Discrete and tremor seismicity near a microseismic stick-slip asperity at the base of an Alpine glacier

• Main Author: Köpfli, Manuela
• Other Authors: Gräff, Dominik, Walter, Fabian Thomas, Fichtner, Andreas
• Language: English
• Published: ETH Zurich 2021
• Subjects: Ice Sheet
• Online Access: https://hdl.handle.net/20.500.11850/544060; https://doi.org/10.3929/ethz-b-000544060

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 ...