NONLINEAR GROUND-MOTION AMPLIFICATION BY SEDIMENTS DURING THE 1994 NORTHRIDGE EARTHQUAKE

It has been known since at least 1898 (ref. 1) that sediments can ampl ify earthquake ground motion relative to bedrock. For the weak ground motion accompanying small earthquakes, the amplification due to sedime nts is well understood in terms of linear elasticity (Hooke's law)(2), but there has been a long-standing debate regarding the amplification associated with the strong ground motion produced by large earthquake s. The view of geotechnical engineers, based largely on laboratory stu dies, is that Hooke's law breaks down at larger strains causing a redu ced (nonlinear) amplification. Seismologists, on the other hand, have tended to remain sceptical of this nonlinear effect, mainly because th e relatively few strong-motion observations seemed to be consistent wi th linear elasticity, Although some recent earthquake studies have dem onstrated nonlinear behaviour under certain circumstances(3,4), the si gnificance of nonlinearity for the type of stiff-soil sites found in t he greater Los Angeles region remains unresolved(5). Here we report th at ground-motion amplification due to sediments for the main shock of the 1994 Northridge earthquake was up to a factor of two less than the amplification observed for its aftershocks, These observations imply significant nonlinearity in such amplification, and bring into questio n the use of measurements of weak ground motion to predict the strong ground motion at sedimentary sites.