BIAS IN SURFACE-WAVE MAGNITUDE M-S DUE TO INADEQUATE DISTANCE CORRECTIONS

We investigate bias in surface-wave magnitude using the complete ISC a nd NEIC datasets from 1978 to 1993. We conclude that although there ar e some small differences between the ISC and NEIC magnitudes, there is no major difference between these agencies for this presentation of t he global dataset. The frequency-distance plot for reported surface-wa ve amplitude observations exhibits detailed structure of the body-wave amplitude-distance curve at all distances; the influence of the surfa ce-wave amplitude decay with distance is much less apparent. This cens oring via the body waves represents a large deficit in the number of p otentially usable surface-wave amplitude observations, particularly in the P-wave shadow zone between Delta = 100 degrees and 120 degrees. W e have obtained two new modified M-s formulas based upon analysis of a ll ISC data between 1978 and 1993. In the first, the conventional loga rithmic dependence of the distance correction is retained, and we obta in M-s(e) = log(A/T)(max) + 1.155 log(Delta) + 4.269. In the second, w e make allowance for the theoretically known contribution of dispersio n and geometrical spreading, to obtain M-s(t) = log(A/T)(max) + 1/3log (Delta) + 1/2log(sin Delta) + 0.0046 Delta + 5.370. Comparison of thes e formulas with other work confirms the inadequacy of the distance-dep endence term in the Gutenberg and Prague formulas, and we show that ou r first formula, as well as that of Herak and Herak, gives less bias a t all epicentral distances to within the scatter of the observed datas et. Our second formula provides an improved overall distance correctio n, especially beyond Delta = 145 degrees. We show evidence that Airy-p hase distance decay predominates at shorter distances (Delta less than or equal to 30 degrees), but for greater distances, we are unable to resolve whether this or non-Airy-phase decay predominates. Assuming 20 -sec surface waves with U = 3.6 km/sec, we obtain a globally averaged apparent Q(-1) of 0.00192 +/- 0.00026 (Q approximate to 500). We argue that our second formula not only improves the distance correction for surface-wave magnitudes but also promotes the analysis of unexplained amplitude anomalies by formally allowing for those contributions that are theoretically predictable. We conclude that there remains systema tic bias in station magnitudes and that this includes the effects of s ource depth, different path contributions, and differences in seismome ter response. For intermediate magnitudes, M-s(t) shows less scatter a gainst log M-0 than does M-s calculated using the Prague formula.