ALBERT

Description: During the TopoIberia experiment, a total of 26 seismic broadband stations were recording in northern Morocco, providing for the first time extended regional coverage for investigating structure and seismotectonics of the southern branch of the Betic-Rif arc, its foreland, and the Atlas domain. Here, we analyze P-to-S converted waves in teleseismic receiver functions to infer gross crustal properties as thickness and Vp/Vs ratio. Strong lateral variations of the crustal thickness are observed throughout the region. Crustal thicknesses vary between 22 and 44 km and display a simple geographic pattern that divides the study area into three domains: entire northwestern Morocco underlain by a thickened crust with crustal thicknesses between 35 and 44 km; northeastern Morocco affected by significant crustal thinning, with crustal thicknesses ranging from 22 to 30 km, with the shallowest Moho along the Mediterranean coast; and an extended domain of 27–34 km thick crust, farther south which includes the Atlas domain and its foreland regions. Vp/Vs ratios show normal values of ∼1.75 for most stations except for the Atlas domain, where several stations give low Vp/Vs ratios of around 1.71. The very sharp transition from thick crust in northwestern Morocco to thin crust in northeastern Morocco is attributed to regional geodynamics, possibly the realm of present-day subcrustal dynamics in the final stage of western Mediterranean subduction. Crustal thicknesses just slightly above 30 km in the southern domain are intriguing, showing that high topography in this region is not isostatically compensated at crustal level.

Description: One of the most fundamental questions in ecology is how many species inhabit the Earth. However, due to massive logistical and financial challenges and taxonomic difficulties connected to the species concept definition, the global numbers of species, including those of important and well-studied life forms such as trees, still remain largely unknown. Here, based on global ground-sourced data, we estimate the total tree species richness at global, continental, and biome levels. Our results indicate that there are ∼73,000 tree species globally, among which ∼9,000 tree species are yet to be discovered. Roughly 40% of undiscovered tree species are in South America. Moreover, almost one-third of all tree species to be discovered may be rare, with very low populations and limited spatial distribution (likely in remote tropical lowlands and mountains). These findings highlight the vulnerability of global forest biodiversity to anthropogenic changes in land use and climate, which disproportionately threaten rare species and thus, global tree richness.

Description: Closure quantities measured by very-long-baseline interferometry (VLBI) observations are independent of instrumental and propagation instabilities and antenna gain factors, but are sensitive to source structure. A new method is proposed to calculate a structure index based on the median values of closure quantities rather than the brightness distribution of a source. The results are comparable to structure indices based on imaging observations at other epochs and demonstrate the flexibility of deriving structure indices from exactly the same observations as used for geodetic analysis and without imaging analysis. A three-component model for the structure of source 3C371 is developed by model-fitting closure phases. It provides a real case of tracing how the structure effect identified by closure phases in the same observations as the delay observables affects the geodetic analysis, and investigating which geodetic parameters are corrupted to what extent by the structure effect. Using the resulting structure correction based on the three-component model of source 3C371, two solutions, with and without correcting the structure effect, are made. With corrections, the overall rms of this source is reduced by 1 ps, and the impacts of the structure effect introduced by this single source are up to 1.4 mm on station positions and up to 4.4 microarcseconds on Earth orientation parameters. This study is considered as a starting point for handling the source structure effect on geodetic VLBI from geodetic sessions themselves.

Description: Ambient seismic noise acquired in the Cerdanya Basin (Eastern Pyrenees) is used to assess the capability of different methodologies to map the geometry of a small-scale sedimentary basin. We present results based on a 1-year long broad-band deployment covering a large part of the Eastern Pyrenees and a 2-months long high-density deployment covering the basin with interstation distances around 1.5 km. The explored techniques include autocorrelations, ambient noise Rayleigh wave tomography, horizontal to vertical spectra ratio, and band-pass filtered ambient noise amplitude mapping. The basement depth estimations retrieved from each of these approaches, based on independent datasets and different implicit assumptions, are consistent, showing that the deeper part of the basin is located in its central part, reaching depths of 600-700 m close to the Têt Fault trace bounding the Cerdanya Basin to the NE. The results from the different methodologies have an overall consistency, although significant differences appear in some zones. The results show also that when high-density seismic data are available, mapping the ambient noise amplitude in a selected frequency band is a valid tool to quickly map the sedimentary 3D geometry. Besides this methodological aspect, our results help to improve the geological characterization of the Cerdanya Basin and will provide further constraints to refine the seismic risk maps of an area of relevant touristic and economic activity.

Description: 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).

Description: The eruption of the Hunga-Tonga volcano produced a high-energy explosion that was recorded globally by different scientific instruments, including broad-band seismometers. Seismic data recorded on global seismic networks are used to explore three main topics; the time evolution of the eruption, the atmospheric waves revolving around the Earth and the low-frequency, long-lasting signals recorded worldwide in the hours after the main event. From the inspection of the data we can assess that the eruption started on January 15 around 04:05 UTC, several minutes before the origin time reported in the seismic catalogs and included two large explosions at 05:30 and 08:35. As for the atmospheric waves, their energy was enough to be detected seismically during more than two complete circumambulations of the Earth, during a time span of three and a half days. The most notable seismic feature has been the low-frequency signals detected over several hours and interpreted as resulting from the excitation of Earth normal modes. To our knowledge, there are no previous examples of atmospheric-solid Earth coupling over such a long time interval and only two examples of normal mode excitation following volcanic eruptions, identified for the 1982 El Chinchón and the 1991 Mount Pinatubo eruptions.

Description: 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.

Description: The study of tsunami hazard is frequently approached from numerical modelling in a deterministic or probabilistic way. When probabilistic assessments are developed, epistemic and aleatory uncertainties are taken into account and incorporated into logic-tree and Monte Carlo approaches. Our approach is based on the use of physics-based earthquake simulators, developed in recent decades to overcome the temporal limitation of the instrumental seismic catalogue in probabilistic seismic hazard assessment (PSHA). Recent development of numerical codes based on the rate-and-state constitutive law for fault slip and frictional behaviour (RSQSim) allows also the modelling of the short-term rupture process based on a quasi-dynamic physical approximation. We have applied this model to the Eastern Betic Shear Zone, where one of the major faults of the western Mediterranean is present, the Carboneras Fault. This fault is a left-lateral transpressive structure oriented with a length of ∼150 km, most of them offshore, and a slip rate of 1.3 mm/yr. This fault has been proposed as source of the 1522 Almeria earthquake (Int. VIII-IX) possibly related to a local tsunami. We present tsunami simulations from seismic ruptures of a 1 Myr synthetic seismic catalogue generated with RSQSim, consistent on 773,893 events with magnitudes Mw 3.3 - 7.6. The Carboneras Fault has the capacity to generate locally damaging tsunamis. The frequency - magnitude distribution of the seismic catalogue departs from the classical Gutenberg-Richter potential relation showing a bell-shaped distribution for the bigger events. The inter-event times for the maximum earthquake magnitudes are between 2000 and 6000 years.