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Spectral features of microearthquakes in volcanic areas: Attenuation in the crust and amplitude response of the site at Mt. Etna, Italy

Istituto Internazionale di Vulcanologia—CNR, Piazza Roma 2, I-95123 Catania, Italy
Dipartimento di Scienze della Terra Università della Calabria, I-87030 Rende, Cosenza, Italy
Gruppo Nazionale per la Vulcanologia—CNR, Piazza Roma 2, I-95123 Catania Italy
Istituto di Geologia e Geofisica Università di Catania, Corso Italia 55, I-95129 Catania, Italy

Abstract

Eighty-five earthquakes recorded in 1991 at the digital surveillance array of Mt. Etna are used for investigating the attenuation of the medium and the amplitude response of the sites. In order to evaluate the influence of the attenuation on seismic spectra, the quality factor Q is estimated for P and S waves by the single-station spectral ratios method, and spectral amplitude ratios calculated between 2 and 20 Hz. The P-wave quality factor Q_p is computed from earthquakes recorded at the whole network, while Q_s is obtained by S data recorded at three-component stations only. The results indicate that Q_p and Q_s increase as a function of the frequency according to the law Q = Q₀fⁿ (with n ranging from 0.5 to 1.6) as a general rule. Reliable Q_s/Q_p ratios are obtained at three, three-component stations only, two of which display values ranging between 1.3 and 1.5, while the third displays anomalous values much lower than unity (0.5). Significant variations of Q_p are found as a function of the depth (much lower Q values are obtained for focal depths less than 5 km) and of the source-station azimuths. After correction of seismic spectra by the geometrical spreading and the quality factor, spectral ratios are computed again at all stations for singling out the site response effects. The amplitude responses differ by up to 10-fold at the different sites. In a very few cases, larger amplification ratios (up to 2 orders of magnitude) are found to affect some frequency bands. Displacement spectra, corrected by the attenuation and the site responses, are then used for inverse determination of the low-frequency level ₀, the corner frequency _c, and the fall-off of spectra beyond _c. The latter typically displays ^–3 behavior. The inverse relationship between seismic moment and corner frequency does not hold for events located at shallow depth since very low Q values may locally mask _c.

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