NONLINEAR RESPONSE OF SOILS TO HORIZONTAL AND VERTICAL BEDROCK EARTHQUAKE MOTION

Abstract

Strong-motion data obtained in the near field during recent earthquakes, notably Loma Prieta in 1989, Northridge in 1994 and Kobe in 1995, have afforded the opportunity to evaluate the validity of certain theoretical developments in basic seismology and in soil dynamics used in ground-motion prediction for engineering purposes. Following a brief preliminary section containing essentially an overview of attenuation mechanisms affecting seismic waves in a weak-motion context and expressed in terms of quality factor, the case of nonlinear soil behavior under high-strain conditions will be addressed, illustrated by data from Loma Prieta and Kobe which display marked differences between transfer functions derived from weak motions and from strong motions. One of the more remarkable observations that can be made is the presence of very high acceleration values on the vertical component as opposed to the horizontal components obtained on alluvial sites. Such unusually high vertical accelerations might result from nonlinear soil behavior, as exemplified in downhole data for the Kobe earthquake. Finally, a mathematical analysis that enables the transition from linear to nonlinear behavior to be detected on an accelerogram is proposed.