Interferometric Redatuming and Imaging of Low Frequency Earthquakes for Fine-Scale Subduction Zone Structure

We investigate the application of interferometric redatuming of seismic waveforms from low frequency earthquakes (LFE's) for fine-scale subduction zone structure. In this approach, seismic waveform data from two LFE sources recorded by an array at the Earth's surface are interferometrically redatumed to replace one of the sources at depth in the Earth by a virtual receiver. With many LFE sources along the top of a subducting plate, virtual source-receiver gathers can be constructed along the top of the plate boundary. Similar, but more involved approaches can also be applied to Wadati-Benioff seismic events which possess a wider depth distribution within the subducting plate. The construction of virtual shot-receiver profiles in the sub-surface has the advantage of effectively removing the distortion effects of the shallow structure above the plate and so affords the potential of providing more detailed images of the subduction zone structure itself. Here we perform initial numerical experiments for LFE sources and stations along a linear profile similar to that found in northern Cascadia. We first redatum synthetic P waveforms of LFE sources for a layered subduction zone structure including a dipping low velocity zone (LVZ) layer. The synthetic waveforms recorded at the surface array from a number of LFE sources are then redatumed to obtain a series of virtual common-source gathers along the top of the plate boundary, which can then be used for fine-scale imaging and velocity analysis of the plate itself. A second series of numerical experiments are performed using P waveforms of surface data from LFE sources to obtain virtual zero-offset profiles redatumed to the top of the plate structure. In these experiments a laterally varying lower boundary of the LVZ structure is imaged by the virtual zero-offset profiles at depth. In order to apply the redatuming approach to three-component seismic data recorded by LFE sources, an elastic formulation is required, however for this case the virtual receiver responses are no longer displacement but rather those of moment-tensor sensors. Here we investigate an asymptotic, elastic redatuming approach that explicitly identifies wave-mode components in the analysis.