Abstract

Effects of Sediment Nonlinearity on Earthquake Ground Motions

I.Beresnev (Department of Geological and Atmospheric Sciences, Iowa State University, Ames, Iowa, USA)

e-mail: beresnev@iastate.edu

The soft sediments composing the upper part of geological media exhibit substantial nonlinearity when subjected to earthquake ground motions. The nonlinearity stems from the complex microstructure of dry sediments, which are filled with voids and loosely packed granular particles. The shear deformation is of primary importance for the dynamics of soil, since the shear waves cause most of the building damage during earthquakes. The shear nonlinearity of soil is of a strain-softening, hysteretic type, which means that the effective shear modulus decreases and damping increases with increasing strain. These effects have a distinct signature in seismic records. For practical purposes, the increase in wave damping due to hysteretic behavior at large strain is the most significant consequence of nonlinearity. It is known that, due to near-surface impedance gradients, sediments amplify earthquake motions, sometimes to damaging levels. Nonlinearity is a potentially mitigating factor, causing the reduction in soil amplification. This role of nonlinearity has remained poorly understood until the records from several significant earthquakes at dense networks have become recently available. The 1994 Northridge, California, earthquake is a clear example. We will review the data on soil nonlinearity inferred from recent earthquake records.

Section : 6