Characterization of the shallow subsurface structure across the Carrascoy Fault System (SE Iberian Peninsula) using P-wave tomography and Multichannel Analysis of Surface Waves

Abstract

The seismicity in the SE Iberian Peninsula is distributed parallel to the coast in a well-developed strike-slip fracture system known as the Eastern Betic Shear Zone (EBSZ). This work focuses on the characterization of the shallow subsurface structure of the Algezares-Casas Nuevas Fault, within the Carrascoy Fault System of the EBSZ. The Carrascoy Fault borders the Guadalentín Depression to the south, which is a densely populated area with extensive agricultural activity. Therefore, this faults system represents a seismic hazard with significant social and economic implications. We have constructed two velocity-depth models based on P-wave tomography and Multichannel Analysis of Surface Waves (MASW) acquired from seismic reflection data. The resulting velocity models have allowed us to interpret the first ~250m depth and have revealed: i) the thickness of the critical zone; ii) the geometry of the Algezares-Casas Nuevas Fault; iii) the depth of the Messinian/Tortonian contact and iv) the presence of blind thrusts and damage zones under the Guadalentín Depression. Our results have also helped us to estimate an apparent vertical slip rate of 0.66±0.06m/ky for the Algezares-Casas Nuevas Fault since 209.1±6.2ka. Our results provide a methodological and backflow protocol to study the shallow subsurface of active faults, complementing previous geological models based on paleoseismological trenches, and can be used to improve the seismic hazard assessment of tectonically active regions around the world.

Geophysical Imaging of the Critical Zone along the Eastern Betic Shear Zone (EBSZ), SE Iberian Peninsula

“…The SW segment is represented by: (i) the Carrascoy fault sensu lato, that uplifted the Carrascoy Range, and (ii) the Algezares-Casas Nuevas fault (ACNF), that runs for 23 km from the villages of Algezares to Casas Nuevas [44,50]. This area was studied by MASW and P-wave tomography, revealing the underground structure, damage zone related to blind faults, and allowed us to propose a vertical slip rate for the Algezares-Casas Nuevas fault of 0.66 ± 0.06 m/kyr since 209.1 ± 6.2 ka [27]-submitted.…”

“…Meanwhile, the thickening of the CZ in the La Salud South profile (Figure 9) appears close to the F3 fault, which is correlated to the S-AMF deformation zone (h3). In the Carrascoy profile (Figure 10), the ACNF fault zone controls the thickness of the CZ, with the thickened layer located near the F1 fault zone (h3) [27,49].…”

“…Some works that focused on the study of the CZ used seismic wave velocities and electrical resistivity to demonstrate the influence of topographic stress and fault activity on bedrock fault distribution [25,26]. Specifically, in the EBSZ, the structure of the CZ was first characterized by P-wave tomography and MASW across the frontal strands of the Carrascoy fault system, namely the Algezares-Casas Nuevas fault (ACNF) [27]-in review. This study revealed the underground structure, the CZ, and the damage zone related to blind faults buried under the Guadalentín Depression.…”