Long and Short Period P‐wave amplitudes from Earthquakes in the Range 0°‐114°

Abstract

Long period (16 s) and short period (1 s) P‐wave amplitudes from shallow earthquakes recorded in the distance range 0°‐114° are analysed to separate the effects of earthquake size, epicentral distance and station structure. The LP and SP vertical component amplitudes are analysed separately so that any period dependent effects can be determined. The results show that distance effects on both LP and SP amplitudes are significant when the whole range 0–114° is considered but between about 30° and 90° the distance effects are not significant. Further the distance effects do depend on period in the range 0–114° but again between 30° and 90° the distance effects are independent of period. In determining magnitude then a single constant correction for distance can be applied to both LP and SP recordings in the range 30°‐90°. Outside this range the correction depends on both distance and period. Station effects are significant for both the LP and SP analyses with little correlation between the LP and SP effects for individual stations. The SP effects can be attributed to variations in Q in the upper mantle; stations recording below average amplitude being underlain by lower Q than those recording amplitudes above average. The SP effects for a particular station will thus also be source effects for shallow events with epicentres near the station. The value of applying corrections for such source effects to magnitude estimates is demonstrated using data from two explosions of the same yield fired at two widely different test sites. Without corrections for source effects the magnitudes differ by 0.6 units, with source effects the difference is 0.02 units. The LP station effects are more difficult to interpret, but for one group of stations on the coastal plains of the Gulf of Mexico and Eastern USA which show large positive effects (above average amplitudes) it is demonstrated by modelling that these effects could be explained by the amplification effects of the large thicknesses of sediment underlying the stations. Comparing the estimates of magnitude (earthquake size) obtained from the LP and SP analyses shows that on average the LP and SP magnitudes are equal but correlation of LP and SP magnitudes for individual earthquakes is poor. This lack of correlation may simply be due to lack of data since only a small number of readings (1–4) contribute to each magnitude estimate, but this needs further study. Copyright © 1974, Wiley Blackwell. All rights reserved