Potential geologic hazards on the eastern Gulf of Cadiz slope (SW Spain)

Geologic hazards resulting from sedimentary, oceanographic and tectonic pro cesses affect more than one third of the offshore Gulf of Cadiz, and are id entified by interpreting high-resolution seismic profiles and sonographs. H azards of sedimentary origin include the occurrence of slope instability pr ocesses in the form of single or multiple slumps occupying up to 147 km(2) mainly concentrated in the steeper, upper slope area. Besides the presence of steep slopes, the triggering of submarine landslides is probably due to seismic activity and favoured by the presence of biogenic gas within the se diment. Gassy sediments and associated seafloor pockmarks cover more than 2 40 km(2) in the upper slope. Hazards from oceanographic processes result fr om the complex system of bottom currents created by the interaction of the strong Mediterranean Undercurrent and the rough seafloor physiography. The local intensification of bottom currents is responsible for erosive process es along more than 1900 km(2) in the upper slope and in the canyons eroded in the central area of the slope, undermining slopes and causing instabilit y. The strong bottom currents also create a mobile seafloor containing bedf orms in an area of the Gulf that extends more than 2500 km(2), mostly in th e continental slope terraces. Hazards of tectonic origin are important beca use the Gulf of Cadiz straddles two major tectonic regions, the Azores-Gibr altar fracture zone and the Betic range, which results in diapir uplift ove r an area of more than 1000 km(2), and in active seismicity with earthquake s of moderate magnitude. Also, tsunamis produced by strong earthquakes occu r in the Gulf of Cadiz, and are related to the tectonic activity along the Azores-Gibraltar fracture zone. (C) 1999 Elsevier Science B.V. All rights r eserved.