band instruments and 190 geophones were continuously operated for 6 months (April to October 2017) covering an area of approximately 1500km2 with a site spacing of approximately 1 to 3km. Joint inversion of HVSR and DC allowed estimating VS and, to some extent density, down to depths of around 1000m. Broadband and short period instruments performed statistically better than geophone nodes due to the latter's gap in sensitivity between HVSR and DC. It may be possible to use HVSR data in a joint inversion with DC, increasing resolution for the shallower layers and/or alleviating the absence of short period DC data, which may be harder to obtain. By including HVSR to DC inversions, confidence improvements of two to three times for layers above 300m were achieved. Furthermore, HVSR/DC joint inversion may be useful to generate initial models for 3D tomographic inversions in large scale deployments. Lastly, the joint inversion of HVSR and DC data can be sensitive to density but this sensitivity is situational and depends strongly on the other inversion parameters, namely VS and VP. Density estimates from a HVSR/DC joint inversion should be treated with care, while some subsurface structures may be sensitive, others are clearly not. Inclusion of gravity inversion to HVSR/DC joint inversion may be possible and prove useful ; This work was funded by TOTAL under the framework of the Orogen project, with funding from the Spanish government through the 'Severo Ochoa Centre of Excellence' accreditation (CEX2019-000928-S) and the research team RNM-194 of Junta de Andalucía (Spain). ; Peer reviewed
Special issue Orogen lifecycle: learnings and perspectives from Pyrenees, Western Mediterranean and analogues.-- 22 pages, 17 figures, supplementary material http://www.bsgf.fr/10.1051/bsgf/2021039/olm ; [EN] We present a 3-D shear wave velocity model of the Mauléon and Arzacq Basins from the surface down to 10 km depth, inverted from phase velocity maps at periods between 2 and 9 s. These phase velocity maps were obtained by analyzing coherent surface wave fronts extracted from ambient seismic noise recorded by the large-N Maupasacq seismic array with a matched filtering approach. This new model is in good agreement with a local earthquake tomography study performed on the same acquisition dataset. Our passive imaging models reveal the upper crustal architecture of the Mauléon and Arzacq Basins, with new details on the basement and its relationship with the overlying sedimentary cover. Combining these new tomographic images with surface and subsurface geological information allows us to trace major orogenic structures from the surface down to the basement. In the basin, the models image the first-order basin architecture with a kilometric resolution. At depth, high velocity anomalies suggest the presence of dense deep crustal and mantle rocks in the hanging wall of north-vergent Pyrenean Thrusts. These high velocity anomalies spatially coincide with a positive gravity anomaly in the western Mauléon Basin. In addition, our models reveal major changes from the Chaînons Béarnais to the western Mauléon Basin across a set of orogen-perpendicular structures, the Saison and the Barlanès transfer zones. These changes reflect the along-strike variation of the orogenic evolution that led to the preservation of the former rifted domain and its underlying mantle in the orogenic wedge of the Western Pyrenees. We discuss the implications of these results for the 3-D architecture of the Mauléon Basin and its underlying basement ; [FR] Nous présentons un modèle 3-D de vitesse des ondes de cisaillement des bassins de Mauléon et d'Arzacq de la surface jusqu'à 10 km de profondeur inversé à partir de cartes de vitesse de phase pour des périodes entre 2 et 9 s. Ces cartes ont été obtenues à partir de l'analyse de fronts d'onde de surface cohérents extraits du bruit sismique ambiant enregistré par le réseau Maupasacq par filtrage adaptatif. Ce nouveau modèle est en bon accord avec la tomographie locale réalisée sur ce même jeu de données. Nos nouvelles images tomographiques révèlent l'architecture supra-crustale des bassins de Mauléon et d'Arzacq, avec des informations nouvelles sur la nature du socle et sa relation à la couverture sédimentaire. En combinant ces nouvelles images tomographiques aux informations géologiques, il est possible de tracer les principales structures orogéniques de la surface jusqu'au socle des bassins. Dans le bassin, les modèles nous fournissent une image de premier ordre des plis et chevauchements à l'échelle kilométrique. En profondeur, les anomalies rapides suggèrent la présence de roches de la croûte inférieure et du manteau dans le toit des chevauchements pyrénéens de pendage nord. Ces anomalies rapides coïncident spatialement avec l'anomalie gravimétrique positive dans la partie ouest du bassin de Mauléon. Nos modèles tomographiques documentent également des changements de structures majeurs entre les Chaînons Béarnais et la partie ouest du bassin de Mauléon à travers des structures perpendiculaires à l'axe de la chaîne, représentées par les structures transverses du Saison et du Barlanès. Ce changement structural reflète les variations latérales de l'évolution orogénique qui a conduit à la préservation des domaines de rift hyper-étirés et du manteau sous-jacent dans le prisme orogénique. Nous discutons les implications de ces résultats concernant l'architecture 3-D du bassin de Mauléon et du socle sous-jacent ; This work was supported by OROGEN, a tripartite research project between the CNRS, TOTAL and BRGM, and by the ANR AAPG program (project CLEARVIEW, ANR-17-CE23-0022). AV received funding from the Spanish government through the "Severo Ochoa Centre of Excellence" accreditation (CEX2019-000928-S) ; Peer reviewed
10 figures, supplemental material https://doi.org/10.1785/0220210049.-- Data and resources: CANALINK-Canarias submarine link S.L. (http://www.canalink.tel/; last accessed February 2021) provided the information on the cable structure. EMODnet Bathymetry Consortium (2020) provided bathymetric data.-- This is a contribution of the Barcelona Center for Subsurface Imaging that is a Grup de Recerca de la Generalitat de Catalunya ; In this study, we investigate 70 days of distributed acoustic sensing (DAS) recordings in the Canary Islands using an undersea fiber‐optic telecommunication cable that links the islands of Tenerife and Gran Canaria. Two DAS interrogators connected to both ends of the cable turned the fiber into an array of 11,968 strain sensors covering a total length of ∼120 km. We present the details of the experiment, noise analysis, and examples of recorded signals. Seismic ambient noise levels assessment indicates poor local coupling of the cable due to the irregular bathymetry that results in high‐amplitude acoustic oscillations in some channels. The DAS array recorded several types of nonseismic (vehicles, surface gravity waves, ships) and seismic signals. Local and regional earthquakes were detected with magnitudes mbLg≥2. Surface waves from teleseismic events at a distance of ∼3000 km were also identified in the strain recordings. Here, we report the first observations with DAS of hydroacoustic T waves generated by oceanic earthquakes located at the Central Mid‐Atlantic Ridge and the Cape St. Vincent region. Events had magnitudes from Mw 4.2 to 6.9, and the hydroacoustic waves were recorded at epicentral distances from 780 to 3400 km. Our findings show that submarine fiber‐optic cables can effectively be used to assess the seismic activity in remote oceanic areas ; With funding from the Spanish government through the "Severo Ochoa Centre of Excellence" accreditation (CEX2019-000928-S), Comunidad de Madrid and FEDER Program under grant SINFOTON2-CM:P2018/NMT-4326, the European Research Council (OCEAN-DAS: ERC-2019-POC-875302), the Spanish Government under projects RTI2018-097957-B-C31, and RTI2018-097957-B-C33. M.R.F.R. and H.F.M. acknowledge financial support from the Spanish MICINN under contract no. IJC2018-035684-I, and IJCI-2017-33856, respectively ; Peer reviewed
The dimensions, the geographical position and the complex geological history of the Iberian Peninsula makes it a unique and singular target to study its crustal and upper mantle structure and geodynamical evolution using geophysical methods. The lithospheric structure beneath Iberia has been investigated since the 1970's using deep multichannel seismic reflection and refraction/wide-angle reflection profiling. Gravimetric and magnetic data were acquired following the deployment of permanent and temporary stations, mostly during the 1990's. Beginning in the late 1990's, the progressive use of Global Navigation Satellite Systems (GNSS) instruments contributed to monitor the present-day motions. During the last decades, numerous geological and geophysical surveys have investigated the Iberian lithosphere and upper mantle in the onshore and offshore regions, the most recent ones related to the TopoIberia project. The approach developed in this contribution is twofold. Firstly, we summarize the available geophysical information over Iberia, from focusing on the upper crust to the lithosphere-asthenosphere boundary and the transition zone marking the bottom of the upper mantle. Results of GNSS data, potential fields, controlled source seismic profiles, magnetotelluric data, body and surface wave tomography, receiver functions and 2D and 3D lithospheric modeling are reviewed and compared. Secondly, we focus on the areas of greater geodynamic interest and the regions where inconsistencies within the geophysical results, or contradictions in their tectonic interpretation exist, identifying the major questions that are still under debate. Besides shedding light to the state of knowledge and pointing out present-day research challenges, this review provides a tool for the integration of the diverse geophysical datasets with the surface geology and geodynamical processes that are interpreted to have built the complex geology of the Iberian Peninsula. ; The authors acknowledge funding from the Generalitat de Catalunya, grant/awards number AGAUR 2017SGR1022, and AGAUR 2017SGR847, the Spanish Ministry of Science, Innovation, and Universities grant numbers RTI2018-095594-B-I00, PGC2018-095154-B-100 and PGC2018-094227-B-I00 and the Spanish Ministry of Economy and Competitiveness grant numbers CGL2017-84901-C2 and PIE-CSIC-201830E039. IP is funded by the Spanish Ministry of Science, Innovation, and Universities and University of Salamanca grant BEAGAL18/00090. AV acknowledges funding from the Spanish government through the 'Severo Ochoa Centre of Excellence' accreditation (CEX2019-000928-S). ; Peer reviewed
