Reactivation of ancient Antarctic rift zones by intraplate seismicity

Buried deep beneath the Antarctic polar ice sheet, the geological structure and tectonic activity of East Antarctica have long remained unknown. The apparent lack of tectonic seismicity was thought to be anomalous relative to other continental interiors and has been attributed to a lack of intraplate stress due to the surrounding spreading ridges and low absolute plate velocity or to the weight of ice sheets increasing the normal stress. Here we report 27 intraplate tectonic earthquakes detected by the AGAP/GAMSEIS seismic array during 2009 in the interior of East Antarctica, which represents locally recorded seismicity in the region. The earthquakes are primarily extensional and located at shallow to mid-crustal depths beneath sedimentary basins aligned linearly adjacent to the Gamburtsev Subglacial Mountains. The basins may be part of an ancient continental rift system, which provides a zone of pre-existing tectonic weakness that focuses the seismicity. These events, when combined with events in published catalogues of Antarctic seismicity, indicate levels of seismicity in East Antarctica of the same order of magnitude as that of other stable cratons, such as the Canadian Shield.