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Using a new biaxial friction apparatus, we conducted experiments of ice-on-rock friction in order to better understand basal sliding of glaciers and ice streams. A series of velocity-stepping and slide–hold–slide tests were conducted to measure friction and healing at temperatures between −20°C and melting. Experimental conditions in this study are comparable to subglacial temperatures, sliding rates and effective pressures of Antarctic ice streams and other glaciers, with load-point velocities ranging from 0.5 to 100 µm s−1 and normal stress σn = 100 kPa. In this range of conditions, temperature dependences of both steady-state friction and frictional healing are considerable. The friction increases linearly with decreasing temperature (temperature weakening) from μ = 0.52 at −20°C to μ = 0.02 at melting. Frictional healing increases and velocity dependence shifts from velocity-strengthening to velocity-weakening behaviour with decreasing temperature. Our results indicate that the strength and stability of glaciers and ice streams may change considerably over the range of temperatures typically found at the ice–bed interface.

Many volcanic earthquakes large enough to be detected globally have anomalous focal mechanisms and frequency content. In a previous study, we examined the relationship between active volcanism and the occurrence of a specific type of shallow, non-double-couple earthquake. We identified 101 earthquakes with vertical compensated-linear-vector-dipole (vertical-CLVD) focal mechanisms that took place near active volcanoes between 1976 and 2009. The majority of these earthquakes, which have magnitudes 4.3 less than or equal to MW less than or equal to 5.8, are associated with documented episodes of volcanic unrest. Here we further characterize vertical-CLVD earthquakes and explore possible physical mechanisms. Through teleseismic body-wave analysis and examination of the frequency content of vertical-CLVD earthquakes, we demonstrate that these events have longer source durations than tectonic earthquakes of similar magnitude. We examine the covariance matrix for one of the best-recorded earthquakes and confirm that the isotropic and pure vertical-CLVD components of the moment tensor cannot be independently resolved using our long-period seismic data set. Allowing for this trade-off, we evaluate several physical mechanisms that may produce earthquakes with deviatoric vertical-CLVD moment tensors. We find that physical mechanisms related to fluid flow and volumetric changes are incompatible with seismological, geological, and geodetic observations of vertical-CLVD earthquakes. However, ring-faulting mechanisms explain many characteristics of vertical-CLVD earthquakes, including their seismic radiation patterns, source durations, association with volcanoes in specific geodynamic environments, and the timing of the earthquakes relative to volcanic activity.

Some of the largest and most anomalous volcanic earthquakes have non-double-couple focal mechanisms. Here, we investigate the link between volcanic unrest and the occurrence of non-double-couple earthquakes with dominant vertical tension or pressure axes, known as vertical compensated-linear-vector-dipole (vertical-CLVD) earthquakes. We determine focal mechanisms for 313 target earthquakes from the standard and surface wave catalogs of the Global Centroid Moment Tensor Project and identify 86 shallow 4.3 ≤ MW ≤ 5.8 vertical-CLVD earthquakes located near volcanoes that have erupted in the last ~100 years. The majority of vertical-CLVD earthquakes occur in subduction zones in association with basaltic-to-andesitic stratovolcanoes or submarine volcanoes, although vertical-CLVD earthquakes are also located in continental rifts and in regions of hot spot volcanism. Vertical-CLVD earthquakes are associated with many types of confirmed or suspected eruptive activity at nearby volcanoes, including volcanic earthquake swarms as well as effusive and explosive eruptions and caldera collapse. Approximately 70% of all vertical-CLVD earthquakes studied occur during episodes of documented volcanic unrest at a nearby volcano. Given that volcanic unrest is underreported, most shallow vertical-CLVD earthquakes near active volcanoes are likely related to magma migration or eruption processes. Vertical-CLVD earthquakes with dominant vertical pressure axes generally occur after volcanic eruptions, whereas vertical-CLVD earthquakes with dominant vertical tension axes generally occur before the start of volcanic unrest. The occurrence of these events may be useful for identifying volcanoes that have recently erupted and those that are likely to erupt in the future.

We calculate two-station phase measurements using single-station measurements made on USArray Transportable Array data for surface waves at periods from 25 to 100 s. The phase measurements are inverted for baseline Love and Rayleigh wave phase velocity maps on a 0.5° × 0.5° grid. We make estimates of the arrival angle for each event at each station using a mini array method similar to beamforming, and apply this information to correct the geometry of the two-station measurements. These corrected measurements are inverted for an additional set of phase velocity maps. Arrival angles range from 0° to ±15°, and the associated corrections result in local changes of up to 4 per cent in the final phase velocity maps. We select our preferred models on the basis of the internal consistency of the measurements, finding that the arrival-angle corrections improve the two-station phase measurements, but that Love wave arrival-angle estimates may be contaminated by overtone interference. Our preferred models compare favourably with recent studies of the phase velocity of the Western United States. The corrected Rayleigh wave models achieve greater variance reduction than the baseline Rayleigh wave models, and the baseline Love wave models, which are more difficult to obtain, are robust and could be used in conjunction with the Rayleigh wave models to constrain radially anisotropic earth structure.

Glacial earthquakes are anomalous earthquakes associated with large ice-loss events occurring at marine-terminating glaciers, primarily in Greenland. They are detectable teleseismically, and a proper understanding of the source mechanism may provide a remote-sensing tool to complement glaciological observations of these large outlet glaciers. We model teleseismic surface-wave waveforms to obtain locations and centroid–single-force source parameters for 121 glacial earthquakes occurring in Greenland during the period 2006–2010. We combine these results with those obtained by previous workers to analyze spatial and temporal trends in glacial-earthquake occurrence over the 18-year period from 1993–2010. We also examine earthquake occurrence at six individual glaciers, comparing the earthquake record to independently obtained observations of glacier change. Our findings confirm the inference that glacial-earthquake seismogenesis occurs through the capsize of large, newly calved icebergs. We find a close correspondence between episodes of glacier retreat, thinning, and acceleration and the timing of glacial earthquakes, and document the northward progression of glacial earthquakes on Greenland's west coast over the 18-year observing period. Our results also show that glacial earthquakes occur when the termini of the source glaciers are very close to the glacier grounding line, i.e., when the glaciers are grounded or nearly grounded.

AGU Fall Meeting 3–7 December 2012, San Francisco, California, Single-frequency GNSS data has not generally been used for high-accuracy geodetic applications since the 1990s, but there are significant advantages if single-frequency GNSS receivers can be usefully deployed for studies of fast-moving outlet glaciers. The cost for these receivers is significantly lower (~50%) than for dual-frequency receivers, a significant benefit given the high spatial density at which these system are deployed on the glacier and the high risk for damage or loss in the glacial environment. In addition, the size of the data files that need to be transferred from extremely remote locations, often at very slow transmission rates, is significantly reduced. Consideration of single-frequency systems for this application is viable because of the relatively small extent (< 50 km) of the entire network to be deployed. Unfortunately, the availability of research-quality software that can perform kinematic solutions on single-frequency data is limited. We have developed the BAKAR software employing a stochastic filter to analyze single-frequency GNSS data. The software can implement a range of stochastic models for time-dependent site position. In this presentation, we describe the BAKAR software, and discuss its strengths and weaknesses. On one hand, chief among the challenges we have encountered are determination of accurate prior positions, and bursts of polar ionospheric activity that impede cycle-slip detection, even over intersite distances as short as 10 km. On the other hand, use of a single-frequency observable is theoretically less sensitive to multipath and signal scattering. We will quantitatively assess these effects, and assess the accuracy of BAKAR in a range of situations and applications

Large calving events at Greenlands largest outlet glaciers are associated with glacial earthquakes and nearinstantaneous increases in glacier flow speed. At some glaciers and ice streams, flow is also modulated in a regular way by ocean tidal forcing at the terminus. At Helheim Glacier, analysis of geodetic data shows decimeter-level periodic position variations in response to tidal forcing. However, we also observe transient increases of more than 100% in the glaciers responsiveness to such tidal forcing following glacial-earthquake calving events. The timing and amplitude of the changes correlate strongly with the step-like increases in glacier speed and longitudinal strain rate associated with glacial earthquakes. The enhanced response to the ocean tides may be explained by a temporary disruption of the subglacial drainage system and a concomitant reduction of the friction at the ice-bedrock interface, and suggests a new means by which geodetic data may be used to infer glacier properties.

While it is agreed that the great Sumatra earthquake of December 26, 2004 was among the largest earthquakes of the past century, there has been disagreement on how large it was, which part of the fault ruptured, and how the rupture took place. We present a centroid-moment-tensor (CMT) analysis of the earthquake in which multiple point sources are used in the inversion to mimic a propagating slip pulse. The final model consists of five point sources, with the southernmost sources accounting for the majority of the moment release. The presumed fault planes of the southern sources strike northwest, while those in the north strike northeast, consistent with the geometry of the subduction trench. Slip on the fault is found to be more oblique in the north than in the south. The inversion with five sources leads to a moment magnitude for the Sumatra earthquake of M_W = 9.3, consistent with estimates from long-period normal-mode amplitudes.