Introduction
Glacial earthquakes, a type of seismic event originating in cold, icy regions, occur when large ice masses detach from glaciers and enter the ocean. First identified in the Northern Hemisphere over two decades ago, these events have been infrequently observed in Antarctica. A recent study, published in Geophysical Research Letters, provides evidence of hundreds of glacial earthquakes in Antarctica between 2010 and 2023. The majority of these events were concentrated at the marine terminus of the Thwaites Glacier, a significant contributor to global sea-level rise.
Mechanism of Glacial Earthquakes
Glacial earthquakes are generated when tall, thin icebergs calve from a glacier's end and subsequently capsize in the ocean. This capsizing process can involve a mechanical interaction with the parent glacier, producing strong ground vibrations, or seismic waves. These waves can propagate thousands of kilometers from their origin. A distinguishing characteristic of glacial earthquakes is their lack of high-frequency seismic wave generation, which differentiates them from typical seismic sources such as tectonic earthquakes, volcanic activity, or nuclear explosions, and historically made their detection more challenging.
Detection in Greenland vs. Antarctica
Historically, most detected glacial earthquakes have been located near the termini of glaciers in Greenland, the largest ice cap in the Northern Hemisphere. These events in Greenland are generally of significant magnitude, with the largest comparable to those produced by nuclear tests. They are routinely detected by a global network of high-quality, continuously operating seismic stations. Greenlandic glacial earthquakes exhibit seasonal variation, occurring more frequently in late summer, and their incidence has increased in recent decades, which may correlate with accelerated warming trends in polar regions.
Despite Antarctica being the Earth's largest ice sheet, direct evidence of glacial earthquakes caused by capsizing icebergs has been limited. Previous efforts to detect Antarctic glacial earthquakes primarily relied on the worldwide seismic network. However, if Antarctic events are of lower magnitude than those in Greenland, this global network might not be sufficiently sensitive for their detection.
New Antarctic Findings
The recent study utilized seismic stations located within Antarctica itself to search for these events. This localized approach identified over 360 glacier seismic events, most of which were not previously documented in existing earthquake catalogues. These events formed two primary clusters: one near the Thwaites Glacier and another near the Pine Island Glacier. Both glaciers are recognized as major sources of sea-level rise originating from Antarctica.
Thwaites Glacier Activity
Approximately two-thirds of the detected events (245 out of 362) were situated near the marine-terminating front of the Thwaites Glacier. The majority of these are interpreted as glacial earthquakes resulting from capsizing icebergs. Unlike the seasonal pattern observed in Greenland, the primary driver for these events at Thwaites does not appear to be the annual cycle of warm air temperatures. Instead, the period of highest glacial earthquake activity at Thwaites, recorded between 2018 and 2020, coincided with an independently verified acceleration in the flow rate of the glacier's ice tongue towards the sea. This speed-up may be linked to oceanic conditions, the specifics of which require further investigation. These findings suggest a short-term influence of ocean states on the stability of marine-terminating glaciers.
Pine Island Glacier Events and Future Research
A second cluster of detections occurred near the Pine Island Glacier. However, these events were consistently located 60–80 kilometers (37–50 miles) inland from the waterfront, making it improbable that they were caused by capsizing icebergs. The origin of these specific events remains a subject for further research.
The detection of glacial earthquakes at Thwaites Glacier provides opportunities to address fundamental research questions regarding the potential instability of the glacier. Specifically, it can aid in understanding the complex interactions between the ocean, ice, and solid ground at the glacier's marine interface. A more comprehensive understanding of these phenomena is critical for refining projections of future sea-level rise over the coming centuries. The complete collapse of the Thwaites Glacier is estimated to contribute up to 3 meters (10 feet) to global sea levels.