Coda attenuation in the area of El Chichón volcano, Chiapas, Mexico

Abstract

Coda-wave attenuation is investigated using the single scattering approximation on analog records from stations of a telemetered network situated close to El Chichón volcano in southern Mexico, as well as from temporary stations deployed during 1982. The single scattering model was applied to the envelope of coda waves to characterize the coda decay rate. We obtained values of Q c which, according to the filter settings of the seismographs, and the average period measured in the traces, correspond to frequencies of about 1.3 ± 0.3 Hz. Results indicate apparently larger attenuation than observed with similar techniques in other regions of Mexico, including the Mexican Volcanic Belt. Within the limitation imposed by the data, the spatial distribution of coda Q with respect to the volcanic edifice was investigated. For paths sampling the near vicinity of the volcano a value of approximately Q c = 30 was obtained. The highest attenuation results are obtained for volumes with a slight E-W tendency, with Q c values of 28 and 25, respectively. The location of a gravity high as well as microseismic activity correlates well with this result, which is interpreted as due to the probable presence of an intrusive magma body. Lower attenuation is obtained for volumes sampling deeper than the Moho, reaching a value of Q c ≈ 87, suggesting that rocks below this boundary play a major role in controlling the overall attenuation of paths traversed by coda waves. The results also suggest a decrease in attenuation towards the south and north possibly reflecting a decrease in magmatic processes in both of these directions.