Seismotectonics of the Gulf of Cadiz: new insights from instrumental seismicity

The Gulf of Cadiz offshore SW Iberia is an area linked with episodic destructive seismic and tsunamigenic events, such as the 1st November 1755 Lisbon earthquake. The association of active faults to this kind of high magnitude event has been intensively studied specially due to the contribution of several international team projects, working in this area for more than two decades. However, the meaning of the persistent small to intermediate magnitude seismicity recognized in this region is still particularly misunderstood. This problem is mostly related to the lack of an accurate hypocenter location of these events resulting from an asymmetrical geographical distribution of the permanent seismic network. To address this problem the EC project NEAREST (Integrated observation from NEAR shore sourcES of Tsunamis: towards an early warning system) conducted a passive seismic experiment in the Gulf of Cadiz where 24 broadband seismometers (plus the GEOSTAR multi-parameter deepsea observatory) were deployed from September 2007 to July 2008. The examination of the continuous data stream allowed the detection of a large number of local events that were not detected by the land networks. The analysis of the complete data set reveals 3 main clusters of earthquakes, two of them coincide with the location of the 3 larger instrumental earthquakes in the area: i) the 28th February 1969 (Mw~8.0); ii) the 12th February 2007 (Mw=6.0) and iii) the 17th December 2009 (Mw=5.5). Many of the small magnitude earthquakes are located in the mantle (at depths between 30 and 60 km), like the hypocenters of these three earthquakes derived from waveform inversion. However, focal mechanisms show a mixed pattern, mostly strike-slip and reverse dip-slip with a very few normal mechanisms, showing us that processes related with this seismicity must be complex probably involving the interaction between different active geological structures and reflecting particular local rheological contrasts.