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Detailed imaging of subsurface crustal structures requires inverse modeling from massive amounts of waveform data, including regional and/or local seismic events of lower magnitudes and high frequencies, aspects not conventionally used in Full-Waveform Inversion (FWI). The use of higher frequency signals notably boosts spatial model resolution but increases the computational cost of inversion, affordable only with HPC infrastructures. Preliminary tomographic models for the Eastern Pyrenees region have been constrained through a visco-elastic FWI using seismic data. Three-component recordings were provided by over 120 stations from a high-density seismic deployment. To optimize coverage, in addition to the far-field waveforms from M>4.5 events, data from 23 smaller magnitude earthquakes were included from a temporary broad-band seismic network deployed between September 2019 and November 2020. Moment tensor solutions of these additional seismic data were determined, and the seismograms were bandpass filtered. Through this computationally intensive workflow, multiple iterations resulted in a better fit with the observed seismograms, and point to improved spatial resolution of shallow structures. This contribution, aims to disclose and discuss the difficulties and challenges faced when using relatively high frequency seismic recordings in visco-elastic FWI applications, as well as addressing the benefits of the resulting models. Improved tomographic models could be the basis for more accurate waveform modeling and we hope that they will contribute to improve ground-motion simulations on a regional scale. This research is funded and contributes to the objectives of the Horizon Europe Project DT-GEO: A Digital Twin for GEOphysical extremes (ID 101058129)., The 28th IUGG General Assembly (IUGG2023) (Berlin 2023)

In slow-moving fault systems, where the observation time window is very small compared with the long seismic cycles and the seismicity rate, the variability of the seismic hazard can be high. To assess this variability, this study develops a PSHA (Probabilistic Seismic Hazard Assessment) using synthetic seismicity. The selected site is the Eastern Betics Shear Zone (EBSZ), located at the SE of the Iberian Peninsula, the most active seismic region of Spain. The synthetic seismicity consists of a million-year catalog generated with the physics-based earthquake simulator RSQSim. This is based on a complex fault model containing slip rates, rakes, and friction properties of major faults. From the synthetic earthquake catalog, we obtained 20 sub-catalogs with random temporal distribution. The duration of each catalog was selected to replicate the historical and instrumental record used in the evaluation of the seismic hazard in the EBSZ, i.e. a thousand years. The results show a significant variability in the maximum magnitude, the slope of the Frequency Magnitude Distribution (FMD) and the annual rate of earthquakes for the different sub-catalogs. This variation has a direct effect on the seismic hazard curves calculated using the PSHA software R-CRISIS. Each return period has associated several PGA (Peak Ground Acceleration) for the different scenarios, showing the dependence of the estimated seismic hazard on the time window analysed., The 28th IUGG General Assembly (IUGG2023) (Berlin 2023)

On 2022-02-01, the mainshock of a seismic sequence that started in 2021 struck between l'Alt Urgell (Catalonia) and Andorra la Vella (Andorra) in the Central Pyrenees area. This magnitude (Mw) 4.0 earthquake, which alerted the population of the vicinities, was followed by low-magnitude aftershocks. In this work, we aim to obtain a new catalog through machine-learning procedures. We used the easyQuake python package, which allowed us to build new catalogs after detecting earthquake arrival times within seismograms recorded by several stations in the study area. Using different pickers, we obtain those machine-learning catalogs and compare them against data from regional agencies. We observed that, depending on the deep-learning model, the new catalog matched a high percentage of the original dataset recorded by the agencies. Besides that, we noted that some earthquakes passed undetected by the routine processing of these agencies. Then, using the new arrival times, we relocated the hypocenters using a 1D velocity model of the area following a non-linear location inversion approach. Our results show low uncertainties, which suggest that the arrival times detected by the machine-learning software are accurate enough to obtain constrained hypocenters. We conclude that this procedure could be advantageous for agencies and organizations that run a regional network in addition to the standard routine procedure carried out by expert scientists and technicians., The 28th IUGG General Assembly (IUGG2023) (Berlin 2023)

[EN]: To produce seismic hazard assessments to the current state-of-the-art, it is essential to characterize the active faults in terms of geometry, interrelation and seismotectonic status. The Guadalentin Depression is the main basin within the Eastern Betic Shear Zone, which corresponds to a NE-SW tectonic corridor bounded by the Carrascoy, Alhama de Murcia and Palomares faults, from north to south. Although a number of active tectonics and paleoseismological studies have been carried out in these faults, almost nothing is known about their geometry at depth. To unveil the deep structure, geometry and upper Neogene deformation history of these faults we have carried out a high-resolution seismic reflection survey. The acquired seismic profiles will allow to improve our understanding of the deep geometry of the known active faults (up to 2 km depth), as well as to identify potential buried branches and will help to reduce the uncertainties in seismic hazard assessment., [ES]: Para producir evaluaciones del peligro sísmico actualizadas al estado del arte actual, es esencial caracterizar las fallas activas de una región en términos de geometría, interrelación y estado sismotectónico. La depresión del Guadalentín es la principal cuenca cuaternaria dentro de la Zona de Cizalla de las Béticas Orientales, la cual corresponde a un corredor tectónico con dirección NE-SW delimitado por las fallas de Carrascoy, Alhama de Murcia y Palomares, de norte a sur. Aunque varios estudios de tectónica activa y paleosismología se han centrado en estas fallas, su geometría en profundidad es bastante incierta. Para revelar la estructura profunda, la geometría y el historial de deformación desde el Neógeno superior de estas fallas, hemos llevado a cabo un estudio de sísmica de reflexión de alta resolución. Los perfiles sísmicos adquiridos permitirán mejorar nuestra comprensión de la geometría profunda de las fallas activas conocidas (hasta 2 km de profundidad), así como identificar posibles ramas ciegas, y ayudarán a reducir las incertidumbres en los cálculos de peligrosidad sísmica, Hector Perea es un investigador postdoctoral del programa "Atracción de Talento" en la Universidad Complutense de Madrid financiado por la Comunidad de Madrid (2018-T1/AMB-11039). Paula Herrero y Júlia Molins han estado contratadas por el proyecto UNrIDDLE (2018-T1/AMB-11039). Juan Alcalde ha recibido financiación de las ayudas IJC2018-036074-I financiadas por MCIN/AEI/10.13039/501100011033.