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Rayleigh waves in Quaternary alluvium from explosive sources: Determination of shear-wave velocity and Q structure

Istituto Nazionale di Geofisica, Roma, Italy
Department of Earth and Atmospheric Sciences Saint Louis University, St. Louis, Missouri
Istituto di Ricerca sul Rischio Sismico C.N.R., Milano, Italy

Abstract

Surface-wave propagation in unconsolidated Quaternary alluvium has been investigated within the valley of the Chiusi Lake tributary (Central Italy). The seismic experiment was originally designed to determine the structural response of an Etruscan tomb to a 200-kg dynamite shot, planned as part of a refraction survey within the CROP-DSS (an Italian project for the investigation of deep crustal structures). The availability of many recording instruments allowed us to design a more complete experiment, and we decided to use surface waves generated by a small explosion (10 kg dynamite) to infer the shear-wave-velocity-attenuation model in the alluvium layer.

We deployed a linear array aligned with the shot point on the river sediments. Sixteen digital, three-component recording stations were used; the minimum distance from the source was 1.9 km, and we chose an interstation separation of 50 m, for a total array length of 750 m. Clear Rayleigh waves were recorded along the array, and we were able to recognize two propagating modes in the waveforms. We calculated the dispersion curves of group velocities using a multiple filter technique. Dispersion curves of phase velocities were calculated using a p- stacking technique.

The whole procedure was run as follows: (1) the inversion of the group-velocity dispersion; (2) the use of synthetic seismograms for mode recognition; (3) the extraction and the inversion of phase-velocity values from the p- stack performed on the observed waveforms; (4) the use of a trial-and-error approach for Q_ß to obtain "good" synthetic waveforms; and (5) the application of a conjugate gradient inversion technique to the anelastic absorption model (no inversion was run on dispersion curves). Values of Q_ß of ranging from 2 to 20, and a shear-wave velocity in the range 280 < ß < 400 m/sec were found in the Holocene alluvium. Moreover, in our modeling we used the relation Q = Q_ß for unconsolidated, water-saturated sediments. Our results suggest that surface waves generated by explosions can be successfully used to determine shear-wave-velocity and attenuation models in soft sediment coverages.

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