Scattered Waveform Modelling of Low Frequency Earthquakes for Fine Scale Structure and Depth Location

In this study, we perform waveform modeling of seismic waves for fine-scale structure of the northern Cascadia subduction using recently identified and located low frequency earthquakes (LFEs). We employ ray theoretical waveform modeling to investigate the LFE waveforms across a profile on southern Vancouver Island. Scattered phases within the LFE waveforms are first identified with subduction zone structure by their occurrence related to event (versus station) location along the profile. The initial ray theoretical modeling yields a structure for the subducting plate that includes a 3-4 km thick, uppermost low-velocity-zone (LVZ) with a high Vp/Vs ratio (2.4-2.7), similar to that found recently by Hansen, Bostock and Christensen (2012) using teleseismic P-waves. The locations of the LFEs relative to the LVZ can also be constrained by the timing of scattered phases generated at the LVZ boundaries. For the eastern portion of the profile, waveform modeling of the scattered waveforms is consistent with an origin of the LFEs at the top of the LVZ and thrust mechanisms aligned with the dip of the LVZ. However, in the western section of the profile, waveform modeling indicates that the LFE locations move from the upper boundary into the LVZ layer. This result is consistent with previous LFE locations mapped onto diffraction tomographic images of subduction zone structure.