STUDY ON THE NEAR-FIELD / FAR-FIELD BOUNDARY IN FINITE-FAULT STRONG MOTION SIMULATIONS

Abstract

Near-field (NF) and far-field (FF) terms represent different properties of the wave-field: the near-source motions are more sensitive to the spatio-temporal details of the rupture process, while far-field terms carry the overall signature of a point-source. However, there is no clear distinction between far-field and near-field terms as their relative importance is frequency dependent. Due to their longer propagation path through complex Earth structure, the far-field wavefield is generally strongly affected by media properties and dominates at high frequencies. In contrast, the near-field terms have higher amplitudes at low frequency. In this study we compute full-wavefield near-field synthetics for a suite of earthquake scenarios, and compare those to corresponding finite-fault ray-theory seismograms that lack the near-field terms. This allows us to define the region around a fault in which NF-terms exert a strong influence on the radiated motions, and hence cannot be ignored. We analyze the simulations in terms of peak shaking levels (PGA, PGV, PGD) and response spectral values, but also consider the waveform characteristics in order to examine the engineering aspects of near-field and far-field components of the seismic wavefield.