Teleseismic attenuation, time delays, and raypath bending, and local reflected phases at Uturuncu volcano, Bolivia

A set of 13 teleseismic earthquakes sourced to the NW (4, Japan Subduction Zone), SE (5, South Sandwich Subduction Zone), and SW (4, Kermadec-Tonga Subduction Zones) was studied to determine how wave propagation was affected by a presumed magma body beneath Uturuncu. 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 33 km, with calculated values of quality factor Qas low as 1.7, suggesting strong seismic attenuation. Relative time delays (between the theoretical and observed travel times) of up to 0.8 sec 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). Results suggest partial melt as high as 16-60% in a region of low Bouguer gravity, high Vp/Vs, persistent seismicity, and overlapping a locus of uplift. Realistically, percent partial melt values above 30% are unlikely and therefore, to account for this, the anomaly would need to have a greater thickness along the raypath for the stations showing 30% or more, thus giving a mean partial melt zone thickness of 24.7 km for an assumed uniform percent partial melt value of 20%. Additionally, there is evidence of reflected phases from some local earthquakes interacting with the top of the mid-crustal magma body. These can be used to reduce depth uncertainty in earthquake locations as well as to determine parameters of the magma body itself, such as determining whether the contact with the country rock is sharp or gradual, resolving the presence and direction of anisotropy, and estimating whether the surface of the magma body is flat or irregular.