Stresses and intermediate frequencies of strong earthquake acceleration

Abstract

The peak of smooth Fourier amplitude spectra, ((FS(T))_max, of strong motion acceleration recorded in California is modelled via dimensional analysis. In this model, the spectrum amplitudes are proportional to ō - (1) the root-mean-square (r.m.s.) amplitude of the peak stresses on the fault surface in the areas of high stress concentration (asperities), and (2) (log₁₀N)^1/2, where N is the number of contributing (sampled) asperities. The results imply simple, one asperity, earthquake events for M ≤ 5, and multiple asperity events for M ≥ 5 (N ~ 10 near M = 7and N ~ 100 near M ~ 8). The r.m.s. value of the peak stress drop on the fault, ō , appears to increase with magnitude for M ≤ 6, and then it levels off near 100 bars, for M ≥ 6. For M < 6, ((FS(T))_max continues to grow with magnitude, because of the larger number of asperities from which the sample is taken (N ~ 100 for M = 8), not because of increasing ō.