Nonlinear Strong Ground Motion in the 2004 Parkfield Earthquake

Justin L. Rubinstein and Gregory C. Beroza

Wednesday 130pm -- Lakeside Ballroom C

Using a moving-window cross-correlation analysis on repeating earthquakes, we identify reductions in seismic velocity coincident with the 2004 Parkfield Earthquake. Based on previous studies (Rubinstein and Beroza, 2004a,b; Schaff and Beroza, 2004), we interpret these velocity reductions as evidence of microcracking induced by the strong shaking in the earthquake (nonlinear strong ground motion). To provide another measure of the nonlinear nature of the strong ground motion, we examine the frequency content of the strong ground motion of the mainshock. We then compare the two measures of nonlinear strong ground motion to gain insight into the nature of nonlinear wave propagation.