- Ground-motion model is:
^{2}= 0.00645 ± 0.00382 and σ^{2}= 0.0527 ± 0.00525 (where τ^{2}is the inter-earthquake variance and σ^{2}is the intra-earthquake variance and ± signifies the standard error of the estimate. - Notes that errors in magnitude determination are one element that contributes to the between-earthquake component of variance and could thus cause apparent differences between earthquakes, even if none existed.
- Develops a method to explicitly include consideration of magnitude uncertainties in a random earthquake effects model so that the between-earthquake component of variance can be split into the part that is due only to magnitude uncertainty (and is therefore of no physical consequence) and the part for which a physical explanation may be sought.
- Applies method to data of Joyner and Boore (1981). Assume two classes of magnitude estimates: those
with estimates of M
_{w}, which assumes to be associated with a standard error of 0.1, and those for which M_{L}was used as a surrogate for M_{w}, which assumes to be associated with a standard error of 0.3. Find that the inter-earthquake variance is much lower than that computed assuming that the magnitudes are exact but that other coefficients are similar. Believes that the high inter-earthquake variance derived using the exact magnitudes model is largely explained by the large uncertainties in the magnitude estimates using M_{L}.