Upper-Crustal Seismic Anisotropy in the Cantabrian Mountains (North Spain) from Shear-Wave Splitting and Ambient Noise Interferometry Analysis

Abstract

The upper-crustal anisotropy of the Cantabrian Mountains (North Spain) has been investigated using two independent but complementary methodologies: (a) shear-wave splitting and (b) ambient seismic noise interferometry. For this purpose, we have processed and compared seismic data from two networks with different scales and recording periods. The shear-wave splitting results show delay times between 0.06 and 0.23 s and spatially variable fast-polarization directions. We calculate that the anisotropic layer has a maximum effective thickness of around 7.5 km and an average anisotropy magnitude of between 4% and 8%. Consistently, our ambient noise observations point to an anisotropy magnitude between 4% and 9% in the first 10 km of the crust. Our results show a clear correlation between the fast directions from both methods and the orientations of the local faults, suggesting that the anisotropy is mainly controlled by the structures. Furthermore, in the west of the study area, fast-polarization directions tend to align parallel to the Variscan fabric in the crust, whereas to the east, in which the Alpine imprint is stronger, many fast directions are aligned parallel to east–west-oriented Alpine features.

The Cantabrian Fault at Sea. Low Magnitude Seismicity and Its Significance Within a Stable Setting

“…The seismic stations were equipped with Worldsensing-Spidernano data loggers in combination with three-component 2 Hz Geospace MiniSeisMonitor sensors. This deployment registered for 19 months, covering an area of 60 km × 50 km (López-Fernández et al, 2018;Acevedo et al, 2020). (2) Later, in May 2019, we installed a second seismic network covering a wider area in the Cantabrian Mountains (160 km × 80 km).…”

“…The role of the CF-VF as a crustal domain boundary structure applies to seismicity, practically absent to the East (Figures 2,4), and to lithospheric and crustal thickness variations based on seismic refraction data, potential field modeling, and tomographic studies (Villaseñor et al, 2007;Torne et al, 2015;Palomeras et al, 2017;López-Fernández et al, 2018;Acevedo et al, 2020).…”

Radial anisotropy and S-wave velocity depict the internal to external zones transition within the Variscan orogen (NW Iberia)

“…The use of ambient noise interferometry is also beginning to provide knowledge about the structure of the upper crust in the study area (e.g. Acevedo et al, 2019Acevedo et al, , 2020Olivar et al, 2020).…”

“…Here we make use of the ambient noise recordings from a new temporary seismic network in NW Iberia to extend to the North and the West an earlier tomographic S-wave velocity model of the eastern sector of the CM (Acevedo et al, 2019), which was mostly affected by the Alpine deformation, and to further determine the 3-D distribution of the crustal radial anisotropy. From a subsequent work (Acevedo et al, 2020), we learnt that the crustal fabric controlled the orientation of fastest seismic velocities, sometimes in accordance with Variscan structures, others with Alpine structures. However, there was coincidence in the orientation of Variscan and Alpine structures, mostly by reworking during the youngest convergence.…”

“…Up to the date, there is limited knowledge on the seismic anisotropy signature of NW Iberia. Díaz et al (2002Díaz et al ( , 2006Díaz et al ( , 2015 studied the upper mantle and lower crust anisotropy from distant earthquake shear wave splitting measurements and Acevedo et al (2020) used local earthquake shear wave splitting and ANI analysis to infer the anisotropic properties of the upper crust in the central part of the CM. Both studies consistently reported an average E-W fast direction, but the rotation of the fast orientations in the vicinity of major Alpine structures suggest that the contribution of the Alpine deformation to the anisotropy can be significant (Acevedo et al, 2020).…”