Seismic discontinuities and order estimation using wavelets : a receiver function approach

Abstract

In this thesis, I explore the use of non-linear wavelet techniques to estimate the order and scale of velocity discontinuties in the mantle transition zone through waveform analysis of Pds converted waves. The converted phases are isolated through a single station/multiple event receiver function technique which uses a wavelet deconvolution and denoising known as WaRD. It is an edge-preserving damped least squares solution with a small water level and subsequent wavelet thresholding. The deconvolved data is then imaged through an imaging technique which maps the conversions to the depth domain. The Pds phases are then isolated through a windowing and weighting, and then matched to a fractional order spline using a greedy matching pursuit algorithm. The data for this study consists of 2 Australian stations, CAN (Geoscope) and WRAB (IRIS), and 5 Japanese stations (JIZ, SGN, TKA, TMR, and TYM) from the F-Net array (formerly Freesia). CAN and WRAB are located in a relatively quiet continental tectonic setting, while the Japanese stations are in a more complex subduction zone environment. TKA (southern Japan) and TMR (northern Japan) are each thought to be underlain by a single subducting slab. JIZ, SGN, and TYM are located in central Japan where the Pacific and Philippine plates meet, and the subduction zone is thought to be very complex, with 2 slabs intersecting directly below these stations. Order and scale estimates for both Pds phases were obtained for CAN, WRAB, and SGN, and only P410s and P660 estimates were obtained for JIZ and TYM, respectively. Signal complexity in the image stacks prevented the determination of order estimates in either Pds phase for TKA and TMR. Order and scale estimates for the 410km discontinuity range between 0.325-0.450, and 18-35, respectively. Estimates for the order and scale of the 660km discontinuity range between 0.225-0.325 and 23-31, respectively. The order estimates for the P410s at CAN and WRAB were lower (0.325) than the estimates at JIZ and SGN (0.400-0.450), while the order estimates for the P660s at CAN and WRAB were higher (0.325-0.350) than the estimates for SGN and TYM (0.225-0.275). The results are consistent with a mixture type model in which the shape of the velocity discontinuity is a cusp-like feature and is caused by a critical density of one mineral phase with another. The ability to determine the order and scale and possible lateral variations could have major implications for the current views of discontinuities in the mantle transition zone.