Seismic velocity modelling of the Carboneras Fault Zone, SE Spain

• Seismic tomography of a fault zone is compared with modelling from field geology.
• Near-surface modelled seismic velocities are corrected for effects of crack arrays.
• Tomographic sections can be predicted from field geology plus lab seismic data.
• Interpretation of in-situ tomography should be constrained by field geological data.

The Carboneras fault zone forms part of a major strike-slip fault system in SE Spain, striking NE–SW, and accommodating up to 40 km displacement. It affects basement metamorphic rocks and unconformably overlying upper Miocene sediments and volcanic rocks. High-resolution shallow seismic tomographic sections were made across the fault zone in two localities. From the same areas, fault rocks and their wallrocks were collected for laboratory seismic velocity measurements. The laboratory data were corrected for the substantial effects of near-surface crack damage. By combining these results with geological cross sections, forward velocity models for the fault zone were constructed to compare with field seismic measurements and hence to ‘ground-truth’ the inferences made from them. These velocity/depth relationships matched moderately well with those extracted from the in-situ tomography results. Aspects of the in-situ seismic sections matched features on the forward-modelled sections, but the comparisons showed that it is important to have some degree of foreknowledge of the geology to be able successfully to interpret seismic tomography sections as an exploration tool.