Fine Structure of Microseismic Glacial Stick-Slip

Frictional instabilities exist in many geological settings, including glaciers and tectonic plate boundaries. However, investigations of suggested analogies between stick-slip "icequakes" and earthquake faulting have been hampered by the noisy, melt-prone and inaccessible nature of glacial environments. Here, we reveal details of stick-slip events beneath an Alpine glacier using seismic sensors within a few meters of a seismically active bed region. We present evidence that widely detected stick-slip events, which are measurable at the ice surface, are in fact dynamic ruptures over many smaller asperities, whose individual seismic failures are usually too small to be recorded at the surface. Characteristic recurrence times of such multi-asperity ruptures and their sizes suggest an analogy to Parkfield earthquakes on the San Andreas Fault, questioning traditional glacier sliding theories. Although several trillion times smaller, glacial seismic sources presented here may therefore be ideal for studying earthquake faulting due to much higher event rates.