Body waves tomography from OBS-recorded earthquakes in the Gulf of Cadiz

Wolfram.Geissler [ at ]


The gulf of Cadiz is a region considered as a complex seismic area, where strong earthquakes occur and where the plate boundary between the African and Eurasian plates is not exactly known. In this paper, we use high resolution seismic data recorded by a network of OBS stations deployed for one year in the Gulf of Cadiz as well as eight permanent Portugal land seismic stations. The OBS network was deployed within an experiment of the NEAREST project. Nearly 600 seismic events are extracted from the recorded data set and their analysis revealed that most of them occur at 20 to 80 km depths, with clusters of seismicity that occur mainly at the Gorringe Bank, within the SW segment of the Horseshoe fault and the Marques de Pombal Plateau and the S. Vicente Fault. A new NW-SE trend of seismicity has been revealed with depths that extend from 35 to 80 km. This seismicity trend is close and nearly parallel to the SWIM (South West Iberian Margin) faults lineament. We further present in this paper, the first regional-scale high resolution P- and S-velocity distributions across the Gulf of Cadiz region. These velocity models are obtained using three-dimensional seismic tomography to invert the OBS data-set. The results show that the patterns of anomalies in the Gulf of Cadiz are in general, oriented in NE-SW and NW-SE directions. They also show the presence of a low velocity zone (LVZ) to the SE of our study area. At shallow depth, this LVZ is interpreted as due to a large accumulation of sediments within the accretionary wedge, while at a greater crustal depth, it may reflect a continental crustal composition rather than an oceanic crust. Moreover, seismic velocity profiles show that under this region of the Cadiz Gulf, the Moho averages a 30-km depth. The Gorringe Bank and the Marquise de Pombal plateau are found to be deeply rooted and represent expressions of mantle uplifting. The association of these deep anomalies with active seismicity that occurs at their levels, indicates that the uplifting of these ridges is still an ongoing process. Furthermore, a NW-SE zone of high velocity is found to the SW of our study area. This zone occurs along and parallel to the SWIM faults zone (SFZ) and appears to support the hypothesis that the SFZ represents the boundary between the Nubia and the Eurasia plates at the Gulf of Cadiz level as previously suggested [1].

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Lbadaoui, A. , Iben Brahim, A. , Matias, L. , Geissler, W. , El Hammoumi, A. , Gueraoui, K. , El Mouraouah, A. , Kasmi, M. , Koulakov, I. , Monna, S. , Zitellini, N. and Toto, E. (2012): Body waves tomography from OBS-recorded earthquakes in the Gulf of Cadiz , International Review of Physics, 6 (2), pp. 232-240 .

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