The Amplitude-Distance Curve for Short Period Teleseismic P-Waves

Abstract

Summary An amplitude-distance curve has been computed for short period P waves in the range A=30-102 (and h=O) using the amplitudes of explosion signals only. Effects of source asymmetry can thus be discounted and the problem of deciding what amplitude to measure is reduced because the maximum amplitude of explosion signals always occurs in the first few cycles. To increase the consistency of the measurements all the amplitudes have been measured by the same person. Assuming the effect of distance is constant over a series of discrete intervals of distance, the amplitude-distance curve with its confidence limits has been estimated by least squares using data from 65 stations which recorded some or all of the explosions fired at six sites. The most striking features of the curve are: (1) a sharp maximum at about A = 35, (2) a minimum followed by a sharp increase at A = 75, and (3) a sharp minimum at A =93. Over the remainder of its length the curve is similar to the Gutenberg and Richter curve. Station magnitude corrections have also been estimated. An amplitude-distance curve derived from explosion data should be more reliable than curves derived from earthquake data: explosions have radially symmetrical radiation patterns and their seismic signals are almost always the same shape with the maximum amplitude occurring in the first few cycles. Differences in the amplitude of signals recorded from a single explosion at two recording stations are unlikely to be due to source asymmetry (a significant factor if earthquakes are shears), and the difficulties of defining what amplitude to measure for magnitude determinations are reduced. We have collected the available World Wide Standard Seismograph Network (WWSSN) and Canadian records of underground explosions and the amplitudes of the explosion signals have been read by one seismologist (Mr P. D. Marshall). The accumulated data has been used to estimate both an amplitude-distance curve and station magnitude corrections for short period teleseismic P waves.