Imaging the upper crust with ambient seismic noise in natural and urban environments

The SANIMS project is focused on the development and application of methods based on seismic ambient noise to image and monitor natural and human-altered environments focusing on two test sites; the Cerdanya Basin in the eastern Pyrenees, and the city of Barcelona. Broad-band and short-period seismometers and a high-density node network have been used to acquire new data.

Broad-band data has been processed using the frequency-dependent phase cross-correlation and time-scale phase-weighted stacking to extract Rayleigh and Love waves. We have obtained Rayleigh and Love group and phase velocities for periods in the 1.5 – 4 s range, that will be inverted to velocity-depth models. The preliminary results show higher velocities to the North, with a well-defined zone with lower than average velocities around the Cerdanya Basin. The geometry of the basin basement has also been investigated using the amplitudes of ambient noise, HVSR methods and RFs, obtaining consistent results. The recently acquired high-density data has already been processed in terms of amplitude variations and will be integrated with the tomographic images.

The data acquired in Barcelona has first been used to monitor human activity during the COVID19 pandemic. Amplitude variations of seismic noise allow to delineate the main geological units of the Barcelona area. HVSR measures using the new data expand the already available results, hence improving the existing seismic hazard maps, and will allow analyzing eventual temporal variations in the measurements. As in the Cerdanya Basin, the data will be used to extract Rayleigh waves and invert for velocities.

Both datasets will also be used to analyze the applicability of the methods based on Rayleigh wave ellipticity inversion of ambient noise and earthquake data to provide S-velocity depth profiles. We expect that the use of ambient noise methods will allow to map the basement and to obtain new higher resolution ambient noise tomographic images of the upper crust in the Cerdanya Basin and to better constrain the subsoil properties of Barcelona. The results in both areas will allow comparing the performance of these methodologies in quiet and noisy areas.

This is a contribution of the SANIMS project (RTI2018-095594-B-I00)