b'Education matters Theses in geophysics completed in 2021PhD theses Joanne Stephenson, Australian Nationalof IC anisotropy in the upper part of the University: Cylindrical anisotropy of EarthsIC. However, the origin of the anomaly Zhengguang Zhao, The University ofinner core re-examined through robustis poorly understood. Research in this Queensland: Automated microseismicparameter search. thesis focuses on a detailed analysis of event detection with machine learning. the waveforms from SSI earthquakes. Novel methods are employed to pick a comprehensive new data set of differential travel times from 36 events. Results show that the significant travel time anomaly present in the data is unlikely to be a result of systematic error in picking of the core phases. Instead, analysis shows that the anomaly is likely a result of strong heterogenous structure elsewhere within the deep The Earths inner core (IC) remains oneEarth. In addition to the new SSI dataset, of the most enigmatic parts of ourthis thesis contributes further data to planet. The structure and dynamics ofthe global PKP differential travel time the IC have been at the forefront of deepdataset, focusing primarily on polar Earth research ever since the discoveryantipodes.of Earths innermost shell in 1936 byMost significantly, this thesis Inge Lehmann. However, the formation,investigates the radial dependence internal structure and dynamics of theof cylindrical anisotropy in the Earths IC still remain a fundamental puzzleIC. Varying models have been created for researchers. The core of the Earthfor an innermost IC (IMIC), with radius is responsible for the geodynamo,anywhere between 300 and 800 km. generating Earths magnetic field andTo investigate the existence of the protecting life on Earth from solarIMIC, the direction of fast/slow axes Automatic microseismic event detectionradiation. and its radial extent, global absolute involves algorithms and/or workflows toGlobal seismology is an excellent toolPKIKP data from the International discriminate genuine microseismic eventsto provide insights into the IC. SeismicSeismological Centre (ISC) is used from noise. Limitations in traditionalwaves travel at different velocitiesin conjunction with the parameter physics-based methods are that user- through media of varying densities andsearch method the Neighbourhood defined thresholds impose too muchreflect off sharp discontinuities. ThisAlgorithm (NA). The merit of the ISC impact on the detection accuracy, andallows us to infer structure below thecatalogue is in the numerous available inappropriate pre-set thresholds are pronesurface of the Earth. In particular, thePKIKP travel times collected over many to bring about low detection accuracy. seismic phase PKIKP traverses the ICdecades, along with improved location In this thesis, we proposed aand is the primary data used throughoutalgorithms. The NA provides a robust Classification Is Detection strategythis thesis. As the number of datameans of testing this hypothesis where that cast microseismic detection as aincreases and computational techniquesit produces ensembles of all models supervised classification problem. Theimprove, the variability and complexitythat satisfactorily fit the data. This well trained support vector machineof IC models have increased dramatically.method can be employed without classifier was used as a microseismicEach model is unique, and interpretationregularisation or needing to employ event detector for distinguishingis often dependent on data choice,any subjective choices such as binning microseismic events from noise andprocessing and methodology employed.of phase data. In addition to the NA, proved to be able to achieve comparable,In particular, this thesis explores thea complementary likelihood ratio if not better, event detection accuracyhypothesis of IC cylindrical anisotropyapproach is used to further constrain than traditional methods. - the idea that seismic rays travelinguncertainty on anisotropy parameters. through the IC parallel to the EarthsThis investigation suggests that spatial rotation axis travel faster than thoseaveraging methods are unnecessary to parallel to the equator. observe the IMIC and may be partially responsible for disagreements between This thesis investigates the anomalouslymodels of IC anisotropy. Results further fast South Sandwich Islands (SSI) toshow that the IMIC is not defined by a Alaska seismic paths. On these paths,change in strength of anisotropy, but PKIKP differential travel times show aby a significant deviation in the slow consistent deviation of 2-4 secondsdirection of propagation at 650 km. from predicted travel times in theSuch a result could be interpreted as a outer region of the IC - an observationphase change in iron or mineralogical arguably responsible for the proposalimpurities at depth within the IC.DECEMBER 2021 PREVIEW 32'