A HIGH-FREQUENCY MAGNITUDE SCALE

A high-frequency magnitude scale (m) is proposed: m = 2 log a(hf) + 3, where a(hf) is the high-frequency level of the Fourier amplitude spec trum of acceleration in cm/sec (average or random horizontal component ), at a hypocentral or closest fault distance of 10 km. m can be deter mined from either instrumental data or the felt area of an earthquake. The definition of m has been arranged such that m = M (moment magnitu de) for events of ''average'' stress drop, in both eastern North Ameri ca (ENA) and California. m provides a measure of the stress drop if M is also known. The observed relationship between m and M indicates tha t the average stress drop is about 150 bars for ENA earthquakes, and a bout 70 bars for California earthquakes. The variability of stress dro p is much larger in ENA than in California. The chief justification fo r the m scale is its utility in the interpretation of the large preins trumental earthquakes that are so important to seismic hazard estimati on in eastern North America. For such events, m can be determined more reliably than can M or m(N) (Nuttli magnitude), and forms a much bett er basis for estimating high-frequency ground motions. When used as a pair, m and M provide a good index of ground motion over the entire en gineering frequency band. If both of these magnitudes can be defined f or an earthquake then a ground-motion model, such as the stochastic mo del, can be used to obtain reliable estimates of response spectra and peak ground motions.