Tomographic three-dimensional seismic velocity structure of the SW Ibero-Maghrebian region

The present tomographic study focuses on SW Ibero-Maghrebian region. To locate the seismic events and find the local velocity structure of epicentral area, the P and S arrivals at 42 stations located at north of Morocco, south of Portugal and Spain are used. The arrival times data used, in this study, were obtained by the “Instituto de Meteorologia” (IM, Lisbon, Portugal), the National Institute of Geophysics (CNRST, Rabat, Morocco) and the “Instituto Geografico Nacional” (IGN, Madrid, Spain) (between 12/1988 and 30/2008). The preliminary estimate of origin times and hypocentral coordinates are determined by the hypocenter 3.2 program. In this study we use a linearized inversion procedure comprising two steps: 1) finding the minimal 1-D model and simultaneous reloca- tion of hypocenters and 2) determination of local velocity structure assuming a continuous velocity field. The earth structure is represented in three dimensions by velocity at discrete points, and velocity at any intervening point is determined by linear interpolation among the surrounding eight grid points. The resolutions tests results indicate that the calculated images give near true structure for the studied region at 15, 30, 45 and 60 km depth. At 5km depth it gives near true structure in the continental region of Portugal, Spain, and Morocco. This study shows that the total crustal thickness varies from 30 to 35 km and contains low-velocity anomalies. A prominent low velocity anomaly that shows a maximum decrease in P-wave velocity of approximately 6 per cent in the Gibraltar region is observed extending down to a depth of approximately 30 km. This low velocity demarcates a small bloc located between Iberia and Nubia plates. The resulting tomographic image has a prominent high velocity anomaly that shows a maximum increase in P-wave velocity of approximately 6 per cent between 45 to 60 km depth beneath South of Portugal and the Golf of Cadiz. High-velocity anomalies could be associated with the location of deep active faults in the uplift and upper crust of South of Portugal. In the Golf of Cadiz, these anomalies could be associated with the seismogenic zone and probably more at the south with the Iberia-Nubia plate boundary

Tomography and geodynamics structure of the Ibero- Maghrebian region

The present study has two main goals: 1) use the most actual seismological data from recent earthquakes in the extended Alboran region to develop a geodynamic-structural model for the region through the application of seismic local tomography techniques; 2) modelling seismogenic sources using specific applications of analysis. The structural scheme detailed in depth will allows us to define possible structural blocks in region between north of Morocco and Alboran sea. Currently the GPS studies show local movements in northern morocco independently of the general movement of the African plate. The present tomographic study focuses on SW Ibero-Maghrebian region. The P and S arrival times at 52 stations located at north of Morocco (National Institute of Geophysics, CNRST, Rabat), south of Portugal (Instituto de Meteorologia, Lisbon) and Spain (Instituto Geografico National, Madrid) are used for the period between 12/1988 and 30/2008. We use a linearized inversion procedure to find a 3D velocity model for the studied region. The resolution tests indicate that the calculated images give near true structure for the Tanger peninsula, the Alhoceima region and southern Spain at 5km depth. At 15, 30, 45 km depth we observe a near true structure in northern Morocco, and southern Spain. At 60 and 100 km, the southern Spain and SW of Alboran Sea gives a near true structure. The resulting tomographic image shows that the total crustal thickness varies between 25 and 35 km and contains low-velocity anomalies. Is defined clearly a prominent negative P- wave velocity anomaly with a maximum decrease of approximately 6 per cent, at 15 km depth, in the northern Morocco. This low velocity demarcates a small bloc located between Iberia and African plate. This bloc is presented by a prominent high velocity anomaly that shows a maximum increase in P-wave velocity of approximately 6 per cent. The area with high velocity values could represent brittle and competent parts of the crust and lithosphere which sustain seismogenic stress where asperities along the faults could exist and probably more with the Iberia-Africa plate boundary. Strong ground motions from major earthquakes depend strongly upon the 3D seismic velocity structure of the crust. Moreover the 3D velocity model is crucial for a better comprehension of structures behavior and has important practical applications toward understanding earthquake hazard in the Ibero-Maghrebian region. In particular, we hope to contribute, with this model, for seismic risk mitigation in north of Morocco

Étude des linéaments du massif de Morin par traitement informatique d'image Landsat

Résumé en anglais inclusMaster's degre

Estimation of Coda Wave Attenuation in Northern Morocco

We studied the attenuation of coda waves and its frequency and lapse-time dependence in northern Morocco. We analysed coda waves of 66 earthquakes recorded in this region during 2008 for four lapse time windows of length 30, 40, 50, and 60 s, and at five frequency bands with central frequency in the range of 0.75–12 Hz. We determined the frequency dependent Q relation for the horizontal (NS and EW) and vertical (Z) component seismograms. We analyzed three-component broadband seismograms of 66 local earthquakes for determining coda-Q based on the single back-scattering model. The Q values show strong frequency dependence in 1.5–12 Hz that is related to high degree of heterogeneity of the medium. The lapse time dependence of Q shows that Q (Q at 1 Hz) significantly increases with lapse time that is related to the depth dependence of attenuation and hence of the level of heterogeneity of the medium. The average frequency-dependent Q(f) values are Q= (143.75 ± 1.09) f, Q= (149.12 ± 1.08) f and Q= (140.42 ± 1.81) f for the vertical, north–south and east–west components of motion, respectively. The frequency-dependent Q(f) relations are useful for evaluating source parameters (Singh et al. 2001), which are the key inputs for seismic hazard assessment of the region.Peer Reviewe