Shear-wave anisotropy and the stress field from borehole recordings at 2.5 km depth at Cajon Pass |
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| Authors: | Yun Liu Stuart Crampin Rachael E Abercrombie |
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| Institution: | Department of Geology and Geophysics, University of Edinburgh, West Mains Road, Edinburgh EH9 3JW, UK. E-mail:;British Geological Survey, West Mains Road. Edinburgh EH9 3LA, UK;Department of Earth Sciences, University of Southern California, Los Angeles, CA 90089-0740, USA |
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| Abstract: | 53 local earthquakes recorded at 2.5 km depth in the Cajon Pass scientific borehole are analysed for shear-wave splitting. The time delays between the split shear waves can be positively identified for 32 of the events. Modelling these observations of polarizations and time delays using genetic algorithms suggests that the anisotropic structure near Cajon Pass has orthorhombic symmetry. The polarization of the shear waves and the inferred strike of the stress-aligned fluid-filled intergranular microcracks and pores suggests that the maximum horizontal compressional stress direction is approximately N13°W. This is consistent with previous results from earthquake source mechanisms and the right-lateral strike-slip motion on the nearby San Andreas Fault, but not with stresses measured within the uppermost 3 km of the borehole. This study suggests that the San Andreas Fault is driven by deeper tectonic stresses and the present understanding of a weak and frictionless San Andreas Fault may need to be modified. The active secondary faulting and folding close to the fault are probably driven by the relatively shallow stress as measured in the 3.5 km deep borehole. |
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| Keywords: | anisotropy Cajon Pass borehole San Andreas Fault shear-wave splitting temporal changes |
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