The Carboneras Fault Zone (SE Spain): Constraints on Fault Zone Properties and Geometry from Controlled-Source-Generated Guided Seismic Waves
Tuesday, 16 December 2014
We combine geophysical data, field-geological mapping and lab measurements to study the Carboneras fault zone (CFZ) in SE Spain. The CFZ is part of the Trans-Alborán Shear Zone which constitutes part of the diffuse plate boundary between Africa and Iberia. The CFZ is inferred to behave as a stretching transform fault with∼40 km left-lateral offset. It was active principally between 12 – 6 Ma BP, and has been exhumed from ca. 1 - 2 km depth. The relatively recent movement history and the semi-arid terrain lead to excellent exposure.The phyllosilicate-rich fault gougeis excellently preserved.
In 2010 we conducted a controlled source seismic experiment at the CFZ in which explosive sources in boreholes (two groups of 3 explosions) were placed in two strands of the fault zone. The signals were observed with dense linear seismic arrays crossing the CFZ at 3.5km and 8.3km distance, respectively. The recordings show clear high-energy P-phases at receivers and from sources located at or near the fault zone. We interpret these phases as P-waves trapped in the low-velocity layer (waveguide) formed by the damage zone of the fault(s).
With waveform modeling (using an analytical solution assuming a straight waveguide embedded in two quarter spaces and a line source at depth) we derive basic models well-explaining the observations. Lab-measurements of the different rocks constrain the possible models. Additionally, we employed extensive three-dimensional finite-difference (3D-FD) modeling with more realistic (curved and anastomosing) waveguide geometries.
It seems that the studied segments of the CFZ form effective waveguides for seismic waves with connectivity over several kilometers. The derived seismic models together with lab measurements of the seismic velocities indicate that the average fault zone core widths are in the order of 15 to 25m which is in good agreement with surface geological mapping.