Tidal and seasonal variations in calving flux observed with passive seismology

Abstract

The seismic signatures of calving events, i.e., calving icequakes, offer an opportunity to examine calving variability with greater precision than is available with other methods. Here using observations from Yahtse Glacier, Alaska, we describe methods to detect, locate, and characterize calving icequakes. We combine these icequake records with a coincident, manually generated record of observed calving events to develop and validate a statistical model through which we can infer iceberg sizes from the properties of calving icequakes. We find that the icequake duration is the single most significant predictor of an iceberg's size. We then apply this model to 18 months of seismic recordings and find elevated iceberg calving flux during the summer and fall and a pronounced lull in calving during midwinter. Calving flux is sensitive to semidiurnal tidal stage. Large calving events are tens of percent more likely during falling and low tides than during rising and high tides, consistent with a view that deeper water has a stabilizing influence on glacier termini. Multiple factors affect the occurrence of mechanical fractures that ultimately lead to iceberg calving. At Yahtse Glacier, seismology allows us to demonstrate that variations in the rate of submarine melt are a dominant control on iceberg calving rates at seasonal timescales. On hourly to daily timescales, tidal modulation of the normal stress against the glacier terminus reveals the nonlinear glacier response to changes in the near‐terminus stress field.

Dynamic Changes at Yahtse Glacier, the Most Rapidly Advancing Tidewater Glacier in Alaska

“…These two factors reach a critical point in October, leading to a full-width terminus retreat of ∼80 m ( Figure 9A). Bartholomaus et al (2015b) observed that Yahtse's calving flux peaks in the fall, coincident with the timing of the full-width terminus retreat observed here. Seasonal terminus retreat at Yahtse shows a dependence on tidewater glacier dynamics governing seasonal changes in ice flux (e.g., Stearns et al, 2015) as well as submarine melting and calving flux (e.g., Bartholomaus et al, 2015b).…”

“…Bartholomaus et al (2015b) observed that Yahtse's calving flux peaks in the fall, coincident with the timing of the full-width terminus retreat observed here. Seasonal terminus retreat at Yahtse shows a dependence on tidewater glacier dynamics governing seasonal changes in ice flux (e.g., Stearns et al, 2015) as well as submarine melting and calving flux (e.g., Bartholomaus et al, 2015b). We interpret that the terminus retreats from the crest of its submarine moraine, which would result in a loss of backstress that could cause the winter acceleration.…”

Cryoseismology

“…This relationship is strictly empirical, because signal attenuation as well as site‐related signal‐to‐noise ratio and local amplification effects do not allow application to other calving fronts and different source station distances. A similar relationship between T and V was later confirmed by observations on the same glacier by O'Neel et al [], on Yahtse Glacier (Alaska, USA) by Bartholomaus et al [] and on the Arctic archipelago of Svalbard (Norway) (A. Köhler and C. Nuth, personal communication, 2016).…”

“…Seismic monitoring revealed that calving mass loss depends on ocean tides and terminus geometry [ O'Neel et al , ; Walter et al , ; Bartholomaus et al , ; Koubova , ]. Recently, Köhler et al [] analyzed 14 year period (2000–2013) of seismicity on Spitsbergen and observed a high number of icequakes (1–8 Hz) associated with tidewater calving glaciers.…”

“…Unfortunately, monitoring of high‐frequency seismicity has not matured toward a global, stand‐alone tool for calving studies. 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.…”