Intermediate‐depth icequakes and harmonic tremor in an Alpine glacier (Glacier d'Argentière, France): Evidence for hydraulic fracturing?

Abstract

We detected several thousand deep englacial icequakes on Glacier d'Argentière (Mont‐Blanc massif) between 30 March and 3 May 2012. These events have been classified in eight clusters. Inside each cluster, the waveforms are similar for P waves and S waves, although the time delay between the P waves and the S waves vary by up to 0.03 s, indicating an extended source area. Although these events were recorded by a single accelerometer, they were roughly located using a polarization analysis. The deepest events were located at a depth of 130 m, 60 m above the ice/bed interface. The clusters are separated in space. The largest cluster extends over about 100 m. For this cluster, the strike of the rupture plane is nearly parallel to the direction of the open crevasses, and the dip angle is 56°. Deep icequakes occur in bursts of activity that last for a few hours and are separated by quiet periods. Many events occurred on 28 and 29 April 2012, during the warmest days, when snowmelting was likely important. The distributions of interevent times and peak amplitudes obey power laws as also observed for earthquakes, but with larger exponents. The polarity of the P waves for all of the events is consistent with tensile faulting. Finally, between 25 April and 3 May, we observed a gliding harmonic tremor with a fundamental resonance frequency that varied between 30 Hz and 38 Hz, with additional higher‐frequency harmonics. During this time we also observed shallow hybrid events with high‐frequency onsets and a monochromatic coda. These events might be produced by the propagation of fractures and the subsequent flow of water into the fracture. The strongest resonance was observed just after a strong burst of deep icequakes and during an unusually warm period when the snow height decreased by 60 cm in 1 week. The resonance frequency shows a succession of several sharp decreases and phases of progressive increases. One of the strongest negative steps of the resonance frequency on 28 April coincides with a burst of deep icequakes. These events appear to be associated with the propagation of fractures, which can explain the decrease in the resonance frequency. Finally, we observed an acceleration of glacier flow on 29 April, suggesting that meltwater had reached the ice/bed interface. These observations suggest that deep icequakes are due to hydraulic fracturing and that they can be used to track fluid flow inside glaciers.

Tremor during ice-stream stick slip

“…Finally, simple models of hydraulic fracture resonance (Lipovsky and Dunham, 2015) and turbulent channel flow (Gimbert et al, 2014) predict spectral signatures that are not consistent with the evenly spaced spectral peaks observed on the WIP, although complex geometrical effects may invalidate the simplifying assumptions of such models. These distinguishing criteria may be useful in analyzing several recently described data sets (Heeszel et al, 2014;Helmstetter et al, 2015a, b;Hammer et al, 2015).…”

Cryoseismology

“…In the authors' view, changing water pressures within englacial conduits displace the conduit walls causing nearly monochromatic wave trains recorded near Alaskan glaciers. The triggering water pressure change may be caused by nearby fracturing, and thus also explains near‐monochromatic coda, which is sometimes observed after deep icequakes [ West et al , ; Röösli et al , ; Helmstetter et al , ]. This phenomenon is again analogous to “hybrid events” in volcanic environments [ White et al , ; Neuberg et al , ; Harrington and Brodsky , ].…”

“…Accordingly, the activity of intermediate icequakes in glacier ablation zones depends on the availability of meltwater. A study of 20–130 m deep icequakes on Glacier d'Argentière (France) showed sporadic bursts in the activity of intermediate icequake clusters [ Helmstetter et al , ]. The waveforms of these icequakes are also consistent with tensile faulting, and variations in depths of icequakes belonging to a single cluster coincided with the warmest periods.…”

Seismic moulin tremor

“…Lawrence and Qamar [] suggested that fluid‐filled fractures cause resonance effects in volcanoes and glaciers. Several subsequent studies have shown that water resonances triggered by fracturing in glacier ice are a common phenomenon in high melt areas [ Métaxian et al , ; West et al , ; Röösli et al , ] and that temporal changes in resonance peaks may reflect geometric changes associated with fracture tip propagation [ Heeszel et al , ; Helmstetter et al , ]. Considering recent advances in theoretical treatments of guided waves in englacial fractures [ Dunham and Ogden , ; Lipovsky and Dunham , ], these findings suggest that seismology is well suited to monitor subsurface water drainage in glaciers and ice sheets.…”