The Altiplano-Puna Volcanic Complex (APVC) in the central Andes is the product of an ignimbrite ''flare-up'' of world class proportions (DE SILVA, 1989). The region has been the site of large-scale silicic magmatism since 10 Ma, producing 10 major eruptive calderas and edifices, some of which are multipleeruption resurgent complexes as large as the Yellowstone or Long Valley caldera. Seven PASSCAL broadband seismic stations were operated in the Bolivian portion of the APVC from October 1996 to September 1997 and recorded teleseismic earthquakes and local intermediate-depth events in the subducting Nazca plate. Both teleseismic and local receiver functions were used to delineate the lateral extent of a regionally pervasive 20-km-deep, very low-velocity layer (VLVL) associated with the APVC. Data from several stations that sample different parts of the northern APVC show large amplitude Ps phases from a lowvelocity layer with Vs £ 1.0 km/s and a thickness of 1 km. We believe the crustal VLVL is a regional sill-like magma body, named the Altiplano-Puna magma body (APMB), and is associated with the source region of the Altiplano-Puna Volcanic Complex ignimbrites (CHMIELOWSKI et al., 1999).Large-amplitude P-SH conversions in both the teleseismic and local data appear to originate from the top of the APMB. Using the programs of LEVIN and PARK (1998), we computed synthetic receiver functions for several models of simple layered anisotropic media. Upper-crustal, tilted-axis anisotropy involving both Vp and Vs can generate a ''split Ps'' phase that, in addition to the Ps phase from the bottom of a thin isotropic VLVL, produces an interference waveform that varies with backazimuth. We have forward modeled such an interference pattern at one station with an anisotropy of 15%-20% that dips 45°within a 20-km-thick upper crust. We develop a hypothesis that the crust above the ''magma body'' is characterized by a strong, tiltedaxis, hexagonally symmetric anisotropy. We speculate that the anisotropy is due to aligned, fluid-filled cracks induced by a ''normal-faulting'' extensional strain field associated with the high elevations of the Andean Puna.

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[1] The Altiplano-Puna volcanic complex (APVC) is located in the central Andes and covers an area over 50,000 km 2 . The style of volcanism is predominately caldera-forming ignimbrite eruptions active between 10 and 1 Ma. Chmielowski et al. [1999] used teleseismic events, recorded from seven portable broadband seismic stations deployed in the APVC, to identify a 1-km-thick very low-velocity zone at a depth of 19 km. Based on the correlation with surface volcanics, the extremely low shear velocities of 1 km/s or less, and the depth of the layer, the low-velocity zone was interpreted to be a ''magma body.'' Using local events recorded during the same seismic deployment, we have improved the crustal model for the APVC and detected seismic anisotropy in the crust above the low-velocity zone. Using a hexagonal symmetry anisotropy code, we computed synthetic receiver functions for many models of layered anisotropic media and found two different models consistent with the data. In both models, approximately 20-30% anisotropy is present in a 3-km-thick surface low-velocity layer, and 15-20% anisotropy is present in the remaining crust above the buried low-velocity zone. The first model has the 1-km-thick low-velocity zone at 17 km, while the alternate model has the low-velocity zone at 14-km depth overlain by a 4-km-thick transition zone of intermediate velocities. This anisotropy may be due to the fracture system by which magma migrates to the surface from the midcrustal sill-like magma body.

Oil and natural gas production from shale formations using hydraulic fracturing or “hydrofracking” techniques has grown rapidly since 2008 and represents a vital and growing domestic energy resource. The waste fluids from increased production (called “brine”) are typically injected into deep underground disposal wells to avoid discharge into streams and other sensitive drinking water supplies. The Environmental Protection Agency estimates that approximately 400 million gallons of brine are currently disposed of on a daily basis in more than 28,000 wells in the United States, and the volume of injection continues to grow dramatically. This process has recently been implicated as the source of some potentially induced seismicity in gas-producing states such as Oklahoma, Texas, Arkansas, and Ohio.

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