Examples of near-field / far field motions Figures 5 and 6 display fault-normal and fault-parallel seismogram, and respective response spectra, for two rupture models (DS45Mod3 and DS75Mod5), considering a fault-normal array of 8 near-fault stations (see Figure 1 for location). Red traces mark full-wavefield COMPSYN synthetics that include near-field contributions, blue traces denote ray-theory ISOCHRONE synthetics. For reference, the (approximate) P- and S-wave arrival times are marked by vertical dotted lines (arrival times are computed with a 3D ray-tracing code; Hole, 1992). Since the near-field terms consist of both P- and S-waves, they arrive between the primary P- and S-wave arrivals. Moreover, they depend on the integral of the moment-release function, and hence exhibit often a characteristic ramp-like appearance in velocity seismograms. In Figure 5 and 6, some clear examples of the near-field term are marked with green circles To facilitate the comparison of the waveforms in Figure 5 and 6, we normalize each seismogram to unit amplitude; corresponding response spectra are computed for normalized waveforms. Examining these near-field pulses and their spectral response, we note that they may not be immediately visible on each component of motion, and they vary in strength. In fact, the relative strength of the near-field ramp seems to be stronger for stations at larger distance, which is probably an effect of the very strong single velocity pulse seen on the very near-source sites. This velocity pulse masks the near-field ramp due to its high amplitude. Note also that there are clear differences in the character of the near-field pulse for the steeply dipping rupture (DS75Mod5, Figure 5) and the more shallowly dipping event (DS45Mod3), although both share a normal-faulting source mechanism. Aside from their expression in the seismic waveforms, the near-field terms also leave a signature on the response spectra. Note that the COMPSYN synthetics (blue) are valid only in a frequency range of 0 – 2 Hz. We thus low-pass the ISOSYN synthetics (red) also at 2 Hz (which introduces occasionally a low-frequency ramp at the onset of the primary S-wave that resembles the near-field pulse). The response spectra should therefore only be considered for periods above 0.5 sec. Nevertheless, we see distinctly different spectral values SAT between the two sets of synthetics, over a period range from roughly 1 – 10 secs, suggesting that the near-field terms are important of a wide range of distanc...
