A search for dilation in low frequency earthquake waveforms

The fault zone process that creates slow earthquakes remains unclear, but one proposed mechanism to limit slip speeds depends on shear-induced dilatancy and the resulting pore pressure changes.  This process would imply that the fault zone dilates during slow earthquakes.  So in this study, we search for the signature of dilation in the focal mechanisms of tremor’s low frequency earthquakes (LFEs).

It is, however, difficult to directly observe dilation in LFE waveforms.  The paths travelled by LFEs’ 1-10 Hz seismic waves are complex, and Earth structure is poorly known at the short wavelengths of interest.   This complexity makes it difficult to disentangle path effects from source properties.  Thus we look for differences in the seismic waves created by two groups of LFEs: groups of events that are in the same location but occur at different times.  The earlier events are smaller and thought to rupture through more solid, low-permeability fault rock and thus may have large dilation, while the later events rupture areas that have recently slipped and thus may have smaller dilation.

In our initial analysis, we stack and analyse waveforms of LFEs identified in Cascadia by Bostock et al (2015).  However, we identify no significant difference in the waveforms or any significant trends in the polarisation of the difference.  Preliminary results suggest that the early and late LFEs have waveforms that differ by less than 1-2%.  

That zero to minimal difference could indicate that there is no dilation.  Perhaps early and late LFEs are exclusively shear slip, and shear-induced dilatancy does not limit LFE slip speeds.  However, it is also possible that dilation is simply small.  The early, potentially large-dilation LFEs could have a dilation-to-shear slip moment ratio of 0.05, and the later, potentially small-dilation LFEs have a dilation-to-shear ratio of 0.04.   Such dilation would be large enough to significantly affect LFE slip rates but would not be visible with our current data and techniques.

In our continuing work, then, we seek to decrease the uncertainty in our observations by identifying and stacking more LFEs and by extracting more information from seismograms including many LFEs.