Attenuation of Coda Waves at the Tres Virgenes Volcanic Area, Baja California Sur, Mexico

Abstract

We estimated the coda-wave attenuation quality factor Q c at the Tres Virgenes volcanic area from 26 local seismic events recorded by a portable network of seven digital stations. The quality factor Q c 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 Q c, six out of seven stations analyzed showed quite similar values at all frequencies. Data from these six stations led to an average Q c given by Q c( 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 Q c values were obtained. For this particular site we found Q c( f ) = (4.3 ± 0.6) f (1.33±0.05). In this zone, we observed an abnormally strong reduction of Q c, 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. Manuscript received 27 February 2000.