The complex tectonic interaction processes between the European and African plates at the Western Mediterranean have left marked imprints in the crustal architecture of this area, particularly ...concerning the lateral variations in crustal thicknesses. The detailed mapping of such variations is hence of large interest, as it provides a major constraint to geophysical and geodynamic modeling at different scales. Controlled-source seismic profiling and receiver functions from natural seismicity are widely considered as major tools to constrain Moho topography. We compile here the Moho depths determined from a comprehensive number of both types of seismic surveys, to end up with a new 3D Moho depth map of the Iberian Peninsula, its continental margins and North Morocco. Since the 1970s, the lithospheric structure beneath this study area has been extensively investigated using multichannel normal incidence seismic reflection and refraction/wide-angle reflection profiling. In the last few years some high-resolution surveys at sea and inland have been acquired, the latter ones involving ~1000 land stations. On the other hand, the TopoIberia-IberArray experiment has triggered the investigations on crustal and lithospheric structure using natural seismicity, providing a homogeneous spatial resolution never achieved before. The availability of good quality results from both methodologies in a common area provides an excellent opportunity to check the consistency of the Moho depth estimations. The integration of both datasets has resulted in a new, high resolution map of the crustal thickness variations. The final grid evidences large Moho topography variations, including crustal imbrication in the Pyrenean range, a large and relatively undisturbed Variscan Massif in the center of Iberia and a probable delamination process beneath the Gibraltar Arc. The crustal thicknesses vary from ~15km in continental margins up to values exceeding 50km beneath the Pyrenees or the Rif Cordillera.
•High resolution crustal thickness model of the Iberia-Western Mediterranean region•Build using all the available estimations derived from active and passive seismic experiments•Large and sharp crustal thickness variations are mapped.•An interactive 3D model allows to better explore such variations.
Summary
Stacks of ambient noise cross-correlations are more and more routinely used to extract empirical Green's functions between station pairs. The success of the cross-correlations is due to waves ...which are recorded by both stations and that constructively sum at lag times which equal their propagation time between the station pair. Stacking cross-correlograms corresponding to different time spans improves the azimuthal noise coverage and further enhances the signals. Here we show how the instantaneous phase coherence can be used for a more efficient signal extraction from ambient noise cross-correlations. The instantaneous phase coherence is obtained by analytic signal processing and can be employed through the phase cross-correlation and/or through the time-frequency domain phase-weighted stack. The phase cross-correlation is more sensitive to waveform similarity but less sensitive to strong amplitude features than the conventional cross-correlation. The time-frequency domain phase-weighted stack cleans the ambient noise cross-correlograms by attenuating incoherent noise and permits an improved signal identification. We show that both approaches are powerful tools in the recovery of signals from ambient noise data and show examples where they improve the extraction of P and Rayleigh waves by considering local and global scale applications.
Résumé
La formation d’un pharmacobézoard (PHB), concrétion formée de médicaments, est une complication peu connue des intoxications médicamenteuses aiguës. Nous rapportons un cas d’intoxication ...médicamenteuse volontaire à la clomipramine compliqué d’un PHB. L’évolution a été favorable après le retrait par voie endoscopique. La présence d’un PHB doit conduire à une prise en charge spécifique pour éviter la survenue de complications toxicologiques et mécaniques. Le diagnostic est évoqué sur des éléments anamnestiques: ingestion de médicaments à libération prolongée, caractère massif de l’intoxication, ralentissement iatrogène du transit. Une évolution clinique et toxicologique non attendue doit également faire suspecter un PHB. Plusieurs thérapeutiques ont été décrites: doses répétées de charbon activé, irrigation intestinale, retrait endoscopique et retrait chirurgical. L’endoscopie occupe une place importante dans la prise en charge diagnostique, pronostique et thérapeutique des PHB.
In the last 10-15 years, the number of high quality seismic stations monitoring the Euro-Mediterranean region has increased significantly, allowing a corresponding improvement in structural ...constraints. We present here new images of the seismic velocity and anisotropy variations in the uppermost mantle beneath this complex area, compiled from inversion of Pn and Sn phases sampling the whole region. The method of Hearn has been applied to the traveltime arrivals of the International Seismological Center catalogue for the time period 1990-2010. A total of 579 753 Pn arrivals coming from 12 377 events recorded at 1 408 stations with epicentral distances between 220 km and 1 400 km have been retained after applying standard quality criteria (maximum depth, minimum number of recordings, maximum residual values ...). Our results show significant features well correlated with surface geology and evidence the heterogeneous character of the Euro-Mediterranean lithosphere. The station terms reflect the existence of marked variations in crustal thickness, consistent with available Moho depths inferred from active seismic experiments. The highest Pn velocities are observed along a continuous band from the Po Basin to the northern Ionian Sea. Other high velocity zones include the Ligurian Basin, the Valencia Trough, the southern Alboran Sea and central part of the Algerian margin. Most significant low-velocity values are associated to orogenic belts (Betics, Pyrenees, Alps, Apennines and Calabrian Arc, Dinarides-Hellenides), and low-velocity zones are also identified beneath Sardinia and the Balearic Islands. The introduction of an anisotropic term enhances significantly the lateral continuity of the anomalies, in particular in the most active tectonic areas. Pn anisotropy shows consistent orientations subparallel to major orogenic structures, such as Betics, Apennines, Calabrian Arc and Alps. The Sn tomographic image has lower resolution but confirms independently most of the features evidenced in the Pn tomography.
We introduce a coherence measure for seismic signal enhancement through incoherent noise attenuation. Our processing tool is designed for densely spaced arrays and identifies signals by their ...coherent appearance. The approach is based on the determination of the lateral phase coherence as function of distance, time, frequency, and slowness. The coherence is derived from the local phases of neighboring stations which we obtain from analytic signals through the S‐transform. The coherence is used to attenuate incoherent components in the time‐frequency representations of the seismic records. No waveforms are averaged in our approach to maintain local amplitude information. This way we construct a data‐adaptive filter which enhances coherent signals using the frequency‐dependent and slowness‐dependent phase coherence. We explain the method and show its abilities and limitations with theoretical test data. Furthermore, we have selected an ocean bottom seismometer (OBS) record section from NW‐Spain and a teleseismic event registered at Spanish broadband stations to show the filter performance on real array data. Incoherent noise has been attenuated in all cases to enable a less ambiguous signal detection. In our last example, the filter also reveals weak conversions/reflections at the 410‐km and 660‐km discontinuities which are hardly visible in the unfiltered input data.
We present a new 3‐D shear velocity model of the western Mediterranean from the Pyrenees, Spain, to the Atlas Mountains, Morocco, and the estimated crustal and lithospheric thickness. The velocity ...model shows different crustal and lithospheric velocities for the Variscan provinces, those which have been affected by Alpine deformation, and those which are actively deforming. The Iberian Massif has detectable differences in crustal thickness that can be related to the evolution of the Variscan orogen in Iberia. Areas affected by Alpine deformation have generally lower velocities in the upper and lower crust than the Iberian Massif. Beneath the Gibraltar Strait and surrounding areas, the crustal thickness is greater than 50 km, below which a high‐velocity anomaly (>4.5 km/s) is mapped to depths greater than 200 km. We identify this as a subducted remnant of the NeoTethys plate referred to as the Alboran and western Mediterranean slab. Beneath the adjacent Betic and Rif Mountains, the Alboran slab is still attached to the base of the crust, depressing it, and ultimately delaminating the lower crust and mantle lithosphere as the slab sinks. Under the adjacent continents, the Alboran slab is surrounded by low upper mantle shear wave velocities (Vs < 4.3) that we interpret as asthenosphere that has replaced the continental margin lithosphere which was viscously removed by Alboran plate subduction. The southernmost part of the model features an anomalously thin lithosphere beneath the Atlas Mountains that could be related to lateral flow induced by the Alboran slab.
Key Points
3‐D Shear velocity model of Iberia and north Morocco
Moho and LAB map of Iberia and north Morocco
Crustal and Upper mantle imaging
We quantify, analyze, and characterize the frequency‐dependent microseismic noise recorded by worldwide distributed seismic stations. Microseismic noise is generated through the interaction of ocean ...waves. It is the strongest ambient noise, and it is observed everywhere on Earth. We introduce a new approach which permits us to detect polarized signals in the time‐frequency domain and which we use to characterize the microseismic noise. We analyze 7 years of continuous seismograms from the global GEOSCOPE network. Microseisms are dominated by Rayleigh waves, and we therefore focus on elliptically polarized signals. The polarized signals are detected in the time‐frequency domain through a degree of polarization measure. We design polarization spectra and show that microseismic noise is more strongly polarized than noise in other frequency bands. This property is used to measure the directions of the polarized noise at individual stations as a function of time and frequency. Seasonal variations are found for the back azimuths and for the number of polarized signals at many stations. We show that the back azimuth directions are robust measurements that point toward the source areas computed from ocean wave models.
Key Points
The t‐f domain detection of polarized signals and microseismic noise characterization
Seasonal variations in number of signals and back azimuthal directions
Back azimuths point to sources obtained from indendent ocean wave models
The S-transform provides a framework for data-adaptive filters which take advantage of time-frequency localized spectra. These filters basically consist in a data transformation to the time-frequency ...domain, the data-adaptive weighting of the localized spectra, and a back transformation. We illustrate that the inverse S-transform of manipulated spectra not necessarily transforms the localized signals as expected from the imposed weighting. The time localization is not directly translated and spurious signals and noise can be generated. We discuss this problem and suggest a new inverse S-transform, which may be helpful to many applications to take more advantage of the time-frequency localization.
The structure and geodynamics of the southern margin of the Bay of Biscay have been investigated from a set of 11 multichannel seismic reflection profiles, recorded also at wide angle offsets in an ...onshore-offshore network of 24 OBS/OBH and 46 land sites. This contribution focuses on the analysis of the wide-angle reflection/refraction data along representative profiles. The results document strong lateral variations of the crustal structure along the margin and provide an extensive test of the crustal models previously proposed for the northern part of the Iberian Peninsula. Offshore, the crust has a typical continental structure in the eastern tip of the bay, which disappears smoothly towards the NW to reach crustal thickness close to 10km at the edge of the studied area (~45°N, 6°W). The analysis of the velocity-depth profiles, altogether with additional information provided by the multichannel seismic data and magnetic surveys, led to the conclusion that the crust in this part of the bay should be interpreted as transitional from continental to oceanic. Typical oceanic crust has not been imaged in the investigated area. Onshore, the new results are in good agreement with previous results and document the indentation of the Bay of Biscay crust into the Iberian crust, forcing its subduction to the North. The interpreted profiles show that the extent of the southward indentation is not uniform, with an Alpine root less developed in the central and western sector of the Basque-Cantabrian Basin. N-S to NE-SW transfer structures seem to control those variations in the indentation degree.
•Crustal structure variations along the S. margin of the Bay of Biscay are documented.•The easternmost sector shows the typical pattern of continental crust.•The NW sector has a transitional character between continental and oceanic crusts.•The results suggest the presence of a partially exhumed mantle domain.
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