Abstract

We recorded ambient noise and weak earthquake ground motion on Holocene sediments in the Tiber Valey, north of Rome, in order to help predict the response of these sediments to strong ground motion. Ambient noise within the valley is high and sharply peaked near 2.5 Hz. Similar peaks are observed along the Tiber Valley towards the south, in and south of Rome. A small local earthquake simultaneously recorded within the valley and at a reference hard-rock site is dominated by energy at 4 to 20 Hz at the reference site but appears to excite low-frequency resonances at the sediment site. We also recorded noise at a small array of four stations within the valley and apply the spatial auto-correlation method of Aki (1957, 1965) to obtain Rayleigh-wave dispersion between 1 and 5 Hz. The observed resonance and dispersion can be used to determine velocity structure in the near-surface sediments; between 1 and 5 Hz, we obtain phase velocities of approximately 400 to 500 m/sec, implying a shear-wave velocity of 440 to 530 m/sec. The Holocene sediment layer is constrained from geotechnical data to have a thickness of 40 m. Our velocity results thus predict a quarter-wavelength resonance of 2.8 to 3.3 Hz, close to that observed in the ambient noise and weak-motion data.

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