Dynamics of the Askja caldera July 2014 landslide, Iceland, from seismic signal analysis: precursor, motion and aftermath

Landslide hazard motivates the need for a deeper understanding of the events that occur before, during, and after catastrophic slope failures. Due to the destructive nature of such events, in situ observation is often difficult or impossible. Here, we use data from a network of 58 seismic stations to characterise a large landslide at the Askja caldera, Iceland, on 21 July 2014. High data quality and extensive network coverage allow us to analyse both long- and short-period signals associated with the landslide, and thereby obtain information about its triggering, initiation, timing, and propagation. At long periods, a landslide force history inversion shows that the Askja landslide was a single, large event starting at the SE corner of the caldera lake at 23:24:05 UTC and propagating to the NW in the following 2 min. The bulk sliding mass was 7–16 × 1010 kg, equivalent to a collapsed volume of 35–80 × 106 m3. The sliding mass was displaced downslope by 1260 ± 250 m. At short periods, a seismic tremor was observed for 30 min before the landslide. The tremor is approximately harmonic with a fundamental frequency of 2.3 Hz and shows time-dependent changes of its frequency content. We attribute the seismic tremor to stick-slip motion along the landslide failure plane. Accelerating motion leading up to the catastrophic slope failure culminated in an aseismic quiescent period for 2 min before the landslide. We propose that precursory seismic signals may be useful in landslide early-warning systems. The 8 h after the main landslide failure are characterised by smaller slope failures originating from the destabilised caldera wall decaying in frequency and magnitude. We introduce the term "afterslides" for this subsequent, declining slope activity after a large landslide

Tidal and thermal stresses drive seismicity along a major Ross Ice Shelf rift

Author Posting. © American Geophysical Union, 2019. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Geophysical Research Letters, 46(12), (2019): 6644-6652, doi:10.1029/2019GL082842.Understanding deformation in ice shelves is necessary to evaluate the response of ice shelves to thinning. We study microseismicity associated with ice shelf deformation using nine broadband seismographs deployed near a rift on the Ross Ice Shelf. From December 2014 to November 2016, we detect 5,948 icequakes generated by rift deformation. Locations were determined for 2,515 events using a least squares grid‐search and double‐difference algorithms. Ocean swell, infragravity waves, and a significant tsunami arrival do not affect seismicity. Instead, seismicity correlates with tidal phase on diurnal time scales and inversely correlates with air temperature on multiday and seasonal time scales. Spatial variability in tidal elevation tilts the ice shelf, and seismicity is concentrated while the shelf slopes downward toward the ice front. During especially cold periods, thermal stress and embrittlement enhance fracture along the rift. We propose that thermal stress and tidally driven gravitational stress produce rift seismicity with peak activity in the winter.NSF grants PLR‐1142518, 1141916, and 1142126 supported S. D. Olinger and D. A. Wiens, R. C. Aster, and A. A. Nyblade respectively. NSF grant PLR‐1246151 supported P. D. Bromirski, P. Gerstoft, and Z. Chen. NSF grant OPP‐1744856 and CAL‐DPR‐C1670002 also supported P. D. Bromirski. NSF grant PLR‐1246416 supported R. A. Stephen. The Incorporated Research Institutions for Seismology (IRIS) and the PASSCAL Instrument Center at New Mexico Tech provided seismic instruments and deployment support. The RIS seismic data (network code XH) are archived at the IRIS Data Management Center (http://ds.iris.edu/ds/nodes/dmc/). S. D. Olinger catalogued and located icequakes, analyzed seismicity and environmental forcing, and drafted the manuscript. D. A. Wiens and B. P. Lipovsky provided significant contributions to the analysis and interpretation of results and to the manuscript text. D. A. Wiens, R. C. Aster, A. A. Nyblade, R. A. Stephen, P. Gerstoft, and P. D. Bromirski collaborated to design and obtain funding for the deployment. D. A. Wiens, R. C. Aster, R. A. Stephen, P. Gerstoft, P. D. Bromirski, and Z. Chen deployed and serviced seismographs in Antarctica. All authors provided valuable feedback, comments, and edits to the manuscript text. Special thanks to Patrick Shore for guidance throughout the research process.2019-11-2

Crack wave resonances within the basal water layer

Hydraulic processes within and beneath glacial bodies exert a far-reaching control on ice flow through their influence on basal sliding. Within the subglacial system, rapid changes in these processes may excite resonances whose interpretation requires an understanding of the underlying wave mechanics. Here, we explore these mechanics using observations from a kHz-sampled pressure sensor installed in a borehole directly above the hard granite bedrock of a temperate mountain glacier in Switzerland. We apply a previously established theory of wave propagation along thin, water-filled structures such as water-filled voids, basal water layers, or hydraulic fractures. Within such structures, short-wavelength waves experience restoring forces due to compressibility and are composed of sound waves. Long-wavelength resonances, in contrast, experience restoring forces due to elasticity and are composed of anomalously dispersed crack waves or Krauklis waves. Our borehole observations confirm the occurrence of both sound and crack waves within the basal water layer. Using both the resonance frequencies and attenuation of recorded crack waves we estimate thickness, aperture and length of the resonating basal water layer patch into which we drilled. We demonstrate that high-frequency observations of subglacial hydraulic processes provide new insights into this evolving dynamic system.ISSN:0260-3055ISSN:1727-564

High Frequency Pressure Oscillations at the Bed of Rhonegletscher

Resonating water-filled fractures in glaciers have been observed repeatedly by their emission of elastic waves with passive seismic instrumentation. At the ice-bedrock interface of glaciers, such hydraulic fractures are often assumed to take the form of thin sheets of water. During a field campaign on the tounge of Rhonegletscher in August 2017, we drilled three boreholes to the glacier bed by using a hot water drill. With a kHz-sampled pressure sensor located approximately one meter above the bedrock, we measured crack waves and sound waves originating from a resonating water-filled fracture at the ice-bedrock interface. Two types of resonance modes were observed: short-lasting oscillations with durations of a few seconds, and long-lasting ones with duration of about one minute.Here we estimate the aperture and length of the resonating hydraulic fracture using both its frequency and attenuation caracteristics

Ocean Coupling Limits Rupture Velocity of Fastest Observed Ice Shelf Rift Propagation Event

Abstract The Antarctic ice sheet is buttressed by floating ice shelves that calve icebergs along large fractures called rifts. Despite the significant influence exerted by rifting on ice shelf geometry and buttressing, the scarcity of in situ observations of rift propagation contributes considerable uncertainty to understanding rift dynamics. Here, we report the first‐ever seismic recording of a multiple‐kilometer rift propagation event. Remote sensing and seismic recordings reveal that a rift in the Pine Island Glacier Ice Shelf extended 10.53 km at a speed of 35.1 m/s, the fastest known ice fracture at this scale. We simulate ocean‐coupled rift propagation and find that the dynamics of water flow within the rift limit the propagation rate, resulting in rupture two orders of magnitude slower than typically predicted for brittle fracture. Using seismic recordings of the elastic waves generated during rift propagation, we estimate that ocean water flows into the rift at a rate of at least 2,300 m3/s during rift propagation and causes mixing in the subshelf cavity. Our observations support the hypotheses that large ice shelf rift propagation events are brittle, hydrodynamically limited, and exhibit sensitive coupling with the surrounding ocean

Monitoring southwest Greenland’s ice sheet melt with ambient seismic noise

The Greenland ice sheet presently accounts for ~70% of global ice sheet mass loss. Because this mass loss is associated with sea-level rise at a rate of 0.7 mm/year, the development of improved monitoring techniques to observe ongoing changes in ice sheet mass balance is of paramount concern. Spaceborne mass balance techniques are commonly used; however, they are inadequate for many purposes because of their low spatial and/or temporal resolution. We demonstrate that small variations in seismic wave speed in Earth’s crust, as measured with the correlation of seismic noise, may be used to infer seasonal ice sheet mass balance. Seasonal loading and unloading of glacial mass induces strain in the crust, and these strains then result in seismic velocity changes due to poroelastic processes. Our method provides a new and independent way of monitoring (in near real time) ice sheet mass balance, yielding new constraints on ice sheet evolution and its contribution to global sea-level changes. An increased number of seismic stations in the vicinity of ice sheets will enhance our ability to create detailed space-time records of ice mass variations

Hydraulic Conditions for Stick‐Slip Tremor Beneath an Alpine Glacier

Accumulating evidence suggests that glacier basal motion can produce seismogenic stick-slip events caused by frictional resistance at the ice-bed interface. Frequent and subsequent failure of multiple stick-slip patches may constitute sliding tremor in seismic recordings. We show that during stick-slip tremor at the base of an Alpine glacier, the released seismic moment increases by an order of magnitude. The tremor emerges during melt-intensive days and is associated with water pressure peaks in a nearby subglacial channel. We propose an extended slider-block model that explains the regular occurrence of stick-slip tremor by coupling seismically sliding patches to surrounding aseismically sliding bed regions. The model predicts that the stick-slip patches control the movement of their surrounding smoothly sliding bed regions via elastic coupling and stress transfer. Thus, tremor producing stick-slip patches could affect large-scale ice flow to a degree, which is underestimated in models, with implications on sea level rise projections.ISSN:0094-8276ISSN:1944-800

Fine structure of microseismic glacial stick‐slip

Frictional instabilities exist in many geological settings, including glaciers and tectonic plate boundaries. However, investigations of suggested analogies between stick-slip ‘icequakes’ and earthquake faulting have been hampered by the noisy, melt-prone and inaccessible nature of glacial environments. Here, we reveal details of stick-slip events beneath an Alpine glacier using seismic sensors within a few meters of a seismically active bed region. We present evidence that widely detected stick-slip events, which are measurable at the ice surface, are in fact dynamic ruptures over many smaller asperities, whose individual seismic failures are usually too small to be recorded at the surface. Characteristic recurrence times of such multi-asperity ruptures and their sizes suggest an analogy to Parkfield earthquakes on the San Andreas Fault, questioning traditional glacier sliding theories. Although several trillion times smaller, glacial seismic sources presented here may therefore be ideal for studying earthquake faulting due to much higher event rates.ISSN:0094-8276ISSN:1944-800

Capturing Glacier-Wide Cryoseismicity With Distributed Acoustic Sensing

Over the past 1-2 decades, seismological measurements have provided new and unique insights into glacier and ice sheet dynamics. At the same time, sensor coverage is typically limited in harsh glacial environments with littile or no access. Turning kilometer-long fiber optic cables placed on the Earth’s surface into thousands of seismic sensors, Distributed Acoustic Sensing (DAS) may overcome the limitation of sensor coverage in the cryosphere. First DAS applications on the Greenland and Antarctic ice sheets and on Alpine glacier ice have highlighted the technique’s superiority. Signals of natural and man-made seismic sources can be resolved with an unrivaled level of detail. This offers glaciologists new perspectives to interpret their seismograms in terms of ice structure, basal boundary conditions and source locations. However, previous studies employed only relatively small network scales with a point-like borehole deployment or < 1 km cable aperture at the ice surface. Here we present a DAS installation, which aims to cover the majority of an Alpine glacier catchment: For one month in summer 2020 we deployed a 9 km long fiber optic cable on Rhonegletscher, Switzerland, and gathered continuous DAS data. The cable followed the glacier’s central flow line starting in the lowest kilometer of the ablation zone and extending well into the accumulation area. Even for a relatively small mountain glacier such as Rhonegletscher, cable deployment was a considerable logistical challenge. However, initial data analysis illustrates the benefit compared to conventional cryoseismological instrumentation: DAS measurements capture ground deformation over many octaves, including typical high-frequency englacial sources (10s to 100s of Hz) related to crevasse formation and basal sliding as well as long period signals (10s to 100s of seconds) of ice deformation. Depending on the presence of a snow cover, DAS records contain strong environmental noise (wind, meltwater flow, precipitation) and thus exhibit lower signal-to-noise ratios compared to conventional on-ice seismic installations. This is nevertheless outweighed by the advantage of monitoring ground unrest and ice deformation of nearly an entire glacier. We present a first compilation of signal and noise records and discuss future directions to leverage DAS data sets in glaciological research