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Title: Shear-wave anisotropy in the New Madrid seismic zone
Author: Rowlands, Helen Jennifer
Awarding Body: University of Edinburgh
Current Institution: University of Edinburgh
Date of Award: 1995
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Seismic anisotropy in the New Madrid seismic zone, the most seismically active area in central and eastern United States, is investigated using shear-wave splitting analysis of microearthquakes. The dataset comprises three-component surface recordings of nearly 1000 earthquakes recorded over 34 months. Three methods of improving shear-wave splitting analysis techniques are developed and applied, and the data are analysed for temporal variations in shear-wave behaviour that may act as a seismic precursor for earthquake prediction. Firstly, I investigate and define the shear-wave window in terms of incidence at an internal interface, rather than the free-surface because of the strong influence of a very low-velocity sediment layer. Secondly, I analyse propagation along repeated raypaths for any bias created in overall results and temporal changes. Thirdly, I determine focal mechanism solutions using a method that provides independent evidence of the presence of splitting, helps verify some individual splitting measurements, and demonstrates that shear-waves can be used to form well constrained solution sets from small local events. The splitting is interpreted in terms of approximately 4.5% anisotropy due to a structure dominated by vertical stress-aligned cracks, oriented N67°E ± 27°, and restricted to the upper 5 km. This direction is consistent with the maximum principal stress direction, approximately N80°E, as determined by superimposition of focal mechanism solutions. No temporal variations of splitting parameters are reliably identified, even using repeated raypaths. This may be due to the lack of sufficiently large and close earthquakes during the recording period, but may also be because the anisotropy is confined to the sedimentary rock above the seismogenic layer which may not experience significant stress changes prior to earthquakes.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available