Seismic site characterization using broadband and DAS ambient vibration measurements on Mt Etna, Italy
Both horizontal-to-vertical (H/V) spectral ratios and the spatial autocorrelation method (SPAC) have proven to be valuable tools to gain insight into local site effects by ambient noise measurements. Here, the two methods are employed to assess the subsurface velocity structure at the Piano delle Concazze area on Mt Etna. Volcanic tremor records from an array of 26 broadband seismometers is processed and a strong variability of H/V ratios during periods of increased volcanic activity is found. From the spatial distribution of H/V peak frequencies, a geologic structure in the north-east of Piano delle Concazze is imaged which is interpreted as the Ellittico caldera rim. The method is extended to include both velocity data from the broadband stations and distributed acoustic sensing data from a co-located 1.5 km long fibre optic cable. High maximum amplitude values of the resulting ratios along the trajectory of the cable coincide with known faults. The outcome also indicates previously unmapped parts of a fault. The geologic interpretation is in good agreement with inversion results from magnetic survey data. Using the neighborhood algorithm, spatial autocorrelation curves obtained from the modified SPAC are inverted alone and jointly with the H/V peak frequencies for 1D shear wave velocity profiles. The obtained models are largely consistent with published models and were able to validate the results from the fibre optic cable.
site characterization, Etna, DAS, ambient vibration, seismic noise, H/V, HVSR, velocity model, SPAC, MSPAC, spatial autocorrelation, Piano delle Concazze, site effects, microzonation, volcanic tremor, joint inversion, ortsverteile faseroptische Dehnungsmessung, Ätna, horizontal-vertikales Spektralverhältnis, räumliche Autkorrelationsmethode, modifizierte räumliche Autkorrelationsmethode, Bodenunruhe, gemeinsame Inversion, Mikrozonierung, seismisches Rauschen, Ortscharakterisierung, Ortseffekte, räumliche Autokorrelation, Geschwindigkeitsmodell, vulkanischer Tremor
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