Source Inversion of Glacial Earthquakes

Abstract:

Glacial earthquakes are very-long-period seismic events originating from fast moving marine-terminating glaciers, primarily in Greenland. They consist in large surface waves with dominant periods between 35 and 150 s that are detectable teleseismically. Several studies report a clear temporal and spatial correlation between major glacial earthquakes and the capsize of large, newly calved icebergs. The teleseismic waveform modeling shows that the seismic data are well-explained by a landslide-type source. Long-period seismic waves would result from the force exerted by the iceberg on the glacier and the underlying earth during its collapse. We propose here a method of waveform inversion to retrieve the source-time function of glacial earthquakes. The inversion is carried out in the frequency domain. Taking the inverse Fourier transform of the frequency components determined by the inversion we then obtain time series of forces in the East, North and vertical directions. The recent installation of high-quality seismic networks in Greenland provides valuable data to improve the analysis of the dynamic of such events. We use broadband data from the GLISN experiment to investigate the source process of major glacial earthquakes in the 20-100 s period band. We test the robustness of the method by showing the stability of the inverted source when different stations with varying epicentral distances are used in the inversion. Since observed waveforms used in the inversion are limited in a particular frequency band, the estimated source-time functions are bandpassed. Though their comparison to filtered classical centroid single force models used in other studies shows that the seismogenic process is more complex. We repeated the inversion for several events around Greenland to obtain statistics on different sources. We give a first order interpretation of the dynamic of the inverted sources when applicable.