Imaging the Carboneras fault zone at depth: preliminary results from reflection/refraction seismic tomography

Understanding and characterizing fault zone structure at depth is vital to predicting the slip behaviour of faults in the brittle crust. We aim to combine detailed field mapping and laboratory velocity/physical property determinations with seismic measurements on the Carboneras fault zone (S.E. Spain) to improve our knowledge of how fault zone structure affects seismic signals. The CFZ is a large offset (10s of km) strike-slip fault that constitutes part of the diffuse plate boundary between Africa and Iberia. It has been largely passively exhumed from ca. 4 to 6 km depth. The friable fault zone components are excellently preserved in the region’s semi-arid climate, and consist of multiple strands of phyllosilicate-rich fault gouge ranging from 1 to 20 m in thickness. In May 2009 we conducted 4 high-resolution seismic reflection and refraction/first break tomography lines. Two of these lines (~1km long) crossed the entire fault zone while the remaining lines (~150 and ~300m long) concentrated on individual fault strands and associated damage zones. For each of the lines a 2 m-geophone spacing was used with a combination of accelerated drop weight, sledgehammer and 100g explosives as seismic sources. Initial seismic reflection processing has been carried out on each of the 4 lines. First breaks have been picked for each of the shot gathers and inputted into a 2D traveltime inversion and amplitude-modeling package (Zelt & Smith, 1992) to obtain first break tomography images. During this field campaign we also carried out numerous fault zone guided wave experiments on two of the dense seismic lines. At the larger offsets (~600-700m) we observe low frequency guided waves. These experiments will capture the various length scales involved in a mature fault zone and will enable the surface mapping and petrophysical studies to be linked to the seismic field observations.