Application and Limitations of Redatuming to Low Frequency Earthquakes in Northern Cascadia

Abstract:

We investigate the application, as well as limitations, of interferometric redatuming to low frequency earthquake templates (LFEs) in northern Cascadia. This approach has the potential of effectively removing the distorting effects of shallow structure and providing more detailed images of the subduction zone structure. We first develop an alternate formulation for redatuming in elastic media where the interferometric integral is decomposed into compressional and shear components. We next test the redatuming approach with synthetic LFE P-waveforms to obtain virtual zero-offset and common offset seismic sections along the plate boundary. We then apply redatuming to observed P-wavetrains from reprocessed and relocated LFE templates that occur beneath a linear subset of stations in northern Cascadia. The effect of the reprocessing is to slightly increase the frequency content and provide better temporal resolution of the extracted LFE waveforms. Although the stations have an average spacing of 7.5 km, the spacing is irregular and multi-step auto-regressive reconstruction is applied to regularize and stabilize the redatuming by interpolating the data onto a denser grid. There are also complications due to the radiation patterns of the direct and scattered waveforms. Since scattered S-to-P wave amplitudes are significant over a restricted range of azimuth and distance, windowing is applied to enhance these arrivals when applying the redatuming. Although the redatuming results are generally consistent with the previous waveform modeling that placed the templates near the top of the Juan de Fuca plate, the increased frequency content and interpolation of the data allows for the identification of upward scattered S-to-P arrivals in several templates indicating that at least a subset of LFEs are occurring within a low velocity zone inferred to be upper oceanic crust of the subducting plate.