Nonlinear strong ground motion in the ML 5.4 Chittenden earthquake:: Evidence that preexisting damage increases susceptibility to further damage -: art. no. L23614

We use 55 repeating earthquake sequences located near the 1989 M-w 6.9 Loma Prieta earthquake to identify time dependent velocity changes in the shallow crust. In addition to large delays caused by the Loma Prieta mainshock, the M-L 5.4 Chittenden earthquake, an aftershock of Loma Prieta, caused direct S wave delays of up to 6ms and S coda delays exceeding 15ms. We attribute the delays to cracks formed or opened during the strong shaking of the Chittenden earthquake, the same mechanism believed responsible for the delays observed following Loma Prieta. The magnitude of the delays caused by Chittenden strongly correlate with those caused by Loma Prieta. This suggests that rocks recently damaged by nonlinear strong ground motion are particularly susceptible to further damage until they are completely healed. Therefore, we expect that the onset of nonlinearity will occur at substantially lower ground motion for large aftershocks than would otherwise be anticipated.