Advances in ultrasonics

Abstract

The occurrence of a seismic series in Deception Island volcano (Antarctica), composed of hundreds of local volcano–tectonic earthquakes, has permitted us to study the seismic attenuation of such a volcanic environment in the short-distance and high-frequency range. This study has been performed using P-waves, S-waves and coda-waves and applying different, frequency dependent and independent, techniques. The methods used for this analysis have been: spectral and broadening-of-the-pulse, for direct P- and S-waves, coda normalization for S-waves, and single back-scattering model for coda-waves. The results show that, in general, Q values are significantly smaller for the entire frequency range used (6–30 Hz) than those found in other volcanic and tectonic areas. The attenuation for P-waves is greater than for S-waves in the frequency independent methods, with a Qβ/QP ratio that ranges between 1.9 and 3.2. Comparing the Q-factor obtained for S-waves we have observed clear differences as a function of the method used; the coda normalization method has supplied significantly higher Q values (Qd) than the other two methods (Qβ). We have interpreted this discrepancy as an effect of the methods: coda normalization and single back-scattering methods eliminate the contribution of the near-surface attenuation in their Q values. Comparing both Qβ and Qd we have estimated the near-surface attenuation under the recording site, named Qκ. On the other hand, we have observed that Qd has anomalous frequency dependence, with a minimum value at 21 Hz. This pattern is interpreted as an effect of strong scattering of the seismic waves in the source area of the earthquakes. Qc values depend clearly with frequency and lapse time and the lapse time dependence could be interpreted as a depth dependence of the seismic attenuation in Deception Island volcano, Antarctica. The obtained Q values have allowed us to separate the contribution of intrinsic and scattering attenuation, deriving that the scattering attenuation is predominant over the intrinsic effects. Finally, in order to investigate how the heterogeneous medium of the volcanic island could produce other effects, we have checked whether it produces polarization of the shear-waves. The preliminary results of the polarization direction indicate a main E–W strain direction. All these evidences reveal the strongly heterogeneous structure of Deception Island volcano.