Polarization analysis of the seismic ambient noise in La Laguna Valley (Tenerife, Spain) and its relationship with the local seismic response

Ambient seismic noise analysis provides an interesting source of information to characterize the subsoil and to investigate local seismic site effects in urban areas. In this study, we present a polarization analysis of ambient noise data acquired in the Aguere Valley (Tenerife), an infilled basin characterized by soft clay-silt deposits and stacked lava flows with pyroclastic and scoria intercalations. We collected a total of 467 ambient noise measurements, covering the entire valley. This dataset has already been analyzed using the standard HVSR method. 

The analysis examines the directional properties of the seismic wavefield to identify preferential azimuths of ground motion and their possible relationship with local heterogeneities and basin geometry. Polarization characteristics are investigated by evaluating the azimuthal dependence of the Horizontal-to-Vertical Spectral Ratio (HVSR) through systematic rotation of the horizontal components over the 0°–180° azimuthal range. This approach allows assessing the azimuthal variability in the H/V ratio and the identification of frequency-dependent polarization features, providing additional constraints on the directional behaviour in a geological complex valley within an urban area.  

The results show that polarization analysis often exhibits: 1) localized azimuthal maxima with high H/V values in a narrow angular range, and 2) broad azimuthal bands in the entire polarization angle range characterized by elevated H/V values without any well-defined preferential direction. In many cases, azimuthal features with elevated H/V values are observed between approximately 50° and 160° at frequencies between 1–3 Hz, forming an eye-shaped pattern in the azimuth–frequency domain. At higher frequencies, between 7 and 20 Hz, the H/V response typically exhibits bands with high values across most of the azimuthal range (0º–180º), indicating weak directional dependence.  

These features generally coincide with the main frequency peaks previously identified in the HVSR curves, suggesting a close relationship between polarization patterns and site resonance frequencies. The observed azimuthal variability likely reflects the complexity of the ambient seismic wavefield and its interaction with the local subsurface geology.