Fabian Walter

Basal icequakes during changing subglacial water pressures beneath Gornergletscher, Switzerland

Using dense networks of three-component seismometers installed in direct contact with the ice, the seismic activity of Gornergletscher, Switzerland, was investigated during the summers of 2004 and 2006, as subglacial water pressures varied drastically. These pressure variations are due to the diurnal cycle of meltwater input as well as the subglacial drainage of Gornersee, a nearby marginal ice-dammed lake. Up to several thousand seismic signals per day were recorded. Whereas most icequakes are due to surface crevasse openings, about 200 events have been reliably located close to the glacier bed. These basal events tend to occur in clusters and have signals with impulsive first arrivals. At the same time, basal water pressures and ice-surface velocities were measured to capture the impact of the lake drainage on the subglacial hydrological system and the ice-flow dynamics. Contrary to our expectations, we did not observe an increase of basal icequake activity as the lake emptied, thereby raising the subglacial water pressures close to the flotation level for several days. In fact, the basal icequakes were usually recorded during the morning hours, when the basal water pressure was either low or decreasing. During the high-pressure period caused by the drainage of the lake, no basal icequakes were observed. Furthermore, GPS measurements showed that the glacier surface was lowering during the basal seismic activity. These observations lead us to conclude that such icequakes are connected to the diurnal variation in glacier sliding across the glacier bed.

Moment Tensor Inversions of Icequakes on Gornergletscher, Switzerland

We have determined seismic source mechanisms for shallow and intermediate-depth icequake clusters recorded on the glacier Gornergletscher, Switzerland, during the summers of 2004 and 2006. The selected seismic events are part of a large data set of over 80,000 seismic events acquired with a dense seismic network deployed in order to study the yearly rapid drainage of Gornersee lake, a nearby ice-marginal lake. Using simple frequency and distance scaling and Green’s functions for a homogeneous half-space, we calculated moment tensor solutions for icequakes with M_w-1.5 using a full-waveform inversion method usually applied to moderate seismic events (M_w>4) recorded at local to regional distances (≈50–700 km). Inversions from typical shallow events are shown to represent tensile crack openings. This explains well the dominating Rayleigh waves and compressive first motions observed at all recording seismograms. As these characteristics can be observed in most icequake signals, we believe that the vast majority of icequakes recorded in the 2 yr is due to tensile faulting, most likely caused by surface crevasse openings. We also identified a shallow cluster with somewhat atypical waveforms in that they show less dominant Rayleigh waves and quadrantal radiation patterns of first motions. Their moment tensors are dominated by a large double-couple component, which is strong evidence for shear faulting. Although less than a dozen such icequakes have been identified, this is a substantial result as it shows that shear faulting in glacier ice is generally possible even in the absence of extreme flow changes such as during glacier surges. A third source of icequakes was located at 100 m depth. These sources can be represented by tensile crack openings. Because of the high-hydrostatic pressure within the ice at these depths, these events are most likely related to the presence of water lenses that reduce the effective stress to allow for tensile faulting.

Evidence for Near-Horizontal Tensile Faulting at the Base of Gornergletscher, a Swiss Alpine Glacier

Abstract Using 3D Green’s functions we determine full and constrained moment tensor solutions of icequakes near the base of Gornergletscher, Switzerland. The seismic events were recorded in the summer of 2004 using a high-density seismometer array. The seismic velocity model used in the generation of Green’s functions is based on radio-echo soundings to approximate the basal topography, which beneath the study site exhibits a strong inclination. As the basal conditions are not well known, we try moment tensor inversions with seismic velocity profiles consisting of two and three media. The former case consists of homogeneous ice resting on bedrock, whereas the latter case includes a thin basal layer with slow seismic velocities representing eroded material or highly fractured ice. Effects of errors in Green’s functions are estimated by sensitivity studies in which we invert 1D and 3D synthetics using Green’s functions of wrong velocity models. The results show that calculations of source types and fault plane orientations of tensile cracks are rather robust with respect to errors in Green’s functions. However, the quality of the waveform fits depends on strike and dip of the synthetic source. When inverting seismograms, Green’s functions of the seismic model that includes the basal slow velocity layer are found to give the most realistic source types as well as the best waveform fits. The fault mechanisms derived from constrained moment tensor inversions are near-horizontal tensile cracks, which suggest a complex time-dependent basal stress field.

Seismic moulin tremor

Through glacial moulins, meltwater is routed from the glacier surface to its base. Moulins are a main feature feeding subglacial drainage systems and thus influencing basal motion and ice dynamics, but their geometry remains poorly known. Here we show that analysis of the seismic wavefield generated by water falling into a moulin can help constrain its geometry. We present modeling results of hour-long seimic tremors emitted from a vertical moulin shaft, observed with a seismometer array installed at the surface of the Greenland Ice Sheet. The tremor was triggered when the moulin water level exceeded a certain height, which we associate with the threshold for the waterfall to hit directly the surface of the moulin water column. The amplitude of the tremor signal changed over each tremor episode, in close relation to the amount of inflowing water. The tremor spectrum features multiple prominent peaks, whose characteristic frequencies are distributed like the resonant modes of a semiopen organ pipe and were found to depend on the moulin water level, consistent with a source composed of resonant tube waves (water pressure waves coupled to elastic deformation of the moulin walls) along the water-filled moulin pipe. Analysis of surface particle motions lends further support to this interpretation. The seismic wavefield was modeled as a superposition of sustained wave radiation by pressure sources on the side walls and at the bottom of the moulin. The former was found to dominate the wave field at close distance and the latter at large distance to the moulin.

Full, constrained and stochastic source inversions support evidence for volumetric changes during the Basel earthquake sequence

Co-seismic volumetric changes are often interpreted as tensile fracturing in response to fluid injection during geothermal reservoir stimulation. Such volumetric changes manifest themselves as isotropic moment tensor components, which may thus constitute a measure for hydraulic stimulation efficiency. Recent analyses found significant isotropic moments of M2+ earthquakes during the 2006 hydraulic stimulation of a geothermal reservoir in Basel, Switzerland. The results contradicted first-motion focal mechanisms, which are in close agreement with shear sources without volumetric changes. Here we revisit the magnitude 1.7+ Basel events with full and stochastic moment tensor inversions in order to provide additional and/or supporting evidences for the occurrence of volumetric sources, if any. We furthermore apply purely deviatoric, and superimposed tensile and shear fault mechanisms, which we believe are meaningful constraints for fluid-induced earthquakes. As a result, we only find a single earthquake with statistically significant volumetric faulting. Spatial and temporal patterns of fluid-induced sources therefore have to be taken as indicative only, even though they suggest a clear relationship between fluid injections and fault mechanisms. On the other hand, we confirm that most inverted moment tensors (including the statistically significant one) show some inconsistencies with first motion focal mechanisms. We suggest that this is a manifestation of a more complicated fault geometry, which none of our moment tensor constraints can describe.

Complex force history of a calving‐generated glacial earthquake derived from broadband seismic inversion

The force applied to the Earth by the calving of two icebergs at Jakobshavn Isbrae, Greenland, has been quantified. The source force history was recovered by inversion of regional broadband seismograms without any a priori constraint on the source time function, in contrast with previous studies. For periods 10-100 s, the three-component force can be obtained from distant stations alone and is proportional to the closest station seismograms. This inversion makes it possible to quantify changes of the source force direction and amplitude as a function of time and frequency. A detailed comparison with a video of the event was used to identify four forces associated with collision, then bottom-out and top-out rotation of the first and second icebergs, and ice melange motion. Only the two iceberg rotations were identified in previous studies. All four processes are found here to contribute to the force amplitude and variability. Such a complete time-frequency force history provides unique dynamical constraints for mechanical calving models.

Automatic Identification of Alpine Mass Movements by a Combination of Seismic and Infrasound Sensors

The automatic detection and identification of alpine mass movements such as debris flows, debris floods, or landslides have been of increasing importance for devising mitigation measures in densely populated and intensively used alpine regions. Since these mass movements emit characteristic seismic and acoustic waves in the low-frequency range (<30 Hz), several approaches have already been developed for detection and warning systems based on these signals. However, a combination of the two methods, for improving detection probability and reducing false alarms, is still applied rarely. This paper presents an update and extension of a previously published approach for a detection and identification system based on a combination of seismic and infrasound sensors. Furthermore, this work evaluates the possible early warning times at several test sites and aims to analyze the seismic and infrasound spectral signature produced by different sediment-related mass movements to identify the process type and estimate the magnitude of the event. Thus, this study presents an initial method for estimating the peak discharge and total volume of debris flows based on infrasound data. Tests on several catchments show that this system can detect and identify mass movements in real time directly at the sensor site with high accuracy and a low false alarm ratio.

Rapid detection and location of debris flow initiation at Illgraben, Switzerland

Heavy precipitation can suddenly mobilize tens to hundreds of thousands of cubic meters of sediments in steep Alpine torrents. The resulting debris flows (mixtures of water, sediments and boulders) move downstream with velocities of 15 several meters per second and have a high destructive potential. Warning schemes for affected communities rely on raising awareness to the debris flow threat, precipitation monitoring and rapid detection methods. The latter, in particular, remain an ongoing challenge, because debris-flow-prone torrents have their catchments in steep and inaccessible terrain, where installing and maintaining instrumentation is difficult. Here, we propose a simple processing scheme for seismic network data. We use debris flow and noise seismograms from Illgraben, Switzerland, a torrent, which produces several debris flow 20 events per year. Automatic in-situ detection is currently based on geophones mounted on concrete check dams and radar stage sensors hung above the channel. The proposed approach has the advantage that it uses seismometers, which can be installed at more accessible locations, and where a stable connection to portable phone networks is available for data communication. Our data processing uses time-averaged ground vibration amplitudes to estimate the location of the debris flow front. Applied to continuous data streams, inversion of the seismic amplitude decay eliminates the need for single25 station-based detection and knowledge of the local seismic velocity model. This makes the approach suitable for automation, as seismic phase identification is unnecessary and the amplitude averaging significantly reduces data volume. We apply our approach to a small debris flow event on 19 July 2011, which was captured with a temporary monitoring network. The processing rapidly detects the debris flow event half an hour before its front arrives at the torrent mouth and 8 minutes before detection by the current alarm system. An analysis of continuous seismic records furthermore indicates that detectability of 30 Illgraben debris flows of this size are unaffected by changing environmental and cultural seismic noise. We therefore propose that our method reliably detects initiation of the Illgraben debris flows and can thus form an important ingredient in the next generation of early warning schemes. Nat. Hazards Earth Syst. Sci. Discuss., doi:10.5194/nhess-2016-321, 2016 Manuscript under review for journal Nat. Hazards Earth Syst. Sci. Published: 13 October 2016 c