Reflected Phases Apparent in Local Event Seismograms at Uturuncu Volcano, Bolivia

Abstract:

Uturuncu volcano is located in the central Andes at 22.27° S, 67.18° W. Despite 270,000 years of volcanic dormancy, a satellite geodetic InSAR survey by Pritchard and Simons (2002) shows an inflation rate of 1.5-2 cm/yr over an area with a width of 70 km (Pritchard and Simons, 2002). The source was modeled, using a Mogi source, to 15-17 km deep located ~3 km to the southwest of Uturuncu’s summit. Current studies have found a source area of decreased density with respect to the crust and with a Vp/Vs ratio of > 1.9, with a depth matching that derived from the deformation signature [Potro et al., 2013; M.E. West, H. McFarlin, D. Christensen written comm.]

Determining the size, geographic location, source depth, and source shape of deformation at Uturuncu volcano is important because this can indicate either (or a combination of) injection of magma from depth into the system, melting of crustal rock from a previous injection, or the build-up of pressure in a hydrothermal system (Pritchard and Simons, 2002). We are using phases arriving between the P and S phases to constrain some of the deformation source parameters. These result from the interaction of seismic waves with the attenuating source, believed to be local thickening of the regional shallow crustal magma body. Analysis of 200 local events spanning the period of PLUTONS seismic network operation (April 2010 to October 2012) reveals that 55 of these events show at least one station with a phase arrival between the P and S phases. If this trend holds as expected during analysis of the other 377 shallow local events, then 25.7% of the local events will generate an anomalous phase. No observable differences in the frequency of phase occurrence have been noted with time; however, the locations of earthquakes generating this phase strongly cluster (39 of 55) within a 10 km radius of the volcano, skewed to the south and west. There is the possibility of a NE-SW trend between the earthquake epicenter and the locations of the stations showing phases, suggesting anisotropy of the phase source body.