S-wave attenuation characteristics in the Galeras volcanic complex (south western Colombia)

Vertical-component, short period seismograms from 435 well located volcano-tectonic earthquakes are used to estimate S-wave attenuation in the Galeras volcanic complex (south western Colombia) using coda waves. The coda magnitudes (Mc) of the events are less than 2. Event depths are less than 10 km and hypocentral distances up to 16 km. Intrinsic absorption (Qi−1) and scattering attenuation (Qs−1) are estimated by means of a fitting procedure between the observed and synthetic coda envelopes in four frequency bands (1–2, 2–4, 4–8, and 8–12 Hz). The observations are compared with the theoretical predictions by an accurate approximate analytical solution of the radiative transfer equation which is based on the assumptions of multiple isotropic scattering, impulsive isotropic point source, and a medium with homogeneous scattering and absorption properties. Results show that scattering is strong and it constitutes the predominant attenuation effect in this region. In the frequency range analyzed in this study the values of the mean free path for scattering of S waves range between 2.7 ≤ l ≤ 8.1 km, which are clearly higher than those obtained in other volcanic regions of the world, but about two orders of magnitude smaller than average estimates for the Earth's crust. The characteristic length scale of intrinsic absorption gives values of 2.5 ≤ la ≤ 77 km, which are on the same order as the usual values for the Earth's crust.