Nonlinear soil response in the vicinity of the Van Norman Complex following the 1994 Northridge, California, earthquake

Ground-motion recordings obtained at the Van Norman Complex from the 1994 N orthridge, California, mainshock and its aftershocks constitute an excellen t data set for the analysis of soil response as a function of ground-motion amplitude. We searched for nonlinear response by comparing the Fourier spe ctral ratios of two pairs of sites for ground motions of different levels, using data from permanent strong-motion recorders and from specially deploy ed portable instruments. We also compared the amplitude dependence of the o bserved ratios with the amplitude dependence of the theoretical ratios obta ined from 1-D linear and 1-D equivalent-linear transfer functions, using re cently published borehole velocity profiles at the sites to provide the low -strain material properties. One pair of sites was at the Jensen Filtration Plant (JFP); the other pair was the Rinaldi Receiving Station (RIN) and th e Los Angeles Dam (LAD). Most of the analysis was concentrated on the motio ns at the Jensen sites. Portable seismometers were installed at the JFP to see if the motions inside the structures housing the strong-motion recorder s differed from nearby free-field motions. We recorded seven small earthqua kes and found that the high-frequency, low-amplitude motions in the adminis tration building were about 0.3 of those outside the building. This means t hat the lack of high frequencies on the strong-motion recordings in the adm inistration building relative to the generator building is not due solely t o nonlinear soil effects. After taking into account the effects of the buil dings, however, analysis of the suite of strong- and weak-motion recordings indicates that nonlinearity occurred at the JFP. As predicted by equivalen t-linear analysis, the largest events (the mainshock and the 20 March 1994 aftershock) show a significant deamplification of the high-frequency motion relative to the weak motions from aftershocks occurring many months after the mainshock. The weak-motion aftershocks recorded within 12 hours of the mainshock, however, show a relative deamplification similar to that in the mainshock. The soil behavior may be a consequence of a pore pressure buildu p during large-amplitude motion that was not dissipated until sometime late r. The motions at (RIN) and (LAD) are from free-field sites. The comparison among spectral ratios of the mainshock, weak-motion coda waves of the main shock, and an aftershock within ten minutes of the mainshock indicate that some nonlinearity occurred, presumably at (RIN) because it is the softer si te. The spectral ratio for the mainshock is between that calculated for pur e linear response and that calculated from the equivalent-linear method, us ing commonly used modulus reduction and damping ratio curves. In contrast t o the Jensen sites, the ratio of motions soon after the high-amplitude port ion of the mainshock differs from the ratio of the mainshock motions, indic ating the mechanical properties of the soil returned to the low-strain valu es as the high-amplitude motion ended. This may indicate a type of nonlinea r soil response different from that affecting motion at the Jensen administ ration building.