FAULT DISPLACEMENT AND SEISMIC MECHANISM IN SHALLOW-FOCUS EARTHQUAKES.
BOEING SCIENTIFIC RESEARCH LABS SEATTLE WASH
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If, as found by Press and Jackson in the 1964 Alaska earthquake, the fault reaches down to depths of 100 or 200 km, the observed fault displacement at the surface corresponds to a stress drop of the order of 10 bar at the fault, far less than is compatible even with the low compressive strengths measured by Raleigh and Paterson on decomposing serpentine. Since the discrepancy cannot be explained by the frictional restraint of the Coulomb layer, it is suggested that the small stress drop has two other causes First, the fault layer hardens with increasing displacement by the branching of cracks and rotation of the blocks between them second, there is an aseismic layer between the Coulomb layer and the Raleigh-Paterson region which is too soft to permit frictional fracture and does not contain materials of low fracture strength such as decomposing serpentine. Calculations on this basis give consistent values both of the fault displacement and the energy release. The mean annual displacement of seismic faults reflects displacements between the adjacent continental and oceanic blocks the mean velocity of displacement is of the order of centimeters per year. If this were the consequence of a contraction or expansion of the earth rather than of convection, the earths radius would change by 1000 km or more in 200 million years, unless seismic activity of the present intensity existed only during a very small fraction of the life of the earth. Author