Glacial earthquakes in Greenland

Tectonic activity is rare in Earth’s polar regions. But in 2003 seismologist Göran Ekström and colleagues discovered dozens of powerful but never-before-detected earthquake-like events hidden in the seismographic record and traceable to those regions–primarily Greenland, where they clustered along fast-moving outlet glaciers. The seismic events, pictured on the map, were produced not from slip on a fault, but from glacial dynamics. To account for their observations, the researchers proposed a bold hypothesis: Huge 10¹⁴-kg chunks of ice the size of Manhattan may sometimes lurch forward a few meters in a time span as short as a minute. Recent local measurements and satellite observations, however, implicate the calving of giant icebergs, not the slipping of the glacier’s bulk, as the source of the seismicity. Harvard University’s Victor Tsai and James Rice and the University of New Hampshire’s Mark Fahnestock have now captured the physics of the problem in a theoretical framework faithful to the observational constraints–the earthquakes’ magnitudes and time scales, among others. Slipping can be an apt mechanism, numerical simulations suggest, only if a glacier’s elastic modulus–effectively its spring constant–is substantially lower than the laboratory value for ice. Iceberg-calving events better match the constraints. In their analysis, the seismic signal originates not from the split of the iceberg from bedrock but from its rolling over in the fjord while pressing against the glacier wall or sea floor. (V. C. Tsai, J. R. Rice, M. Fahnestock, J. Geophys. Res. 113, F03014, 2008; figure adapted from V. C. Tsai, G. Ekström, J. J. Geophys. Res. 112, F03S22, 2007.) — R. Mark Wilson