Uturuncu volcano in southern Bolivia last erupted around 250 ka but is exhibiting signs of recent activity, including over 50 yr of surface uplift, elevated seismic activity, and fumarolic activity. We studied the spatial and temporal scales of surface deformation from 1992 to 2021 to better understand subsurface activity. We tracked Uturuncu’s recent deformation using interferometric synthetic aperture radar (InSAR) data and the global navigation satellite system (GNSS) station UTUR, located near Uturuncu’s summit. We observed a spatially coherent signal of uplift from 2014 to 2021 from Sentinel-1 A/B satellites that indicates the Altiplano-Puna magma body, located 19–24 km below ground level, and previously noted as the source of the large region of deformation, is still active. The ground is now uplifting at a rate of ~3 mm/yr compared to prior rates of ~10 mm/yr. We corroborated this waning uplift with in situ data from station UTUR. We combined the Sentinel-1 data with TerraSAR-X interferograms to constrain an ~25 km2 region of subsidence located 11 km SSW of Uturuncu, with a source depth of 2.1 km below ground level to an active period of ~2.5 yr with ~5 mm/yr subsidence. We developed a conceptual model that relates these varying depths and time scales of activity in a transcrustal magmatic system. We associate the surface uplift with pressurization from ascending gases and brines from magmatic reservoirs in the midcrust. We infer the existence of brine lenses in the shallow hydrothermal system based on low subsurface resistivity correlated with surface subsidence.
The recent identification of unrest at multiple volcanoes that have not erupted in over 10 kyr presents an intriguing scientific problem. How can we distinguish between unrest signaling impending eruption after kyr of repose and non-magmatic unrest at a waning volcanic system? After ca. 250 kyr without a known eruption, in recent decades Uturuncu volcano in Bolivia has exhibited multiple signs of unrest, making the classification of this system as “active”, “dormant”, or “extinct” a complex question. Previous work identified anomalous low resistivity zones at <10 km depth with ambiguous interpretations. We investigate subsurface structure at Uturuncu with new gravity data and analysis, and compare these data with existing geophysical data sets. We collected new gravity data on the edifice in November 2018 with 1.5 km spacing, ±15 μGal precision, and ±5 cm positioning precision, improving the resolution of existing gravity data at Uturuncu. This high quality data set permitted both gradient analysis and full 3-D geophysical inversion, revealing a 5 km diameter, positive density anomaly beneath the summit of Uturuncu (1.5–3.5 km depth) and a 20 km diameter arc-shaped negative density anomaly around the volcano (0.5–7.5 depth). These structures often align with resistivity anomalies previously detected beneath Uturuncu, although the relationship is complex, with the two models highlighting different components of a common structure. Based on a joint analysis of the density and resistivity models, we interpret the positive density anomaly as a zone of sulfide deposition with connected brines, and the negative density arc as a surrounding zone of hydrothermal alteration. Based on this analysis we suggest that the unrest at Uturuncu is unlikely to be pre-eruptive. This study shows the value of joint analysis of multiple types of geophysical data in evaluating volcanic subsurface structure at a waning volcanic center.
We present evidence of volcano‐tectonic interactions at Sabancaya volcano that we relate to episodic magma injection and high regional fluid pore pressures. We present a surface deformation time series at Sabancaya including observations from ERS‐1/2, Envisat, Sentinel‐1, COSMO‐SkyMed, and TerraSAR‐X that spans June 1992 to February 2019. These data show deep‐seated inflation northwest of Sabancaya from 1992–1997 and 2013–2019, as well as creep and rupture on multiple faults. Afterslip on the Mojopampa fault following a 2013 MW 5.9 earthquake is anomalously long lived, continuing for at least 6 years. The best fit fault plane for the afterslip is right‐lateral motion on an EW striking fault at 1 km depth. We also model surface deformation from two 2017 earthquakes (MW 4.4 and MW 5.2) on unnamed faults, for which the best fit models are NW striking normal faults at 1–2 km depth. Our best fit model for a magmatic inflation source (13 km depth, volume change of 0.04 to 0.05 km3 yr−1) induces positive Coulomb static stress changes on these modeled fault planes. Comparing these deformation results with evidence from satellite thermal and degassing data, field observations, and seismic records, we interpret strong pre‐eruptive seismicity at Sabancaya as a consequence of magmatic intrusions destabilizing tectonic faults critically stressed by regionally high fluid pressures. High fluid pressure likely also promotes fault creep driven by static stress transfer from the inflation source. We speculate that combining high pore fluid pressures with sufficiently large, offset magmatic inflation can promote strong earthquakes during volcanic unrest.
Fluids are present in much of Earth's crust. Mapping where and why these fluids accumulate, as well as identifying their composition are critical questions in the earth sciences. For example, understanding where magma resides is important for volcanic hazard assessment, mapping the extent of geothermal systems is pertinent for maximizing production efficiency, and identifying the location and properties of metal-rich brines is relevant for mineral exploration. The question that this study addresses is: can seismic attenuation combined with seismic anisotropy be used to map the location of fluids and, in combination with auxiliary data, identify fluid composition? Here, we test this hypothesis at Uturuncu volcano, Bolivia.Uturuncu volcano sits within the Bolivian Andes. The volcano last erupted 250,000 yr ago (Muir et al., 2015), yet has exhibited significant uplift at rates of up to 1 cm/yr (Gottsmann et al., 2018;Pritchard et al., 2018). Uturuncu lies ∼20 km above the Altiplano-Puna Magma (or Mush) Body (APMB), Earth's largest body of silicic partial melt (Pritchard et al., 2018). This melt heats the crust above and potentially provides a source of ascending metalrich volatiles (Blundy et al., 2021). Uturuncu provides an ideal location for attempting to image and identify fluids, since the host crust isolated from the volcanic system is likely predominantly unsaturated except near surface rivers/lakes, while a shallow, partially saturated hydrothermal system likely exists under the volcano that is sustained via heat and volatiles from the APMB (Gottsmann et al., 2022).
for laterally and vertically complex volcanic plumbing system at Sabancaya • High fluid pressure at Sabancaya promotes strong seismicity during 2012-2019 eruptive period • High fluid pressure and static stress transfer from deep inflation promote long-lived fault creep
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