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Detection and Source Characterisation of Crevasse Icequakes at an Alpine Glacier Using Distributed Acoustic Sensing

Description: 

Crevassing is an important mechanism that controls the stability of glaciers. If ice flows over suitable topography or pre-existing fractures are exposed to meltwater, then the ice can fracture. If these fractures open all the way to the bed, then they can provide pathways for meltwater to reach the bed, potentially driving further speed up of the glacier downhill and enhanced melting. Crevasse fracturing generates icequakes. However, studying the source characteristics of such icequakes is challenging as it is not normally feasible to deploy seismic instrumentation in crevasse fields. Fibreoptic sensing now makes deployments in crevasse fields logistically feasible. We present results from a dense 2D grid deployment of fibre within a crevasse field at Gornergletscher, Switzerland. Icequakes are first detected and located using back-migration, with 951 icequakes detected and located. We then explore new full-waveform inversion methods to refine event depths and obtain focal mechanisms. Furthermore, we quantify fracture mode and volumetric opening extent. We find that events typically exhibit tensile crack opening, consistent with expected crevasse fracture mechanisms. As well as direct P-wave and surface-wave energy, the waveforms contain strong coda. We attempt to isolate the origin of this coda, to decipher if it is associated with either: fluid resonance at the crevasse fracture site, or wavefield scattering off other crevasses within the wider crevasse field. Coincident seismic nodes were also deployed as part of the experiment, allowing us to compare the value of a dense, horizontally-sensitive 2D fibreoptic grid to an array of seismic nodes sensitive only to the vertical wavefield. As well as providing observations for assessing glacier damage due to crevassing, our work exemplifies the application of a new generation of tools for interrogating seismic source properties using fibreoptic sensing techniques. These tools will likely be applicable for studying fracture in other geological settings, from geothermal stimulation to CO2 storage integrity monitoring.

Session: Fiber-optic Sensing Applications in Seismology - I

Type: Oral

Date: 4/15/2025

Presentation Time: 08:30 AM (local time)

Presenting Author: Thomas

Student Presenter: No

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