Shear waves characterization of the sub-surfaceimaging active fault and critical zone along the eastern betic shear zone, se iberian peninsula

Resumen

The Earth System is formed by four components that sustain life: the atmosphere, the biosphere, the hydrosphere and the geosphere. The complex region in which these elements interact is known as the ‘Critical Zone’. The critical zone (CZ) is crucial for our society, as it is host of a diverse range of hydrological, geochemical, and biological processes that operate on numerous scales and shape landscapes, support ecosystems, and control resource availability. The CZ and by extension the shallow subsurface is also the host to human activity and infrastructure, and it is therefore vulnerable to natural hazards, such as extreme weather events, volcanic activity or earthquakes. Thus, it is key to investigate and monitor the shallow subsurface, to understand its geometry, extent and physical properties in order to produce comprehensive hazard assessments that could take place in connection to this important layer. The southeastern part of the Iberian Peninsula is characterized by moderate but intense earthquake activity that has caused significant damage since historical times. The seismicity in that area is distributed within a relatively broad deformation band parallel to the coast that includes a well-developed strike-slip fracture system. This deformation band is known as the Eastern Betic Shear Zone (EBSZ) and, runs along the southern border of the Guadalentín Depression, it is a densely populated area with extensive agricultural activity. Therefore, the activity of the faults in the EBSZ represents a seismic hazard with a very significant social and economic impact potential. This memoir focuses in the characterization of the shallow sub-surface structure including the CZ and, characteristics of the Carrascoy and Alhama de Murcia fault systems, SE Iberian Peninsula. To achieve this characterization, different shear wave seismic velocity-depth models based on Multichannel Analysis of Surface Waves (MASW) were constructed. These models were complemented with investigations involving P-wave tomography and/or Electrical Resistivity Tomography (ERT) methods, as well as surface geological observations (e.g. mapping and paleoseismological trench studies) to constrain physical properties models of the subsurface. S-wave velocities were estimated from the surface waves recorded within the controlled-source shot records of conventional normal incidence seismic reflection data. P-wave velocity models were determined from first arrival travel-time tomography. The resulting velocity-depth models revealed: i) the location, geometry and extent of shallow fault zones, and their associated damaged unconsolidated zones, new blind faults were also identified; ii) the thickness of the critical zone and its relation with fault zones in the target areas; and, iii) provide valuable information about the fault network for seismic hazard assessments. The seismic signature of the fault zones, including these of the multiple blind faults and fractured zones identified in the different profiles, was well demonstrated using 2D velocity depth models (S- and P- waves). Seismic signatures of fault zones and blind faults are indicated in the models by low-velocity anomalies. Vs velocities in the range of 500-1000 m/s and Vp within the range of 1300-1700 m/s. These low values indicate a reduced bulk- and shear-modulus along and around the fault plane. The velocity model images across the Carrascoy and La Torrecilla reveal higher number of faults than what is imaged in the La Salud South and La Salud North profiles. This might be indicative of differences in the maturity degree of the fracture zones involved. Being the former more mature than the latter. Facultat de Ciències de la Terra Página 4 de 4 The models obtained have resolved the geometry and characteristics of the CZ in the neighborhood of faults/fracture zones. It is characterized consistently by relatively low seismic velocities Vs < 600 m/s and Vp < 1300 m/s, which is consistent with the expected low resistivity layer of Quaternary alluvial unconsolidated sediments in the La Torrecilla, La Salud North, and La Salud South profiles, as well as the Unidad Roja (Red Unit) and the Pleistocene-Quaternary alluvial in the Carrascoy profile. The thickness of the CZ in La Salud North profile lies within the range from 35-45 m and in La Salud South profile within 40-47.5 m. Furthermore, the thickest CZ layer constrained in the La Torrecilla profile features a thickness of ca. 40-50 m. The thinnest CZ is located in the Carrascoy profile with a thickness of ca. 30-40 m. In addition, the results also reveal a relation between fracture/fault zones and the thickness of the CZ. The presence of faults can contribute to the observed topography of the CZ in two ways: changes in bedrock elevation as a consequence of fault geometry changes and/or an increase in the number of regolith layer fragments, with the CZ layer near the fault zone becoming relatively thicker than in places far away from the fault zone. The shear wave characterization of the sub-surface presented in this memoir provides a methodological and workflow protocol to study the shallow subsurface in areas with active deformation, providing geometry and depth constraints to the structures interpreted in the geological models based on paleoseismological trenches, and, therefore, these can be used for improving the seismic hazard assessment and provide a detailed characterization of the CZ of this and other tectonically active regions in the world.