This research is designed as a rigorous test of the hypothesis that variations in normal stress are a primary factor controlling stick-slip behavior in the laboratory. The hypothesis has been suggested by recent work on stick-slip behavior in foam rubber, and, if true, will have a major impact on current ideas about rupture nucleation and the characteristics of premonitory slip. The existence of normal stress variations during stick-slip has implications for rupture mechanics, fault behavior, and short- term earthquake prediction. Estimates can be made about the spectra of seismic waves recorded during a slip event which involves normal motions; these estimates will be compared with the actual foam rubber stick-slip data in order to assess whether normal motions have occurred. This work is a component of the National Earthquake Hazard Reduction Program.
