Crust‐mantle coupling in the Gulf of Cadiz (SW‐Iberia)

Abstract

[1] In the Gulf of Cadiz and Cape St. Vincent region at the SW-Iberian margin, seismic catalogues report seismicity down to $100 km depth. According to structural models for the region, this includes earthquakes that occur in the uppermost continental mantle. We invert regional waveforms to estimate moment tensors and centroid depths for moderate earthquakes, obtaining eight solutions (Mw 3.8 to 5.3) between 6 and 60 km depth. Moment tensor solutions and previous source estimates show predominately reverse and strike-slip faulting style with $NNW-SSE oriented P-axes, close to plate convergence between Africa and Eurasia. We obtain similar stress tensor estimates from crustal and upper mantle earthquakes, both with sub-horizontal, $NNW s 1 -axes. This indicates analogous deformation in both domains, and that earthquakes are unrelated to any subduction process.

QP and QS in the upper mantle beneath the Iberian peninsula from recordings of the very deep Granada earthquake of April 11, 2010

“…More recently, Spakman and Wortel [2004] describe the 3-D geometry of a continuous slab extending to 660 km depth, supporting models of westward slab roll-back in the BeticRif-Alboran region [Lonergan and White, 1997;Faccenna et al, 2004;Jolivet et al, 2008]. Ongoing eastward subduction has been proposed by Gutscher et al [2002], however no signature of such a process shows up in the regional seismotectonic patterns and the GPS velocity field [Stich et al, 2005[Stich et al, , 2006. Other subduction geometries have been proposed by other authors to explain the formation of the Gibraltar arc and the occurrence of deep seismicity [e.g., Zeck, 1996], as well as entirely different geodynamic models, ranging from convective removal of the lithospheric root [Platt and Visser, 1989] to delamination of the upper mantle [Seber et al, 1996;Calvert et al, 2000].…”

Scenarios for earthquake-generated tsunamis on a complex tectonic area of diffuse deformation and low velocity: The Alboran Sea, Western Mediterranean

“…Lopez Casado et al (2001) clearly show the almost exclusive presence of shallow seismic events « 15 km) in the Alboran domain, ignoring the deeper earthquakes related to the Arc of Gibraltar, and not related to the deformation of the Alboran domain. These shallow depths are typical of earthquakes occurring on the continental crust; in the oceanic crust the events often reach depths of up to 40-50 km (Watts and Burov, 2003), such as in the Gulf of Cadiz, with oceanic crust and involving the elastic deformation of the lithospheric mantle Stich et al (2005).…”

Recent and active tectonics in the western part of the Betic Cordillera

“…Along the Iberia-Maghreb sector, earthquakes are of low-to-moderate magnitude (M<5.5) and occur at shallow depth, except in two localized areas of intermediate (40-120 km) and deep (>620 km) seismicity around Malaga and Granada, respectively (Buforn et al, 1991;Morales et al, 1997) as well as subcrustal earthquakes (< 60 km) in old oceanic lithosphere in the Atlantic (Stich et al, 2005;Geissler et al, 2010). Within the Betic Cordillera, faulting style changes from predominately strike-slip in the eastern part to normal faulting in the central Betics and to reverse and strike-slip faulting in the western Betics .Within the western Betics, seismicity is usually of lower magnitudes (rarely exceeding magnitude 4) and less frequent than in the eastern and central sectors.…”