A group of geoscientists led by Dr Tavi Murray of Swansea University, UK, has shown that during the glacier edge breaking process, known as calving, the glacier moves rapidly backward and downward, causing the so-called ‘glacial earthquakes’ which until now have been poorly understood.

Glacial earthquakes have increased seven-fold since the 1990s and have been migrating north suggesting an increase in rates of mass loss from the Greenland Ice Sheet through calving.

In order to explain the unexpected movements of glaciers in the minutes surrounding the calving events, Dr Murray and co-authors used cameras, GPS sensors and a global seismographic network to closely monitor the Helheim Glacier – one of the largest glaciers in southeast Greenland (6 km wide and over 200 km long) – for almost two months in 2013.

The glacier retreated about 1.5 km during that time, and the team was able to capture details of 10 large-scale calving events.

Their findings published in the journal Science illustrate how calving icebergs fall away from the glacier and temporarily reverse the glacier’s course, causing glacial earthquakes that register about magnitude five on the Richter Scale.

“We were really surprised to see the glacier flowing backwards in our GPS data. The motion happens every time a large iceberg is calved and a glacial earthquake is produced. A theoretical model for the earthquakes and the laboratory experiments have allowed us to explain the backwards and downwards movement,” Dr Murray said.

Understanding this glacier behavior and the associated glacial earthquakes is a crucial step towards measuring calving events and their contribution to sea-level change remotely.

This tool has the potential to provide unprecedented, global and near real-time estimates of iceberg loss from the ice sheet.

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T. Murray et al. Reverse glacier motion during iceberg calving and the cause of glacial earthquakes. Science, published online June 25, 2015; doi: 10.1126/science.aab0460