AbstractsEarth & Environmental Science

Studies of seismic deconvolution and low-frequency earthquakes

by Alexandra Amélie Royer




Institution: University of British Columbia
Department: Geophysics
Degree: PhD
Year: 2014
Record ID: 2029715
Full text PDF: http://hdl.handle.net/2429/49978


Abstract

Deconvolution of seismic data is an important component of signal processing that aims to remove the seismic source from seismograms, thereby isolating the Green’s function. By considering seismograms of multiple earthquakes from similar locations recorded at a given station and that therefore share the same Green’s function, we investigate a system of equations where the unknowns are the sources and source durations. Our solution is derived using direct linear inversion to recover the sources and Newton’s method to recover source durations. For the short seismogram durations considered, we are able to recover source time functions for noise levels at 1% of the direct P -wave amplitude. However, the nonlinearity of the problem renders the system expensive to solve and sensitive to noise; therefore consideration is limited to short seismograms with high signal-to-noise ratio (SNR). When SNR levels are low, but a large multiplicity of seismograms representing a common source-receiver path are available, we can apply a different deconvolution approach to recover the Green’s function. In an application to tectonic tremor in northern Cascadia, we implement an iterative blind deconvolution method that involves correlation, threshold detection and stacking of 1000’s of low frequency earthquakes (LFEs) that form part of tremor to generate templates that can be considered as empirical Green’s functions. We exploit this identification to compute hypocentres and moment tensors. LFE hypocentres follow the general epicentral distribution of tremor and occur along tightly defined surfaces in depth. The majority of mechanisms are consistent with shallow thrusting in the direction of plate motion. We analyze the influence of ocean tides on the triggering of LFEs and find a spatially variable sensitivity to tidally induced up-dip shear stress (UDSS), suggesting that tidal sensitivity must partially depend on laterally heterogeneous physical properties. The majority of LFEs fail during positive and increasing UDSS, consistent with combined contributions from background slow slip and from tides acting directly on LFEs. We identified rapid tremor reversals in southern Vancouver Island with higher sensitivity to UDSS than the main front and which at least partially explains an observed increase in LFE sensitivity to UDSS with time.