Determining the timing of Helheim glacial earthquakes from glacier-based GPS time series

GPS data acquired on Helheim glacier, East Greenland, in the Arctic summer 2007 demonstrate significant temporal variations in glacier flow during glacial earthquakes. There is no coseismic offset in GPS position time series, but there is a significant change in the velocity of the glacier near the time of the earthquake. It would be useful to use the GPS time series to obtain a constraint on the timing of the earthquake that is independent from the seismic data, in order to investigate the mechanism of the glacial earthquake. We have tested several approaches for estimation of the time of the velocity change, including matched filter, curve-fitting, and Kalman estimator. However, providing an accurate (few minutes or better) constraint has been difficult, in part because of the level of systematic noise in the time series (due most likely to the poor multipath environment of the glacier), and also because of semidiurnal signals in the glacier due presumably to the ocean tidal flexure of the glacier. Attempts to separate the tidal signal from the earthquake signal using an EOF analysis have not been successful, because the EOF analysis results in the two signals, of presumably different origin, are described by the same temporal eigenfunction, unexpectedly indicating that these signals have the same spatial variability. We present our analyses, and discuss the particular challenges for GPS in this application.