Shear-wave splitting is observed in a three-years dataset recorded at the Parkfield borehole digital seismic network, central California, and also at Cajon Pass with data recorded at a 2.5 km deep borehole, southern California. Shear-wave polarizations at five out of seven stations at Parkfield are aligned in the direction which is consistent with the direction of horizontal maximum compressive stress. In the fault zone, shear-wave polarizations are aligned approximately parallel to the fault. Temporal variation of time delays between two split shear-waves is detected before and after a M<SUB>L</SUB>=4 earthquake at Parkfield. The analysis of temporal variation based on the best quality data shows that the temporal change is significant at the 68% confidence level. The study of earthquake multiplets also shows similar variations. This means that there was probably a change in stresses, which modified the geometry of microcracks pervading the rockmass before and after this event, so that time delays between faster and slower shear-waves for the similar events travelling along nearly the same wave paths and occurred at different times display a distinct increase before the time of the M<SUB>L</SUB>=4 earthquake, and a decrease afterwards. Normalised time delays in the fault zone are found to be about twice as large as those in the intact rocks. This suggests that fluid-filled cracks and fractures within the fault zone are more extensive than those in the surrounding rocks, and that the alignment of fault-parallel shear-wave polarizations in the fault zone is attributed to the fault internal structures. Studying the causes of the temporal variation indicates that stress concentration before the M<SUB>L</SUB>=4 earthquake had probably increased the number of cracks or the radius of the cracks; after the release of strain energy after the earthquake, stress reduced to a lower level, accompanied by healing and flattening of the cracks. The change of crack density and aspect ratio associated with the M<SUB>L</SUB>=4 earthquake is interpreted as the cause of the observed temporal variation.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:664191 |
| Date | January 1995 |
| Creators | Liu, Yun |
| Publisher | University of Edinburgh |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Source | http://hdl.handle.net/1842/12445 |
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