Physics
Scientific paper
Apr 1983
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1983pepi...31..334o&link_type=abstract
Physics of the Earth and Planetary Interiors, Volume 31, Issue 4, p. 334-347.
Physics
10
Scientific paper
The short-period seismic phases known as P and Lg are often recorded at distances of 200-1000 km on long-range refraction profiles and are usually the largest-amplitude features on record sections for this distance range. P and Lg propagate as multiply reflected compressional and shear waves in a crustal waveguide whose principal boundaries are the Moho and the free surface. Equivalently, they can be interpreted as the interference pattern produced by a superposition of higher-mode P, SV and SH waves propagating in a leaky waveguide. For compressional waves, the waveguide efficiency is a strong function of frequency and depends on the presence or absence of low-velocity layers within a few kilometers of the surface, such as deep sedimentary sections commonly found in active tectonic areas. Such low-velocity surface layers create constructive interference effects for upcoming P waves incident at near grazing angles at the free surface and lead to efficient P propagation. Several good examples of strong P phases can be found on long-range refraction profiles for the tectonically active western United States; the 550 km profile eastward from SHOAL to Delta, UT is analyzed here. We have used a modified reflectivity-method computer program to model crustal phases for the SHOAL-Delta profile. The reflectivity technique accounts for all body and surface waves contributing to the short-period seismograms. It is found that the synthetic waveforms realistically model the observed P characteristics. In this case, the decay of P amplitudes with distance appears to be dominated by surface-reflection leakage from the waveguide rather than by anelastic attenuation due to Q of crustal rocks.
Braile Lawrence W.
Olsen Kenneth H.
Stewart John N.
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