Neotectonic fault mapping at the Gibraltar Strait Tunnel area, Bolonia Bay (South Spain)

Abstract

A Map of Fault Activity has been developed for preliminary engineering planning related to the construction of an underwater fixed link-route between Africa and Europe at the Gibraltar Strait Sector, in South Spain (the Gibraltar Tunnel). Main criteria used in fault activity evaluation have been seismicity, surface faulting, geomorphology of fault traces, identification of paleoseismic ground failures, and evaluation of differential uplift rates on coastal segments separated by large-scale inland faults. The aim of this work is the development of comprehensive methodology for the graphic representation (chart-format) of geo-hazards related to ground conditions, seismicity and fault activity. The criteria management is mainly focused on the establishment of the seismic or aseismic character of individual fault traces. For this the age of the last deformational event, and slip rates (where possible) are taken into account, but their classification as active or inactive structures is avoided. From this study, the Cabo de Gracia strike-slip fault (NE–SW) can be catalogued as active during at least the last 128 ka BP, and as a probable seismic source of moderate events (mb < 5), with relevant incidence in the ancient Roman city Baelo Claudia. Engineering planning also needs to take into account the occurrence of unstable units such as, low-cohesive bedrock (large landslides) and unconsolidated littoral deposits (liquefaction). Expected horizontal ground acceleration of 0.07 g at the mapped zone can reach average accelerations of 0.16 g on the unstable clayey units of the Gibraltar Flysch (NCS-94), which are capable of inducing near-surface destabilization on unstable ground, as evidenced by the geologic, geomorphologic and archaeological records.