ON THE INFERENCE OF ABSOLUTE STRESS LEVELS FROM SEISMIC RADIATION

This paper determines the conditions under which it is possible to learn the absolute stress level at a subset of points on a fault from observation of the fault kinematics. Specifically, the points on a rupturing fault can be divided into two groups, those points at which the rake rotates during the rupture process, and those points at which the rake does not rotate. If it is true that sliding frictional traction is collinear with the instantaneous velocity of one side of a fault, then at the former group of points (having rotating rakes) there is a unique absolute stress consistent with the motion, while at the latter group of points (having rakes that do not rotate) the absolute stress is unspecified by the motion. Published dislocation solutions for the 1979 Imperial Valley, 1986 North Palm Springs, 1987 Whittier Narrows, and 1989 Loma Prieta, California, earthquakes are examined for evidence of temporal rotations of rake at the hypocenters and largest asperities. The 1986 North Palm Springs earthquake shows evidence of a possible rake rotation at the hypocenter. This may imply a low absolute stress level.