“Western Mediterranean” (WM) broad band network: permanent seismic monitoring at the Ibero-Maghrebian region

The “Ibero-Maghrebian” region, a western part of the Eurasia-Africa plate boundary, is a broad deformation area without a well defined plate boundary line, comprising the Southern part of the Iberian Peninsula, the Northwestern part of Africa, the Gulf of Cadiz and the Alboran Sea, with a convergence between Iberia and Africa at some mm/year (Nuvel-1A model, DeMets et al, 1994) in a NNW-SSE direction, and a dif- ferential motion at Alboran Sea area (Buforn et al, 1995). Although seismic activity is characterized by a low to moderate magnitude and shallow depth earthquakes, big events are also well documented, among others: Lisbon earthquake (1755, I0= X), Gulf of Cadiz (1969, Ms= 8.1), or, more recently Bourmedes (2003, Mw= 7.1), some them with an associated tsunami, like Lisbon or Bourmedes earthquakes. An interme- diate seismic activity is also clearly registered from Gulf of Cadiz to mid Alboran Sea (Martin Davila and Pazos, 2003), and very deep earthquakes (h> 650 km) have been registered at Granada basin (1954, M= 7.0). In order to better study the seismic characteristics of this area, from 1996 on, ROA and the University Complutense of Madrid (UCM), with the collaboration of Geo- forschungZentrum of Potsdam (GFZ), have deployed a broad band seismic net with stations located at Southern Spain and Spanish possessions located Northern Africa, mainly surrounding the Alboran Sea, complementing the previously installed ROA SP and LP stations. This net has been named as “Western Mediterranean net” (WM FDSN code). As net was evolving, new Institutes and stations joined WM net, such a way at present is formed by ten stations located at: San Fernando (SFS), Málaga (MALA), Cartagena (CART), and Evora (UEVO, University of Evora, Portugal) at Iberian peninsula, Mahón (MAHO) at Minorca island, three stations at Melilla (MELI), Peñón de Vélez-Gomera (PVLZ), and Ceuta (CEU) at Spanish villages located Northern Africa in South Alboran Sea area, and Averroes (AVE) and Ifrane (IFR) installed at Morocco mainland as a joined effort among ISRABAT (Institut Scientifique, Univer- sité Mohammed V), ROA and UCM. Most stations are collocated with permanent geodetic GPS stations (Gárate et al, 2004). Next future plans include the installation of five stations by U. Evora at Southern Portugal area, a new station at Morocco by ISRABAT, ROA and UCM and also a new station at Oran (Algeria), as a collaboration among Université d’Oran (Algeria), ROA and UCM. All them will be associated to WM net. All WM network stations include Streckeisen STS-2 sensor, a Quanterra or Earth Data digitizer, and a SeiscomP process system (Heinloo, 2004), and all them are available in real time via phone modem or Internet, except PVLZ and CEU, which will be in short. In this work, partly funded by the Spanish Ministry of Education and Science (MEC) through the project REN2006-10311-C03-01/02 (RISTE), we will present the present status, the next future plans and some related activities of WM net

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

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

Finite-frequency Rayleigh wave tomography of the western Mediterranean: Mapping its lithospheric structure

We present a 3-D shear wave velocity model for the crust and upper mantle of the western Mediterranean from Rayleigh wave tomography. We analyzed the fundamental mode in the 20¿167 s period band (6.0¿50.0 mHz) from earthquakes recorded by a number of temporary and permanent seismograph arrays. Using the two-plane wave method, we obtained phase velocity dispersion curves that were inverted for an isotropic Vs model that extends from the southern Iberian Massif, across the Gibraltar Arc and the Atlas mountains to the Saharan Craton. The area of the western Mediterranean that we have studied has been the site of complex subduction, slab rollback, and simultaneous compression and extension during African-European convergence since the Oligocene. The shear velocity model shows high velocities beneath the Rif from 65 km depth and beneath the Granada Basin from ¿¿70 km depth that extend beneath the Alboran Domain to more than 250 km depth, which we interpret as a near-vertical slab dangling from beneath the western Alboran Sea. The slab appears to be attached to the crust beneath the Rif and possibly beneath the Granada Basin and Sierra Nevada where low shear velocities (3.8 km/s) are mapped to >55 km depth. The attached slab is pulling down the Gibraltar Arc crust, thickening it, and removing the continental margin lithospheric mantle beneath both Iberia and Morocco as it descends into the deeper mantle. Thin lithosphere is indicated by very low upper mantle velocities beneath the Alboran Sea, above and east of the dangling slab and beneath the Cenozoic volcanics.This research was funded by the U.S. National Science Foundation EAR-0808939. The deployment of the IberArray broadband seismic network is part of the CONSOLIDER CSD2006-00041 (Geosciences in Iberia: Integrated studies on Topography and 4-D Evolution) grant from the Spanish Ministry of Science and Innovation. Additional funding was provided by the Spanish ministry under grants CGL2010-17280 and by Generalitat de Catalunya under grant 2009 SGR 6.Peer Reviewe

Coda wave attenuation tomography in Northern Morocco

In this study we focused on seismic attenuation (1/ Q) tomography in Northern Morocco. For this purpose, two different models are employed: The Single Backscattering model hypothesis of Aki and Chouet (1975) to calculate values of Coda Q (Q) and the Back-projection technique of Xie and Mitchell (1990) to estimate lateral variation in Q via a tomographic inversion. For this investigation, the Coda Q method is applied to a number of 94 local earthquakes with a magnitude between Ml=0.7 and Ml=4. The digital seismograms of these earthquakes were recorded during the year 2008 by both local temporary and permanent broadband seismic station network deployed in Northern of Morocco. The Q quality factor values have been computed at central frequencies 0.75, 1.5, 3, 6 and 12 Hz. The lapse time windows are restricted to 30s in order to sample the earth's crust only. The Q results indicate that strong frequency dependence follow a power law for the entire area. The preliminary results of seismic Coda Q attenuation tomography shows a dependence at each frequency band, between seismic attenuation and the geology structure units in the study area, especially in the region of Al Hoceima and the eastern part of the Rif which are characterized by high attenuation values due to active faults area, while low attenuation values are seen in the west and the south of the Rif in high frequencies.Peer Reviewe

FROM THE ATLAS TO THE RIF A CRUSTAL SEISMIC IMAGE ACROSS MOROCCO

The velocity structure of the crust and the geometry of the Moho across Morocco has been the main target of two recently acquired wide-angle seismic reflection transects. One is the SIMA experiment which provided seismic constraints beneath the Atlas Mountains and the second has been the RIFSEIS experiment which sampled the RIF orogen. Jointly these controlled source wide-angle seismic reflection data results in an almost 700 km, seismic profile going from the the Sahara craton across the High and Middle Atlas and Rif Mountain till the Gibraltar-Arc (Alboran). Current work on the interpretation of the seismic data-set is based on forward modeling, ray-tracing, as well as low fold wide-angle stacking. The data has resulted in a detailed crustal structure and velocity model for the Atlas Mountains and a 700 km transect revealing the irregular topography of the Moho beneath these two mountain orogens. Results indicate that the High Atlas features a moderate crustal thickness and that shortening is resolved at depth through a crustal root where the Saharan crust under-thrusts below the Moroccan crust, defining a lower crust imbrication which locally places the Moho boundary at, approximately, 40 km depth. The P-wave velocity model is characterized, in averaged, by relatively low velocities. These low deep crustal velocities together with other geophysical observables such as: conductivity estimates derived from Mt measurements; moderate Bouguer gravity anomaly; surface exposures of recent alkaline volcanics; lead the interpretation to propose that partial melts are currently emplaced in the deep crustal levels and in the upper mantle. The Moho discontinuity defines a crust which is in average relatively thin beneath the Atlas which is almost a 4000 m high orogenic belt. The resulting model supports existence of mantle upwelling as a possible mechanism that contributes, significantly, to maintain the High Atlas topographyPeer Reviewe

The contribution of the seismic component of Topo-Iberia to the imaging of the deep structure of the Iberian Peninsula and North Morocco

Topo-Iberia has been a large-scale Spanish project running from 2007 to 2013 that integrated more than 150 researchers on Earth Sciences. One of its key assets was the management of an observatory platform, named IberArray, aimed to provide new geophysical datasets (seismic, GPS, MT) to constrain the structure of Iberia with unprecedented resolution. The IberArray seismic pool was composed by 70+ BB stations, covering the study area in 3 deployments with a site-density of 60km x 60km. The data base holds ~300 sites, including the permanent networks in the area. Hence it forms a unique seismic database in Europe that allows for multiple analyses to constrain the complex geodinamics of the Western Mediterranean. A summary of new results coming from different techniques is presented here. The SKS splitting analysis has provided a spectacular image of the rotation of the fast velocity direction along the Gibraltar Arc. In central and northern Iberia, the fast polarization directions are close to EW, consistently with global mantle flow models considering contributions of surface plate motion, density variations and net lithosphere rotation. Those results suggest an asthenospheric origin of the observed anisotropy related to present-day mantle flow. Receiver functions have revealed the crustal thickness variations beneath the Atlas, Rif and southern Iberia, evidencing a relevant crustal root beneath the Rif, in agreement with recent, high- density active seismic experiments. The Variscan Iberian massif shows a flat Moho discontinuity, while the areas reworked in the Alpine orogeny show a slightly thicker crust. Beneath N Iberia, the imbrication of the Iberian and Eurasian crusts results in complex receiver functions. Depths exceeding 45 km are observed along the Pyrenean range, while the crust thins to values of 26-28 km close to the Atlantic coasts. The geometry of the 410-km and 660-km discontinuities has been investigated using novel cross-correlation/stacking techniques. Ambient noise tomography allows to identify the main sedimentary basins and to discriminate between the Variscan and the Alpine reworked areas. Local body-wave tomography in North Morocco has improved the location of the small magnitude events on the area and the details of the crustal structure. Teleseismic tomography has confirmed, using an independent data set, the presence of a high-velocity slab beneath the Gibraltar Arc.Peer Reviewe

The Crust beneath Morocco: From the surface topography to the upper mantle a 700 km long seismic section across Morocco.

The most characteristic topographic features of Morocco are the Atlas Mountains and the Rif Coordillera. These two orogenic belts are the response of different geodynamic processes acting at lithospheric scale caused by a complex plate interaction. Both are located within the diffuse plate boundary zone separating Africa and Europe. The boundary zone is characterized by a relatively broad zone of deformation that includes mountain chains in southern Iberia, the Betics and in Morocco, the Rif Cordillera, separated by the Alboran basin. The zone delineates an arcuate arc system known as the Gibraltar arc. The area is characterized by a relatively large amount of earthquake activity at various depths and with a broad spectra of focal mechanisms. Within the last decade a large international effort have been devoted to the area. The topic has fostered a strong collaborations between Spanish and international research teams form Europe and USA. Key multi-seismic projects have been developed that aim to constrain the structure, composition and tectonic scenario from south of the Atlas to the Betics, across the Rif cordillera and the Alboran basin. The multidisciplinary research program included: natural source (earthquakes) recording with temporal deployments of broad band (BB) instrumentation and, controlled source seismic acquisition experiments where, spatially dense recording of wide-angle seismic reflection shot gathers were acquired. The natural source experiments consisted on a transect from Merzouga across the Gibraltar Arc and into the Iberian Peninsula (until south of Toledo) and, a nearly regular grid of BB. The controlled source data-sets were able to constrain the crustal structure and provide seismic P-wave propagation velocity models from the coast across the Rif and the Atlas. From south to north the crust features a relatively moderate crustal root beneath the Middle Atlas which can reach 40 km clearly differing from the 35 km thickness value observed at both sides of this root. Travel time inversion results position the crustal root just south of the High Atlas defining a thrusted mantle wedge and, also a limited crustal imbrication is suggested in the Middle Atlas. The most surprising feature is a prominent and unexpected crustal root (over 50 km) located beneath the external Rif and identified by both the wide-angle data and receiver function studies. To the east of this feature the crust thins rapidly by 20 km across the Nekkor fault zone, suggested to be related to the sharp change in crustal thickness. On shore-offshore recording of marine shots reveal further complexities in the transition to the Alboran basin. The low values of the Bouguer gravity anomalies beneath the Rif Cordillera are consistent with the crustal models derived from the new seismic data. The detailed knowledge on the crustal structure achieved by this high resolution imaging geophysical techniques is an asset to evaluate the earthquake and potential tsunami hazard for the coasts of North Africa and western Europe.This work has been primarily funded by the Spanish MEC project CGL2007–63889. Additional funding was provided by projects CGL2010–15416, CSD2006-00041, and CGL2009–09727 (Spain), CGL2008–03474-E, 07- TOPO_EUROPE_FP-006 (ESF Eurocores) and EAR-0808939 (US, NSF).Peer Reviewe

From the Surface Topography to the Upper Mantle, Seismic constraints on the Crustal structure Across Morocco

The most characteristic topographic features of Morocco are the Atlas Mountains and the Rif Coordillera. These two orogenic belts are the response of different geodynamic processes acting at lithospheric scale caused by a unique driver, the collision between two tectonic plates. Both are located within the diffuse plate boundary zone separating Africa and Europe. The boundary zone is characterized by a relatively broad zone of deformation that includes Mountain chains in southern Iberia, the Betics and the Rif cordillera in Morocco. The zone delineates the arcuate arc system of Gribraltar. Within the last decade a large international effort have been devoted to the area mostly leaded by Spanish groups with the collaboration of international research teams (including scientist form Europe and USA). Key multi-seismic projects have been developed that aim to constrain the structure, composition and tectonic scenario from south of the Atlas to the Betics, across the Rif cordillera and the Alboran basin. The multidisciplinary research program includes: natural source (earthquakes) recording with temporal deployments of broad band (BB) instrumentation and, controlled source seismic acquisition experiments where, spatially dense recording of wide-angle seismic reflection shot gathers were acquired. The passive experiments consisted on: a transect from Merzouga across the Gibraltar arc and into the Iberian peninsula (untill south of Toledo); a nearly regular grid of BB which was achieve by multiple deployments of a number of BB. The controlled source datasets were able to constrain the crustal structure and provide seismic P-wave propagation velocity models from the coast across the Rif and the Atlas. Travel-time inversion of the controlled source seismic data across the Atlas constraints a crustal root to the south of the High Atlas, and reveals mantle wedge. A limited crustal imbrication also appears in the Middle Atlas. The crustal thickness, does not exceeded 40 km in the root zone and is less than 35 km elsewhere. Approaching the Rif, the controlled source data reveals a prominent crustal root over 50 km depth which is located where the topography does not exceed 1400 m. These features indicate that complex structure and processes beneath the crust play a key role in supporting the particular geometry of the surface topography of this part of the western Mediterranean. On one hand the Atlas is being supported by the mantle, on the other the abrupt change in crustal thickness at the Nekkor fault and the deep Rif crustal root can be attributed to interaction of the subducting Alboran slab with the North African Neo-Tethys passive margin.Peer Reviewe