Tremor during ice stream stick-slip

Abstract

Abstract. During the 200 km-scale stick slip of the Whillans Ice Plain (WIP), West Antarctica, seismic tremor episodes occur at the ice–bed interface. We interpret these tremor episodes as swarms of small repeating earthquakes. The earthquakes are evenly spaced in time and this even spacing gives rise to spectral peaks at integer multiples of the recurrence frequency ~ 10–20 Hz. We conduct numerical simulations of the tremor episodes that include the balance of forces acting on the fault, the evolution of rate- and state-dependent fault friction, and wave propagation from the fault patch to a seismometer located on the ice. The ice slides as an elastic block loaded by the push of the upstream ice, and so the simulated basal fault patch experiences a loading velocity equal to the velocity observed by GPS receivers on the surface of the WIP. By matching synthetic seismograms to observed seismograms, we infer fault area ~ 10 m2, bed shear modulus ~ 10 MPa, effective pressure ~ 10 kPa, and state evolution distance ~ 1 μm. Large-scale slip events often occur twice daily, although skipped events have been increasing in frequency over the last decade. We observe that tremor seismic particle velocity amplitudes are greater during the double wait time events that follow skipped events. The physical mechanism responsible for these anomalously high seismic amplitudes may provide a window into near-future subglacial conditions and the processes that occur during ice stream stagnation.

Possible stick‐slip behavior before the Rausu landslide inferred from repeating seismic events

“…The occurrence of repeating events resembles stick‐slip behavior observed in rock mechanics experiments [ Brace and Byerlee , ] and block‐slider models [ Burridge and Knopoff , ]. Similar apparent stick‐slip behavior has also been observed during movement of glaciers and ice sheets at the contact between ice and bedrock [ Caplan‐Auerbach and Huggel , ; Thelen et al , ; Allstadt and Malone , ; Helmstetter et al , ; Lipovsky and Dunham , ]. Note that the maximum soil‐frost depth in this area is less than 0.5 m [ Kinoshita et al , ], so ice is not involved in this landslide.…”

Ice Shelf Rift Propagation and the Mechanics of Wave‐Induced Fracture

“…Larour et al (), citing laboratory studies such as those by DeFranco and Dempsey (), invokes constitutive instability as a possible mechanism for episodic rift activity. Constitutive instability gives rise, for example, to the stick‐slip instability that is responsible for basal stick‐slip motion of glaciers and ice sheets (Lipovsky & Dunham, ). Such behavior is a typical pathology of laboratory experiments conducted on samples which are too thin to achieve a state of plane strain (Bažant, ; Broek, ).…”

“…Banwell et al () used a nearby seismometer located on bedrock to show that a rift propagation event on the McMurdo Ice Shelf occurred during the arrival of large amplitude ocean waves from a distant storm. Finally, a constitutive instability, essentially the opening‐mode equivalent of the shearing‐mode stick‐slip instability (Lipovsky & Dunham, ), has been proposed to be important for episodic rift motion (Larour et al, ). One of the goals of this paper is to develop a theoretical framework within which to compare the predictions of these hypotheses.…”

Glacier sliding, seismicity and sediment entrainment

“…Specifically, seismicity occurs when the effective pressure exceeds a critical value (Rice and others, 2001),for seismogenic patch rigidity k , rate-weakening parameter b − a (Eqn (1)) and state evolution distance L . These parameters are discussed in detail by Lipovsky and Dunham (2016) and Lipovsky and Dunham (2017). As an example of typical values, we note that if k = 10 MPa/m, b − a = 0.01 and , then seismicity is expected to be possible only when the effective pressure is greater than 10 kPa.…”

“…As an example of typical values, we note that if k = 10 MPa/m, b − a = 0.01 and , then seismicity is expected to be possible only when the effective pressure is greater than 10 kPa. We note that the state evolution distance L is often inferred to vary with the size of the sliding region; our choice of a relatively small value in this example calculation is typical of ~ meter-scale sliding regions (Lipovsky and Dunham, 2016). Lastly, we note that dynamic variations in normal stress (Linker and Dieterich, 1992; Selvadurai and others, 2018) and stiffness (Leeman and others, 2015) are both neglected here in order to focus on the most basic relationship between friction and entrainment.…”