Seismic Attenuation, Time Delays and Raypath Bending of Teleseisms Beneath Uturuncu Volcano, Bolivia

A set of 14 teleseismic earthquakes was studied to determine how wave propagation was affected by a presumed magma body beneath Uturuncu volcano, Bolivia. Teleseisms are suitable for study because they are relatively long period, contain purely P waves, and have near-vertical incidence angles. The number of events is small but the events have good signal-to-noise ratios and very similar waveforms for each event so that reliable measurements could be made of arrival times and amplitudes. Attenuation of amplitudes occurs in a NW-SE trend beneath the volcano, 14 by 34 km (long axis NW-SE). Calculated values of the quality factor Qp are an average of 12.4, with extreme values as low as 1.8. These calculations are based on the assumption that the highest amplitude observed is the “true” amplitude, and all others have been attenuated. The average thickness of the anomaly is 10.2 km, and the center is ~20 km SE of the summit, within the area of surface uplift measured geodetically. Time delays of up to 0.8 s were also observed. The pattern of attenuation and relative time delays together showed four trends: fast and not attenuated (normal crust), slow and attenuated (partial melt), fast and attenuated (likely high fracture density), and slow but not attenuated (possible deep low Vp structure). Back azimuth differences of up to 60° were observed. In nearly all cases, azimuths were rotated into directions parallel to local rock fabric, suggesting that shallow crustal properties affected near-surface wave propagation. Overall results suggest partial melt as high as 10%–20% in a region of varying thickness, low Bouguer gravity and resistivity, high Vp/Vs, persistent seismicity, and overlapping a locus of recent uplift