We estimated the coda-wave attenuation quality factor Qc at the Tres Vírgenes volcanic area from 26 local seismic events recorded by a portable network of seven digital stations. The quality factor Qc was estimated using the single-scattering attenuation model of Sato (1977), in the frequency range from 6 to 24 Hz. The magnitudes of the analyzed events range from 1 to 3. These events occurred at hypocentral depths ranging from 3 to 6 km and were recorded at epicentral distances of up to 30 km. Within the variation of Qc, six out of seven stations analyzed showed quite similar values at all frequencies. Data from these six stations led to an average Qc given by Qc(f) = (50 ± 3) f(0.65±0.20) in the indicated frequency range. For the seventh station, which is located near a steam production well, anomalous low Qc values were obtained. For this particular site we found Qc(f) = (4.3 ± 0.6)f(1.33±0.05). In this zone, we observed an abnormally strong reduction of Qc, leading to attenuation values that are three to four times larger than for the whole area of study. Results from other geophysical studies suggest the presence of a highly conductive body below this particular area. A correlation of the observed coda attenuation with a geothermal anomaly is suggested.

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