Interactions between heterogeneous slip and a locked asperity on a glacier fault

Abstract:

Because seismogenic faults are typically buried at great depth and aseismic slip processes are strongly attenuated by the lowpass filter of the earth, high spatial resolution observations of interactions between slow and fast slip are limited. The Whillans Ice Plain is a large (~100x150 km²), thin (< 1km), stick-slipping glacier in West Antarctica where these interactions are much easier to observe. Here we present analysis of many repeat glacier stick-slip events, or ice stream earthquakes, which we find are usually triggered by periods of accelerated slow slip in creeping regions surrounding a primary locked asperity. Low effective stresses and relatively compliant fault materials (ice vs. rock) make this system a valuable natural analog to tectonic faults by magnifying and decelerating slip processes before and during daily fault-wide ice stream earthquakes, in which the ice slides ~0.5-1 m in ~25 min. Kinematic GPS on the ice surface readily distinguish spatial patterns of background locking and creep, revealing a large, locked, and isolated central asperity, as well as patterns of heterogeneous creep in surrounding un-locked regions.

We use data from 9 GPS and 76 ice stream earthquakes in 2014 to investigate timing of failure of the locked central asperity in relation to tidally-controlled heterogeneous slip rates in surrounding creeping regions. We find that the main ice stream earthquake, in which the central asperity fails and the entire fault ruptures, is slip-predictable, with total slip, peak velocity, and peak acceleration all scaling with elapsed time since the previous event and therefore with accumulated stress from a constant upstream loading. However, event recurrence times range from 8-28 hrs, and for >80% of events, and in contrast to previous studies, GPS reveal that failure of the central asperity is triggered within minutes to several hours by accelerated slow slip caused by falling tide in a region between the asperity and the floating ice shelf. In this simple system, ice stream earthquake moment depends on large scale stresses on the asperity from constant upstream loading, while timing depends on local stress concentration from heterogeneous slip in surrounding regions. These results are relevant for understanding interactions between locked and slowly slipping regions on tectonic faults.