Metamorphic bedrock geometry of Santorini using HVSR information and geophysical modeling of ambient noise and active-source surface-wave data

Abstract

Santorini is the most active volcano of the Southern Aegean Volcanic Arc, with volcanic material that has accumulated during the last 600+ Kyrs on top of the pre-volcanic Santorini island. The geometry of the pre-volcanic basement not only provides constraints on the volcanic history, but is also fundamental for seismic hazard assessment, as it partly controls site-effects on strong ground motions. We investigate the geometry of the metamorphic (Cycladic) basement of Santorini using information from both passive (noise) and active source data. We performed a large number (∼280) of single-station ambient noise measurements and collected additional data from 42 previous studies to compute the Horizontal-to-Vertical Spectral Ratio (HVSR) curves. The majority of the recovered HVSR curves show prominent spectral ratio peaks (locally larger than 7–8) indicating a large impedance contrast between pyroclastic volcanic strata and bedrock metamorphic formations. We also performed active (MASW) and passive (Noise Array) surface wave measurements at 4 selected sites to determine local dispersion curves. To find the 1D Vs structure from the surface down to the metamorphic bedrock we performed a joint Monte Carlo inversion of the dispersion curves and the fundamental frequency (f0HVSR) values from the HVSR curves. These models, as well as deep borehole information and HVSR data along the caldera cliff, where the thickness and stratigraphy of the volcanic formations above the bedrock can be directly observed, allowed us to determine a linear relationship between the thickness of the pyroclastic formations and their fundamental resonance period for central-southern Santorini. The resulting geometry of pre-volcanic island shows deep basins around the pre-Alpine bedrock outcrop (Profitis Ilias) and generally agrees with recent results from larger-scale tomographic and gravity data but with some significant local differences. The basins are 200+ meters thick in the Kamari and Perissa area (southeastern Santorini) and >400 m thick in the central (Fira-Imerovigli) area, while there is a thick (∼70-130 m) pyroclastic layer on top of a metamorphic basement “plateau” in the Megalochori area.