Abstract
A study of teleseismic, short-period (1 s) P-wave travel-time and amplitude variations in western North America reported here reveals a high level of correlation: early arrivals relate to low amplitudes and late arrivals to higher amplitudes. This trend is opposite to observations noted for the USA and southern Canada as a whole1. With some exceptions, seismic stations east of the Rocky Mountain front generally record faster travel times2–8 and higher amplitudes9–15 than those to the west. These differences have been attributed to higher attenuation and lower velocities in the upper mantle beneath the tectonically active western region11,16. The correlation in the west between P-wave travel-time residuals and relative amplitudes is consistent with elastic focusing and defocusing effects and is found for both World Wide Standardized Seismograph Network (WWSSN) and Long Range Seismic Measurement (LRSM) stations. These results are relevant to the estimation of yields of nuclear explosions because by reciprocity similar elastic focusing and defocusing effects can alter the amplitudes and travel times of P-waves from explosions.
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