Icequake locations and discrimination of source and path effects with small aperture arrays, Bering Glacier terminus, AK

Abstract

[1] The complex source processes associated with both glacier calving and the breakup of icebergs, combined with commonly heterogeneous periglacial seismic velocity structure, can result in complicated seismic records. Key features of the waveforms, which are typically characterized by low-amplitude or emergent first-arrivals and long-duration, narrow-band codas, have been attributed to either source processes or propagation path effects. This uncertainty must be addressed in order for seismic data to be effective for studying the calving process as it relates to terminus dynamics. In this study, we use sets of 3-element arrays of 3-component geophones and infrasound sensors to locate calving and iceberg breakup events and isolate path effects in the seismic records obtained near the Bering Glacier terminus in the summer of 2010. Using waveform correlation, we treat each array as an antenna and determine the direction to the source and apparent velocity of the wavefield across the array. The initial few ($3) cycles of P waves recorded from an array, beam formed to identify coherent arrivals for each event, are useful for deriving a propagation azimuth and apparent velocity, allowing for location of events using a small number of similar arrays. We locate 125 calving and iceberg breakup events near the terminus with this method. We also demonstrate that the longer-lived narrow-band coda is not coherent across individual arrays, suggesting that the narrow-band coda observed at the Bering Glacier is attributable to a path effect rather than to the source process. The large number of iceberg breakup events that we located has important implications for other calving glaciers where icebergs are present, and calving rates may be erroneously overestimated from the seismic data if their contribution is not taken into account.

Cryoseismology

“…Detection algorithms require site‐specific tuning [ O'Neel et al , ; Bartholomaus et al , ]. Iceberg disintegration [ Richardson et al , , ] and collision with obstacles [ MacAyeal et al , ; Dziak et al , ] produce signals within a seismic spectrum, which includes 1–5 Hz calving band. Furthermore, the generation of seismic signals within the proglacial water is highly sensitive to the iceberg's free‐fall height [ Bartholomaus et al , ].…”

Seismicity within a propagating ice shelf rift: The relationship between icequake locations and ice shelf structure

“…Understanding rift propagation is required to understand the process of iceberg calving. Identifying and locating fracturing events using passive seismic observations provide a powerful means to study the physical processes associated with calving [ Bartholomaus et al ., ; Bassis et al ., , ; Richardson et al ., , ]. Because seismic waves also contain information that constrain rupture mechanics [e.g., Walter et al ., ; Wiens et al ., ], passive seismology‐derived results provide information about the temporal variability in the stress state of the ice shelf on time and spatial scales shorter than is possible to determine with satellite monitoring.…”

A Pilot Experiment on Infrasonic Lahar Detection at Mount Adams, Cascades: Ambient Infrasound and Wind-Noise Characterization at a Quiescent Stratovolcano

“…5d). These events may be icequakes (comparable to those described by Lombardi et al, 2019, with Richardson et al, 2012, and Allstadt and Malone, 2014, being other useful references) or possibly large tree-fall events. We rule out electromagnetic interference, given the acoustic travel times, general absence of thunderstorms, and similar amplitudes on each sensor (vs. Haney et al, 2020).…”