Consequences of Sequence of Motion: Implications of Site Characteristics

Abstract

Traditionally, seismic safety is assessed for mainshock (MS) of a record as its damaging potential is perceptibly dominant than the foreshocks (FSs) and aftershocks (ASs) of the said event. However, nonlinearity in soil may be induced at very low strain levels, and hence, a sequence of motion may influence the characteristics of MS and structural response as well. Against this backdrop, the influence of FS-MS-AS sequence has been studied herein for three different soil strata representative of loose, medium, and dense sand employing 1D nonlinear site response analysis. The nonlinear behavior of soil is modeled with general quadratic or hyperbolic model (GQ/H) which satisfies both the small-strain and large-strain modeling of the backbone (initial stress–strain) curve. The hysteric behavior is modeled using extended unload-reload Masing behavior in order to properly represent damping at large strains. Effect of excess pore water pressure generation has been neglected. Earthquake sequences chosen from a number of real earthquakes include two FS-MS, five MS-AS, and two FS-MS-AS sequences. Limited case studies show that the changes in soil characteristics owing to seismic FS may hardly influence the characteristics of the subsequent MS; whereas, the properties of AS may appreciably be altered by the changes in soil characteristics attributed to the MS alone as well as FS-MS sequences. Energy dissipation in soil strata may be also increased substantially due to the consideration of FS and AS. The study provides useful insights on the implications of sequence of motions for structural safety assessment accounting for site characteristics.