Magnetic Rossby waves in the solar tachocline and Rieger-type periodicities

Apart from the 11-year solar cycle, another periodicity around 155-160 days was discovered during solar cycle 21 in high energy solar flares, and its presence in sunspot areas and strong magnetic flux has been also reported. This periodicity has an elusive and enigmatic character, since it usually appears only near the maxima of solar cycles, and seems to be related with a periodic emergence of strong magnetic flux at the solar surface. Therefore, it is probably connected with the tachocline, a thin layer located near the base of the solar convection zone, where strong dynamo magnetic field is stored. We study the dynamics of Rossby waves in the tachocline in the presence of a toroidal magnetic field and latitudinal differential rotation. Our analysis shows that the magnetic Rossby waves are generally unstable and that the growth rates are sensitive to the magnetic field strength and to the latitudinal differential rotation parameters. Variation of the differential rotation and the magnetic field strength throughout the solar cycle enhance the growth rate of a particular harmonic in the upper part of the tachocline around the maximum of the solar cycle. This harmonic is symmetric with respect to the equator and has a period of 155-160 days. A rapid increase of the wave amplitude could give place to a magnetic flux emergence leading to observed periodicities in solar activity indicators related with magnetic flux.Comment: 34 pages, 5 figures, accepted in Ap

Testing the applicability of ambient noise methods in zones with different degree of anthropogenic sources

EGU2020: Sharing Geoscience Online, 4-8 may 2020The general objectives of the ¿Seismic Ambient Noise Imaging and Monitoring of Shallow Structures¿ (SANIMS) project, funded by the Spanish Ministry of Science, Research and Innovation (Ref.: RTI2018-095594-B-I00), are focused into the application and development of methods based on ambient noise seismic data recorded by dense networks to image and monitor natural and human-altered environments. To achieve this objective, temporal seismic networks have been installed since late 2019 in two very different settings; the Cerdanya Basin, a sedimentary basin located in the eastern Pyrenees and the city of Barcelona. Regarding the Cerdanya Basin, a relatively unaltered setting, a network of up to 25 broad-band stations has been installed for a period of one year. Additionally, a high resolution grid of seismic nodes will be deployed for 2 months in the central part of the basin, with interstation distances of 1.5 km. In order to constraint the uppermost crustal structure using ambient noise, vertical component recordings will be processed using the phase cross-correlation and time-frequency domain phase-weighted stacking to extract fundamental mode Rayleigh waves. The surface waves will then be used to measure inter-station group and phase velocity dispersion curves that will be inverted using the Fast Marching Surface Tomography method. Depending on data quality, we will also process the horizontal components to extract Love waves for joint inversions with Rayleigh waves to constrain radial anisotropy and/or the application of new strategies to perform attenuation tomography. Regarding areas strongly altered by human activity, we have deployed a network of 15 short-period stations within the city of Barcelona, in most of the cases installed in the basement of secondary schools, for a duration of 9-12 months. The objective of this deployment is twofold; acquire new valuable scientific data and introduce the students in an Earth Science research project. Although the Barcelona area has been investigated using MHVSR methods by different authors, the new data acquired by the SANIMS project will expand the available data and will allow to analyze the time variability of the measurements. This new dataset will also be used to analyze the applicability of the methods based on Rayleigh wave ellipticity inversion of ambient noise and earthquake data to provide S-velocity depth profiles. Under the assumption of an isotropic horizontally layered medium, the ellipticity inversion is not affected by the directivity of the diffusive noise wave field and seems therefore to be a good option to determine local S-velocity depth profiles in areas with little lateral inhomogeneities and uneven distribution of noise sources. We expect that the use of ambient noise methods will allow to map the basement and to obtain new higher resolution ambient noise tomographic images of the upper crust in the Cerdanya Basin and to better constrain the subsoil properties of Barcelona, hence improving the existing seismic hazard maps. Besides, comparing the results in both areas will allow to compare the performance of the different methods based on ambient noise in quiet and noisy areas

Constraining the crustal root geometry beneath Northern Morocco

Consistent constraints of an over-thickened crust beneath the Rif Cordillera (N. Morocco) are inferred from analyses of recently acquired seismic datasets including controlled source wide-angle reflections and receiver functions from teleseismic events. Offline arrivals of Moho-reflected phases recorded in RIFSIS project provide estimations of the crustal thicknesses in 3D. Additional constraints on the onshore-offshore transition are inferred from shots in a coeval experiment in the Alboran Sea recorded at land stations in northern Morocco. A regional crustal thickness map is computed from all these results. In parallel, we use natural seismicity data collected throughout TopoIberia and PICASSO experiments, and from a new RIFSIS deployment, to obtain receiver functions and explore the crustal thickness variations with a H-κ grid-search approach. This larger dataset provides better resolution constraints and reveals a number of abrupt crustal changes. A gridded surface is built up by interpolating the Moho depths inferred for each seismic station, then compared with the map from controlled source experiments. A remarkably consistent image is observed in both maps, derived from completely independent data and methods. Both approaches document a large crustal root, exceeding 50. km depth in the central part of the Rif, in contrast with the rather small topographic elevations. This large crustal thickness, consistent with the available Bouguer anomaly data, favors models proposing that the high velocity slab imaged by seismic tomography beneath the Alboran Sea is still attached to the lithosphere beneath the Rif, hence pulling down the lithosphere and thickening the crust. The thickened area corresponds to a quiet seismic zone located between the western Morocco arcuate seismic zone, the deep seismicity area beneath western Alboran Sea and the superficial seismicity in Alhoceima area. Therefore, the presence of a crustal root seems to play also a major role in the seismicity distribution in northern Morocco.Funding for this project has been available from the Spanish Ministry of Science and Innovation under grants: CGL2009-09727 (RIFSIS), CSD 2006-00041 (TopoIberia), CGL2007-63889 (SIMA), and CGL2008-3474 (TopoMed), and by Generalitat de Catalunya grant: 2009SGR996. We have also used data from the PICASSO project, founded by the U.S. NSF grant EAR0808939.Peer reviewe

In memoriam of Andrés Pérez-Estaún

Non

Geometría de la raíz cortical bajo la Cordillera del Rif

The seismic experiments conducted in northern Morocco during the last decade have provided detailed information on the geometry of the crust beneath the Rif Cordillera. In the mainframe of the RIFSIS project, 2D models of velocity / depth along two 300 km-long profiles oriented NS and EW, have revealed the presence of a thickened area, with depths of Moho near 50 km under the External Rif. The crustal geometry of this area has also been investigated using passive seismic data using methods based on the analysis of receiver functions and obtaining consistent values. The origin of the thickening under the External Rif is related to the slab of Alboran, which would still be connected to the lithosphere under this area, hence causing the crustal deformation.La financiación para este trabajo proviene del Ministerio de Economía mediante los proyectos CGL2009-09727 (RIFSIS), CSD 2006-00041 (TopoIberia), CGL2007-63889 (SIMA), CGL2008-3474 (TopoMed) y de la subvención de la Generalitat de Catalunya 2009SGR996. Hemos utilizado también datos del proyecto PICASSO, financiado por el proyecto EAR0808939 de la NSF de Estados UnidosPeer Reviewe

Finite-difference approximation of wave equation: a study case of the SIMA velocity model

Synthetic seismograms enable to model the theoretical seismic response of the Earth interior due to different structural features and changes in the physical properties of crust and mantle. This approximation provides a best understanding of the real seismic data recorded in field experiments. In this paper, we are showing the development and application of a new scheme based on a multi-order explicit finite-difference algorithm for acoustic waves in a 2D heterogeneous media. The results of the modeling are compared with the seismic data acquired within the SIMA project providing new insight about the internal structure of the subsurface allowing improving the velocity model obtained in previous works.Peer Reviewe