Relative importance of near-, intermediate- and far-field displacement terms in layered Earth synthetic seismograms

Waveform modeling and moment tensor inversion commonly assume only the far- field response to the seismic source. However, for certain frequencies and distances the near- and intermediate-field terms of the Earth's response be comes important. We developed a criterion to estimate the relative importan ce of these terms for horizontally layered Earth models by comparing reflec tivity synthetic seismograms, computed with just the far-field response (fa r-field terms only), with the complete response (all terms included). The f requency and range dependent differences of these two responses are quantif ied by measuring the difference between synthetics for regional and local e arthquake scenarios, the 1992 Little Skull Mountain, Nevada (M-W 5.7) and 1 997 Calico Hills, Nevada (M-W 4.0) earthquakes. For these crustal earthquak es, we found that the near-field term becomes important when the minimum fr equency, f(n) (Hz) of the simulation at hypocentral distance r (km) is f(n) = gamma cr(-1) where a reasonable crustal velocity for c is 5.8 km/sec and gamma is the fraction of a wavelength estimated in this study to be approx imate to 0.6. The far-field response converges to the complete response for frequencies greater than f(n).