Mark D. Zoback, Gregory C. Beroza; Evidence for near-frictionless faulting in the 1989 (M 6.9) Loma Prieta, California, earthquake and its aftershocks. Geology ; 21 (2): 181–185. doi: https://doi.org/10.1130/0091-7613(1993)021<0181:EFNFFI>2.3.CO;2
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Analysis of the unusually diverse right-lateral, left-lateral, reverse, and normal faulting aftershocks of the Loma Prieta earthquake reveals a systematic relation between fault-plane orientation and slip direction; right-lateral aftershock planes strike slightly more north-ward (10°-15°) than the main shock, left-lateral planes strike slightly more westward, reverse planes dip slightly less steeply, and normal planes dip slightly more steeply than the main shock. We demonstrate that these slip patterns are consistent with an approximately uniaxial stress field acting nearly perpendicular to the main-shock fault plane. These results imply an extremely weak fault zone, possibly resulting from near-lithostatic pore pressure.