Active faulting and slope failure in the Iberian margins: towards offshore geohazard mitigation

Abstract

Several high-resolution marine geological and geophysical surveys have been carried out to investigate goehazards in the South and East Iberian Margins associated with slow-moving seismic imaging, physical properties and dating methods, submarien fault systems and slope failure processes have been characterized with unprecedented resolution. Direct investigations of selected faults from the Mediterranean Sea and the external part of the Gulf of Cadiz yield their fault seismic parameters, suggesting than these faults are active and capable of generating large magnitude (Mw>6) seismic events, representing an earthquake and tsunami hazard for the surrounding coastal areas. Off fault paleoseismic evidence in the SW Iberian Margin based on widespread synchronous turbidite deposits yields a regional recurrence rate of large magnitude earthquakes (Mw>8) of 1800--2000 years. Submarine landslides are also ubiquitous in the Mediterranean margins of Iberia. Passive margings tend to have larger, but fewer landslides when compared to active margins. Most submarine landslides in the western Mediterranean and Gulf of Cadiz originate beyond 2000 m water depth in slopes not exceeding 2º display little vertical runout. Thus, only some infrequent large events are capable ot tsunami generation. So far there are very little constraints on ages of most events. However, of these events have been dated it is remarkable that a large number of them are reported as Holocene, which suggests a link to natural climate fluctuations. Detailed investigation of a few events suggest overpressures at deph in siliciclastic wedges and fluid seepage from sub-Messinian formations as important contributors for slope instability.