Discrete and tremor seismicity near a microseismic stick-slip asperity at the base of an Alpine glacier
Creators: name: Köpfli, Manuela
Publisher: name: ETH Zurich
Year: 2021

description: Frictional resistance at the base of glacial ice controls ice sheet dynamics but is still incompletely understood. While basal sliding theories are traditionally based on slow and smooth sliding on a friction-less bed caused by viscous creep and regelation, frictional sliding at distinct patches of the glacier bed ('asperities') may also play a significant role. With the help of a dense borehole seismometer network within 30\,m of an active microseismic stick-slip asperity at the glacier bed of Rhonegletscher, I collected a novel data set. This data set makes it possible to gain a detailed view on frictional processes, that resist smooth sliding and lead to episodic stress release at the glacier bed.

The high spatial and temporal resolution of the seismic array helps to investigate basal sliding from a new perspective. I locate distinct basal stick-slip events with phase arrivals and successive events (basal stick-slip sliding tremor) with amplitude source location. Location with the double-difference method shows that weak stick-slip events cluster at distinct asperities that lie in slip direction of the location where strongest events cluster ($M_w=-2.49^{+0.4}_{-0.05}$). This indicates that the distribution of the weak clusters is not random, but is related to the stronger cluster.  All clusters locate along a linear feature which could be a subglacial water channel. Similarly, the location of successive events at the same location as discrete events suggests that stress on the glacier bed can be dissipated in different sliding styles.

I compare the cumulative moment magnitude distribution of a three-day period with an active strong cluster to a three-day period without any strong cluster. The comparison shows a similar abundance of weak events. The b-value of the Gutenberg-Richter relation, which describes the relative abundance of weak and strong events is found to be 1.647\,$\pm$\,0.030 for the weak events during the first three-day period, and 1.72\,$\pm$\,0.07 for the second. For earthquakes at plate boundaries, b-values larger than one are often found in a creeping regime where several seismic events arise without a main shock. Besides, relative b-values give information about stresses at the strike-slip interface.

The transient occurrence of frequency gliding lines in spectrograms of the continuous recordings, called tremor, correlates with measurements of water pressure at the glacier bed. Tremor is always active in the afternoon when meltwater input is high. However, these daily fluctuations alone are not sufficient to explain the occurrence of the tremor. An inefficient englacial drainage system is necessary to cause sufficiently high water pressures. After two cool days on Rhonegletscher with only little surface melt, the subglacial drainage system closed enough that in the following days the water pressure subsequently increased. Thus, high  water pressures enabled the occurance of tremor. By comparing the basal water pressure at the beginning and the end of the tremor a pressure difference of up to 4\,bar is found. This difference in basal water pressure could be explained by healing which is described by the rate-and-state friction formalism.