Icequakes at Sermeq Kujalleq, West Greenland, located by combining borehole distributed acoustic sensing and geophones

Abstract

The rate of ice loss from marine terminating glaciers is governed by both surface runoff and ice dynamics. Although ice loss due to runoff can be relatively easily studied using in-situ and remote sensing methods, ice dynamics are more difficult to constrain and are the largest contributor to uncertainty in ice loss estimates. By recording and locating the source of seismic signals released during (e.g.) crevasse opening and stick-slip events, we can obtain a spatial and temporal distribution of icequakes. This will allow for estimates of stress and strain within Sermeq Kujalleq (Store Glacier), West Greenland. The emerging technology of distributed acoustic sensing (DAS) offers the ability to perform seismic surveys at higher spatial sampling resolutions than is feasible with conventional geophone deployments. This is especially true when instruments are required to be deployed in a logistically challenging environment such as Store Glacier. Here, we present an icequake source location method that exploits the dense spatial sampling of DAS alongside the directionality and increased signal to noise ratio of 3-component geophones. Both instruments were deployed in closely drilled boreholes on Store Glacier in July 2019, as part of the RESPONDER project. The DAS fiber optic cable was deployed in a 1043 m deep borehole. The three geophones are at depths of 100 m, 250 m and 400 m. The data set includes controlled-source vertical seismic profiles (VSPs) and a 3-day passive record of cryoseismicity recorded by both instrument types. Previous work on the passive DAS dataset created a convolutional neural network-based method that performs efficient signal detection in the frequency-wavenumber domain and provided a catalogue of detections at a mean rate of 4074 per hour. The next step is to locate the catalogued seismic sources in targeted time periods containing arrivals. Our borehole DAS deployment only records the vertical component of a seismic signal because of the fiber optic cable’s sensitivity only to strain along its longitudinal axis. This renders it impossible to determine the backazimuth of arrivals solely using the DAS data. To overcome this limitation, we analyse the particle motion recorded by the 3-component geophones. The 3D source location can be derived using well-resolved estimates of source depth and distance from the DAS data, plus the backazimuth gained from geophones. Our initial results include an estimated location of the source of one large icequake at ~800 m offset from the borehole and ~300 m depth. The backazimuth of these arrivals is yet to be determined, but the depth of this event suggests it may originate from an englacial shear zone related to a temperature anomaly that we have inferred in Store Glacier from distributed temperature sensing. Once efficiently automated, we aim to combine the seismic observations to build a catalogue of seismicity, including event time and origin location.  Additional work aims to estimate moment tensors of these detected and located events to improve our understanding of Store Glacier’s dynamics.