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Do iceberg collisions hasten deglaciation?
« on: July 28, 2020, 11:05:46 AM »
There appear to be two types of iceberg collisions, directly, or indirectly through seismicity/hydroacoustic tremor.

Icebergs calve off glaciers all the time. But most don’t pitch backward, capsize, and send seismic waves radiating out for thousands of kilometers.

New research reports that such “glacial earthquakes” have now been detected for the first time on Antarctica’s Thwaites Glacier. These observations confirm that Thwaites’s floating ice shelf is degrading. That’s bad news, scientists agree, because the glacier helps hold back the West Antarctic Ice Sheet from flowing into the sea.

For icebergs to capsize, they must be taller than they are wide. That’s common in Greenland because most glaciers there don’t contain floating ice shelves, said Winberry. “The edge of a glacier is grounded or close to touching the bedrock.” That ice thickness translates into icebergs being taller than they are wide, which renders them unstable in the water.

But Antarctic glaciers tend to have floating ice shelves, so their iceberg progeny are typically wider than they are tall and, accordingly, don’t produce glacial earthquakes. Thwaites appears to be an anomaly.

Notice that the distinction between Greenland and Antarctica will change with more breakups.

Seismic and hydroacoustic tremor generated by colliding icebergs
To identify the IHT source mechanism and to understand its relevance to iceberg calving, evolution, and breakup, we deployed seismometers on a giant (25 by 50 km) tabular iceberg called C16 in the Ross Sea, Antarctica, during a uniquely accessible period (austral summer, 2003–2004) when it was aground against the northern shore of Ross Island. During the deployment period, C16 was in sporadic contact with another giant tabular iceberg, B15A, that was moving under the influence of local ocean currents. This study reveals that the C16‐associated IHT was a manifestation of extended episodes of discrete, repeating stick‐slip icequakes (typically thousands of individual subevents per hour) produced when the cliff‐like edges of the tabular icebergs underwent glancing, strike‐slip collisions. The IHT signal that we observed is thus not a phenomenon associated with iceberg elastic or fluid resonance modes, but is instead the consequence of long sequences of very regularly spaced and similar pulses of seismic radiation from these constituent stick‐slip subevents. IHT represents a newly identified glaciogenic source of seismicity that can be used to improve our understanding of iceberg dynamics and possibly of ice shelf disintegration processes.

B15A fractured from multiple strikes against Davey Shoal and the adjacent Possession Islands; these strikes were driven by the combination of tidal currents and the coastal mean flow. The periods of iceberg‐sourced seismic radiation were correlated with the strikes. The iceberg‐ and land‐based seismic signals showed that the iceberg fracture, its sliding across the shoals, and the ice‐on‐ice stick‐slip contacts among the postbreakup iceberg fragments generated the strong chaotic and harmonic tremor episodes that were observed at distances as far as the South Pole, where these signals propagated as seismically coupled hydroacoustic T phases.
At the bottom of this study are wav's, part of the supporting information. Will later post a video on this topic.
Collision Calves Iceberg from Mertz Glacier Tongue, Antarctica
« Last Edit: July 28, 2020, 11:30:16 AM by oren »