The detection and source characteristic of glacial earthquakes or “icequakes”.

Cryoseismic events are distinct from the earthquakes we have been characterizing and modeling so far in class. This wasn’t recognized until the latter part of the last century when a number of earthquakes were giving unusual results due to their atypical amplitude spectra.

What was obvious in these events, however, was their lack on high-frequency energy. As earthquakes scale in magnitude, typically the larger events lack in high-frequency signal; these smaller events, though, were breaking the rules. The answer to the puzzle was actually quite simple. The reason larger events lack the high-frequency energy is due to their long duration. Small events with short durations are very efficient at releasing short-period energy. Think about the Fourier Transform of an impulse function as an extreme. While these events (4.6 < M < 5.0) weren’t scaling up in magnitude, they were in duration.

As it turns out, a typical duration of an icequake is between 30 and 60 seconds – much longer than even many large earthquakes, which is great at quelling the high-frequency signal and the reason these events weren’t caught on traditional monitoring equipment using previous methods.

Later, attempts were made by Ekström et al. to invert the seismograms using the global-moment-tensor method to characterize the slip event. As expected, they ran into hurdles and the inversions were unstable. Their solution was to parameterize their inversion in terms of a centroid single force (CSF). A centroid single force model is a distribution of single forces equal but opposite of the slip direction. This is related to the event energy source being gravitational potential energy as opposed to elastic strain energy. Centroid single forces are also used in the characterization of landslides. CSF analysis can provide the product of mass and sliding distance, but neither independently.

Can anyone give a better description of a CSF?

Reference:   Ekström, Göran, Meredith Nettles, and Geoffrey A. Abers. “Glacial earthquakes.” Science 302.5645 (2003): 622-624.