A document by Miriam Gauntlett. Click on the document to view its contents.

Microseismicity, induced by the sliding of a glacier over its bed, can be used to characterize frictional properties of the ice-bed interface, which are a key parameter controlling ice stream flow. We use naturally occurring seismicity to monitor spatiotemporally varying bed properties at Rutford Ice Stream, West Antarctica. We locate 230000 micro-earthquakes with local magnitudes from –2.0 to –0.3 using 90 days of recordings from a 35-station seismic network located ~40 km upstream of the grounding line. Events exclusively occur near the ice-bed interface and indicate predominantly flow-parallel stick-slip. They mostly lie within a region of interpreted stiff till and along the likely stiffer part of mega-scale glacial landforms. Within these regions, micro-earthquakes occur in spatially (<100 m radius) and temporally (mostly 1-5 days activity) restricted event-clusters (up to 4000 events), which exhibit an increase, followed by a decrease, in event magnitude with time. This may indicate event triggering once activity is initiated. Although ocean tides modulate the surface ice flow velocity, we observe little periodic variation in overall event frequency over time and conclude that water content, bed topography and stiffness are the major factors controlling microseismicity. Based on variable rupture mechanisms and spatiotemporal characteristics, we suggest the event-clusters relate to three end-member types of bed deformation: (1) continuous creation and seismogenic destruction of small-scale bed-roughness, (2) ploughed clasts and (3) flow-oblique deformation during landform-formation or along bedrock outcrops. This indicates that multiple processes, simultaneously active during glacial sliding, can accommodate stick-slip behaviour and that the bed continuously reorganizes.

Icequakes, microseismic earthquakes at glaciers, offer critical insights into the dynamics of ice sheets. For the first time in the Antarctic, we explore the use of fibre optic cables as Distributed Acoustic Sensors (DAS) as a new approach for monitoring basal icequakes. Fibre was deployed on the ice surface at Rutford Ice Stream, in two different configurations. We compare the performance of DAS with a conventional geophone network for: microseismic detection and location; resolving source and noise spectra; source mechanism inversion; and measuring anisotropic shear-wave splitting parameters. The DAS arrays detect fewer events than the geophone array. However, DAS is superior to geophones for recording the microseism signal, suggesting the applicability of DAS for ambient noise interferometry. We also present the first full-waveform source mechanism inversions using DAS anywhere, successfully constraining the horizontal stick-slip nature of the icequakes. In addition, we develop an approach to use a 2D DAS array geometry as an effective multi-component sensor capable of accurately characterising shear-wave splitting due to anisotropy of the ice fabric. Although our observations originate from a glacial environment, the methodology and implications of this work are relevant for employing DAS in other microseismic environments.

The crystal orientation fabric of glacier ice severely impacts its strength and flow. Crystal fabric is therefore an important consideration when modelling ice flow. Here, we show that shear wave splitting (SWS) of glacial microseismicity can be used to invert for seismic anisotropy and ice fabric at Rutford Ice Stream (RIS). RIS is a fast-flowing Antarctic ice stream, a setting crucial for informing flow models. We present ~2000,000 SWS measurements from glacial microseismicity, registered at a 38-station seismic network located ~40 km upstream the grounding line. A representative subset of this data is inverted for ice fabric. Due to the character of SWS, which accumulates along the ray path, our method works best if additional information on the depth structure of the ice is available, which are radar measurements in our case. We find that the following three-layer model fits the data best: a broad vertical cone near the base of RIS (500 m thick), a thick vertical girdle, orientated perpendicular to flow, in the middle (1200 m thick) and a tilted cone fabric in the uppermost 400 m. Such a fabric causes a depth-dependent strength profile of the ice with the middle layer being ~3.5 times harder to deform along flow than across flow. At the same time, the middle layer is a factor ~16 softer to shear than to compression or extension along flow. If such a configuration is representative for fast-flowing ice streams, it would call for a more complex integration of viscosity in ice sheet models.

Uturuncu volcano in southern Bolivia is something of a “zombie” volcano – presumed dead, but showing signs of life. The volcano has not erupted in 250 kyr, but is exhibiting unrest in the form of ground deformation, seismicity, and active fumaroles. Elucidating the subsurface structure of the volcano is key for interpreting this recent unrest. Magnetotelluric measurements revealed alternating high and low resistivity anomalies at depths <10 km beneath the volcano, with a low-resistivity anomaly directly beneath Uturuncu. A key question is, what is the nature of this anomaly? To what extent is it partial melt, a hydrothermal brine reservoir, or a mature ore body? Knowing the density of this anomaly could distinguish between these scenarios, but existing density models of the area lack sufficient resolution. To address this issue, we collected additional gravity measurements on the Uturuncu edifice with 1.5 km spacing in November 2018. Gradient analysis and geophysical inversion of these data revealed several features: a 5 km diameter, high density anomaly beneath the summit of Uturuncu (1 – 3 km elev.), a 20 km diameter ring-shaped negative density anomaly around the volcano (-3 – 4 km elev.), a NNE trending, positive density anomaly northwest of the volcano (0 – 4 km elev.), and a NW trending, negative density anomaly to the southeast. These structures often (but not always) align with resistivity anomalies, features in new seismic tomography models, and relocated earthquake hypocenters. Based on a joint analysis of these data, we interpret the positive density anomaly as a crystallizing dacite pluton, and the negative density ring anomaly as a zone of hydrothermal alteration. Earthquakes around the edges of the crystallizing pluton may represent escaping fluids as the magma cools. The high density anomaly to the northwest likely represents a solidified pluton, and the low density anomaly to the southeast may represent a fractured fault zone. We posit that the alternating zones of high and low resistivity anomalies represent zones of low and high fluid/brine content, respectively. 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.

Uturuncu volcano in southern Bolivia is a member of a distinctive class of volcanoes – systems that show unrest despite not having erupted in the Holocene. Uturuncu has not erupted in 250 kyr, but has been deforming (uplift with a moat of subsidence) for several decades, along with seismic swarms and active, sulfur-encrusted fumaroles. Our work builds on previous geophysical imaging at Uturuncu by jointly analyzing multidisciplinary datasets, focusing on imaging the shallow (<15 km depth below surface) structure of the system with geophysical and geochemical data. Whereas previous research pointed to andesite melt at depths >15 km depth, results were ambiguous as to what proportions of melts vs. brines are present at shallower depths. Identifying fluids (melt, brine, etc.) and structures at shallow depths is key for evaluating the hazard potential of the volcano and understanding the source of the unrest. We present new results from gravimetry, seismology (hypocenter relocation, seismic velocity and attenuation tomography), gas geochemistry, and InSAR observations. The results point to an extensive and active hydrothermal system extending ~20 km laterally and ~10 km vertically from Uturuncu, with possible connections at depth to the deeper magmatic system. A combined view of the new density, seismic velocity and attenuation models, and the existing resistivity model is crucial for revealing key features of the hydrothermal system: a vapour-rich conduit beneath Uturuncu (low resistivity/high attenuation column extending from 1.5 to 12.5 km depth), an extensive alteration zone surrounding Uturuncu (complex zone of annular shaped anomalies surrounding Uturuncu from 1.5 to 12.5 km depth), and a possible zone of sulfide deposition just below the western flank of Uturuncu at 1.5 km depth (high density/low resistivity/high attenuation). High fluxes of diffuse CO2 degassing at sub-magmatic temperatures and a small area directly above a low resistivity anomaly subsiding from 2014 to 2017 show that the hydrothermal system is currently active. Analyzed jointly, this multidisciplinary data set suggests that current activity within the shallow structure at Uturuncu is dominated by hydrothermal, rather than magmatic processes.

Hydraulically-forced crevassing is thought to reduce the stability of ice shelves and ice sheets, affecting structural integrity and providing pathways for surface meltwater to the bed. It can cause ice shelves to collapse and ice sheets to accelerate into the ocean. However, direct observations of the hydraulically-forced crevassing process remain elusive. Here we report a new, novel method and observations that use icequakes to directly observe crevassing and determine the role of hydrofracture. Crevasse icequake depths from seismic observations are compared to a theoretically derived maximum-dry-crevasse-depth. We observe icequakes below this depth, suggesting hydrofracture. Furthermore, icequake source mechanisms provide insight into the fracture process, with predominantly opening cracks observed, which have opening volumes of tens to hundreds of cubic meters. Our method and findings provide a framework for studying a critical process, key for the stability of ice shelves and ice sheets, and hence rates of future sea-level rise.

Uturuncu volcano is situated in the Bolivian Andes, directly above the world’s largest crustal body of silicic partial melt, the Altiplano-Puna Magma Body (APMB). Uturuncu last erupted 250,000 years ago, yet is seismically active and lies at the centre of a 70 km diameter uplifted region. Here, we analyse seismicity from 2009 to 2012. Our earthquake locations, using a newly developed velocity model, delineate the top and bottom of the APMB, reveal individual faults, and reconcile differences in depth distribution between previous studies. Spatial clustering analysis of these earthquakes reveals the orientations of the faults, which match stress orientations from seismic anisotropy. Earthquake b-values derived from moment magnitudes (1.4) differ significantly from those using local magnitude measurements (0.8). We suggest that, if possible, moment magnitudes should always be used for accurate b-value analysis. We interpret b-values > 1 in terms of fluid-enhanced seismicity. Shallow seismicity local to Uturuncu yields b-values > 1.1 with some temporal variation, suggesting fluid migration along pre-existing faults in a shallow hydrothermal system, likely driven by advection from the APMB. Intriguingly, events deeper than the APMB also yield large b-values (1.4), mapping the ascent into the lower crust of fluids originating from a subducting slab. Cumulatively, these results provide a picture of an active magmatic system, where fluids are exchanged across the more ductile APMB, feeding a shallow, fault-controlled hydrothermal system. Such pathways of fluid ascent may influence our understanding of arc volcanism, control future volcanic eruptions and promote the accumulation of shallow hydrothermal ore deposits.

Improving our understanding of glacial sliding is crucial for constraining basal drag in ice dynamics models. We use icequakes, sudden releases of seismic energy as the ice slides over the bed, to provide geophysical observations that can be used to aid understanding of the physics of glacial sliding and constrain ice dynamics models. These icequakes are located at the bed of an alpine glacier in Switzerland and the Rutford Ice Stream, West Antarctica, two extremes of glacial settings and spatial scales. We investigate a number of possible icequake source mechanisms by performing full waveform inversions to constrain the fundamental physics and stress release during an icequake stick-slip event. Results show that double-couple mechanisms best describe the source for the events from both glacial settings and the icequakes originate at or very near the ice-bed interface. We also present an exploratory method for attempting to measure the till shear modulus, if indirect, reflected icequake radiation is observed. The results of this study increase our understanding of how icequakes are associated with basal drag while also providing the foundation for a method of remotely measuring bed shear strength.

Mapping fluid accumulation in the crust is pertinent for numerous applications including volcanic hazard assessment, geothermal energy generation and mineral exploration. Here, we use seismic attenuation tomography to map the distribution of fluids in the crust below Uturuncu volcano, Bolivia. Seismic P-wave and S-wave attenuation, as well as their ratio (QP/QS), constrain where the crust is partially and fully fluid-saturated. Seismic anisotropy observations further constrain the mechanism by which the fluids accumulate, predominantly along aligned faults and fractures in this case. Furthermore, subsurface pressure-temperature profiles and conductivity data allow us to identify the most likely fluid composition. We identify shallow regions of both dry and H2O/brine-saturated crust, as well as a deeper supercritical H2O/brine column directly beneath Uturuncu. Our observations provide a greater understanding of Uturuncu’s transcrustal hydrothermal system, and act as an example of how such methods could be applied to map crustal fluid pathways and hydrothermal/geothermal systems elsewhere.