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Ligabue – Large Induced Ground Amplitudes by Urban Excitations, as Recorded by a 7c-Station

Description: 

We deployed a 7C-station as a multiparameter recording setup, composed by a classical broadband seismometer (Nanometrics Trillium Compact Horizon 120s), a rotational motion sensor (iXBlue, blueSeis-3A) plus an infrasound sensor (Chaparral 64 UHP2). Here, we understand Urban Excitations as the daily anthropogenic noise (car and railway traffic, power lines, hydraulic pumps, washing machines, etc.) in the city of Florence, temporarily superposed by large oscillations generated during a public performance of the Italian rock musician Luciano Ligabue. The 7-component station, installed approximately 650 m eastwards from the concert hall “Nelson Mandela Forum”, include three translational and three rotational components, which recorded large amplitudes of seismic ground motion. These 6 degrees-of-freedom, in combination with an additional infrasound sensor (0.015-50 Hz, Nyquist frequency), allowed us to identify the generated wave types, to distinguish between the respective propagation paths, and to discriminate between the seismic and the acoustic source(s).

The largest seismic amplitudes observed during the two-hours rock concert appear as discrete monochromatic wave-trains in a predominant frequency range of 2-3 Hz. A parallel and synchronized video recording of the musical event proves that these oscillations correspond exactly to the master beat of the respective song (refrain) and are caused by the rhythmically jumping spectators. The seismic beat amplitudes are not amplified due to local site effects, as suggested by the spectral ratios curve computed on the quiet part of ambient noise, but can be correlated with the number of enthusiastic spectators. Being recorded on the vertical and the radial (E-W) component, the master beat signals are composed by Rayleigh waves. This is confirmed by data from the rotational sensor, which hardly records signals below 8-9 Hz due to its self-noise level. However, the large seismic amplitudes on the HJ2-component between 2-3Hz stand out from the self-noise-dominated rotation rate spectra. Unexpected was the recording of the departing traffic after the end of the concert.

Session: Special Applications in Seismology - I

Type: Oral

Date: 5/1/2024

Presentation Time: 04:45 PM (local time)

Presenting Author: Thomas

Student Presenter: No

Invited Presentation: 

Authors