Backward-moving glacier key to glacial earthquakes

When an iceberg breaks off into the ocean, the glacier moves rapidly backward and downward, causing the characteristic glacial earthquakes, says a new research.

This new insight into glacier behaviour should enable scientists to measure glacier calving, when an iceberg breaks off into the ocean, remotely and will improve the reliability of models that predict future sea-level rise in a warming climate.

Glacial earthquakes have increased seven-fold in the last two decades and have been migrating north suggesting an increase in rates of mass loss from the ice sheet through calving.

"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," said lead author professor Tavi Murray from Swansea University.

"A theoretical model for the earthquakes and the laboratory experiments have allowed us to explain the backwards and downwards movement," he added.

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

For the study, the team studied the Helheim Glacier, one of the largest glaciers in southeast Greenland.

Icebergs calving from Helheim Glacier can be very large and have been measured up to 4 km across and with a volume of about 1.25 km.

The researchers installed a robust wireless network of Global Positioning System (GPS) devices on the chaotic surface of Helheim in order to measure velocity and displacement of the glacier surface.

Earthquake data was used to explain the unexpected movements of the glacier in the minutes surrounding the calving events.

Understanding this glacier behaviour is a crucial step towards measuring calving events and their contribution to sea-level change remotely, the authors said.

The study appeared in the journal Science Express.