Spatial variation of coda wave attenuation in the AL Hoceima region (Earthquakes of 24 Frebruary 2004), Morocco

On 24th February 2004 a significant earthquake (Md = 6.4) occurred in the north of Morocco causing great damage in the vicinity of Al Hoceima region. This area is characterized by a complex faulting system as a result of compressional tectonic forces. Three short period stations are set in this area of interest and recordings from these stations were used in this study. In order to complete our knowledge of attenuation, 60 local earthquakes are recorded a few days after the great earthquake with magnitude Ml 2.6-5.0 to estimate seismic attenuation. For this purpose, we applied the single backscattering model of Aki & Chouet 1975 in the frequency range for 1 to 4 Hz. The study of coda waves was limited to a relatively short lapse time (10 Seconds) in order to sample the earth’s crust only .The values of Qc estimated for all the three stations show a strong frequency dependent relationship of the form Qc=Q0fn, where Q0 is Qc at 1Hz , and n represents the degree of frequency dependence , and reflect the level of crustal heterogeneities to varying degrees. The average frequency dependent attenuation relationship has been obtained which indicates that the attenuation is high in this region. Finally to conclude our work, the values of Q0 suggest that Al Hoceima area is highly heterogeneous and the n parameter indicates a meaning frequency dependence of Qc

Quality factor of seismic coda waves from earthquakes in northern Morocco

The main objective of this work is to analyze seismic attenuation (1/Qc) using a single backscattering model hypothesis of Aki and Chouet (1975). For this purpose, the recordings of 66 local earthquakes (epicentre distance < 100 km) during 2008 in Northern Morocco have been used with a magnitude (Ml ) less than 4. The Qc quality factor values have been computed at four central frequencies 0.75, 1.5, 3, 6 and 12 Hz and analyzed for two horizontal and vertical components for performing the average values. Four lapse time windows seconds from 30 to 60 duration with a difference of 10 seconds have been analyzed to study the lapse time dependence of Qc. We obtained a strong average frequency dependence follow a power law Qn=Q0fn where Q0 is Qc at 1Hz and n is the power of frequency dependent .The frequency dependent relationships obtained are Qc=(143.75±1.09)f(0.864±0.006) for the vertical component , Qc=(149.12±1.08)f(0.85±0.05) and  for the N component , and Qc=(140.42±1.81)f(0.902±0.04) for the E component. The values estimated of coda Q shows independent on the component of wave motion consistent with (Jen-Kuang Chung 2009 and Priyamvada Singh 2012 ), thus only one component sufficient to treat the attenuation in this region. The mean values of the estimated Qc of the vertical component vary from 76 (at 0.75) to 1147.6 (at 12 Hz) for 30  seconds coda window length, for 40 seconds coda window length Qc vary from 122.48 (at 0.75) to 1255 (at 12 Hz ) while for 50 seconds coda window length Qc vary from 141.4 (0.75) to 1420.8 (at 12 Hz ) . Similarly for 60 seconds coda window length Qc vary from 173.89 to 1495. The increase in Qc values with lapse time shows the depth dependence which agree with many studies. The results obtained with this model are presented and then compared to results from the literature

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

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

Subduction and volcanism in the Iberia-North Africa collision zone from tomographic images of the upper mantle

New tomographic images of the upper mantle beneath the westernmost Mediterranean suggest that the evolution of the region experienced two subduction-related episodes. First subduction of oceanic and/or extended continental lithosphere, now located mainly beneath the Betics at depths greater than 400 km, took place on a NW-SE oriented subduction zone. This was followed by a slab-tear process that initiated in the east and propagated to the west, leading to westward slab rollback and possibly lower crustal delamination. The current position of the slab tear is located approximately at 4°W, and to the west of this location the subducted lithosphere is still attached to the surface along the Gibraltar Arc. Our new P-wave velocity model is able to image the attached subducted lithosphere as a narrow high-velocity body extending to shallow depths, coinciding with the region of maximum curvature of the Gibraltar Arc, the occurrence of intermediate-depth earthquakes, and anomalously thick crust. This thick crust has a large influence in the measured teleseismic travel time residuals and therefore in the obtained P-wave tomographic model. We show that removing the effects of the thick crust significantly improves the shallow images of the slab and therefore the interpretations based on the seismic structureThis is a contribution of the Team Consolider-Ingenio 2010 TOPO-IBERIA (CSD2006-00041). Additional fundingwas provided by the SIBERIA (CGL2006-01171), RIFSIS (CGL2009-09727) and ALERTES-RIM (CGL2013-45724-C3-3-R) projects.Peer reviewe

The crustal domains of the Alboran Basin (Western Mediterranean)

The Alboran Basin in the westernmost Mediterranean hosts the orogenic boundary between the Iberian and African plates. Although numerous geophysical studies of crustal structure onshore Iberia have been carried out during the last decade, the crustal structure of the Alboran Basin has comparatively been poorly studied. We analyze crustal‐scale images of a grid of new and reprocessed multichannel seismic profiles showing the tectonic structure and variations in the reflective character of the crust of the basin. The nature of the distinct domains has been ground‐truthed using available basement samples from drilling and dredging. Our results reveal four different crustal types ‐domains‐ of the Alboran Basin: a) a thin continental crust underneath the West Alboran and Malaga basins, which transitions to b) a magmatic arc crust in the central part of the Alboran Sea and the East Alboran Basin, c) the North‐African continental crust containing the Pytheas and Habibas sub‐basins, and d) the oceanic crust in the transition towards the Algero‐Balearic Basin. The Alboran Basin crust is configured in a fore‐arc basin (West Alboran and Malaga basins), a magmatic arc (central and East Alboran), and a back‐arc system in the easternmost part of the East Alboran Basin and mainly Algero‐Balearic Basin. The North‐African continental crust is influenced by arc‐related magmatism along its edge, and was probably affected by strike‐slip tectonics during westward migration of the Miocene subduction system. The distribution of active tectonic structures in the current compressional setting generally corresponds to boundaries between domains, possibly representing inherited lithospheric‐scale weak structures

Crust structure of northern Morocco and southern Iberian Peninsula from local earthquake tomography

We have estimated the P-wave velocity structure under northern Morocco and Alboran Sea and Southern Spain using an iterative simultaneous inversion method of local earthquake arrival-time data for velocity and hypocentral parameters. For this investigation we applied this tomographic method to 40714 P-wave arrival times from 2429 local events recorded by 124 both temporary and permanent seismic stations of local and regional networks from January 2000 to June 2009. The P wave arrival times used are calculated by the finite difference technique which allows a flexible parameterization of the velocity model. Twenty layers with a thickness of 4km for each one were postulated to obtain the three-dimensional P-wave structure along the complex Ibero-Maghribean boundary region. The hypocenter location of the global earthquake dataset has been remarkably improved by the obtained three-dimensional velocity model (RMS reduced to 27.3%). At the uppermost level of the crust the results suggest that the most prominent feature is the very low velocity zone associated with flysch units north of the Strait of Gibraltar, and in northern Morocco extending from Al-Hoceima region to the Alboran ridge. Conversely, a high velocity anomaly is observed in the area of the Ronda Peridotites, but a similar structure is not observed in the Beni-Boussera region in Morocco. The inverted velocity model is generally consistent with geology structure of the entire area and yields more details at depth of the geology structures and tectonic units. Moreover, it shows an accurate identification at depth of the shape and the geometry of the geology structures in the area. The tomographic cross section profiles reveal a vertical downgoing highly velocity materials in the whole area and show a thick crust in either the western part of the Alboran sea or northern Morocco region compared with the eastern one. keywords: local earthquake, P arrival-time, simultaneous inversion, hypocenter relocation, tomograPeer Reviewe

The IberArray BB seismic network of Topo-Iberia: new constraints revealing 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 allow 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 Rif and southern Iberia, including a crustal root beneath the Rif. 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