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Relative Importance of Near-, Intermediate- and Far-Field Displacement Terms in Layered Earth Synthetic Seismograms

Geophysics and Global Security Division
Lawrence Livermore National Laboratory
Mail Code L-206
Livermore, California 94550

Manuscript received 30 September 1999.

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 becomes important. We developed a criterion to estimate the relative importance of these terms for horizontally layered Earth models by comparing reflectivity synthetic seismograms, computed with just the far-field response (far-field terms only), with the complete response (all terms included). The frequency and range dependent differences of these two responses are quantified by measuring the difference between synthetics for regional and local earthquake scenarios, the 1992 Little Skull Mountain, Nevada (M_w 5.7) and 1997 Calico Hills, Nevada (M_w 4.0) earthquakes. For these crustal earthquakes, we found that the near-field term becomes important when the minimum frequency, f_n (Hz) of the simulation at hypocentral distance r (km) is f_n = cr^-1 where a reasonable crustal velocity for c is 5.8 km/sec and is the fraction of a wavelength estimated in this study to be 0.6. The far-field response converges to the complete response for frequencies greater than f_n.

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