In general, the depths of shallow earthquakes are poorly resolved in current catalogues. Variations in depth of ±10 km can significantly alter the tectonic interpretation of such earthquakes. If the depth of a seismic event is in error then moment tensor estimates can also be significantly altered. In the context of nuclear-test-ban monitoring, a seismic event whose depth can be confidently shown to exceed say, 10km, is unlikely to be an explosion. Surface wave excitation is sensitive to source depth, especially at intermediate and short periods, owing to the approximate exponential decay of surface wave displacements with depth. The radiation pattern and amplitude of surface waves are controlled by the depth variations in the six components of the strain tensor associated with the surface wave eigenfunctions. The potential exists, therefore, for improvements to be made to depth and moment tensor estimates by analysing surface wave amplitudes and radiation patterns. A new method is developed to better constrain seismic source parameters by analysing 100-20s period amplitude spectra of fundamental-mode surface waves. Synthetic amplitude spectra are generated for all double-couple sources over a suitable depth range and compared with data in a grid-search algorithm. Best fitting source parameters are calculated and appropriate bounds are placed on these results. This approach is tested and validated using a representative set of globally-distributed events. Source parameters are determined for 14 moderately-sized earthquakes (5.4 ≤ M<sub>w</sub> ≤ 6.5), occurring in a variety of tectonic regimes with depths calculated between 4-39km. For very shallow earthquakes the use of surface wave recordings as short as 15s is shown to improve estimates of source parameters, especially depth. Analysis of aftershocks (4.8 ≤ M<sub>w</sub> ≤ 6.0) of the 2004 great Sumatra earthquake is performed to study the depth distribution of seismicity in the region. Three distinct tectonic regimes are identified and depth estimates calculated between 3-61km, including the identification of one CMT depth estimate to be in error by some 27km.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:491494 |
| Date | January 2007 |
| Creators | Fox, Benjamin Daniel |
| Contributors | Woodhouse, J. H. |
| Publisher | University of Oxford |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Source | http://ora.ox.ac.uk/objects/uuid:6b89e41d-8dd0-4286-9bf0-d22c4a349bb7 |
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