New frequency-based real-time magnitude proxy for earthquake early warning

The two frequency-based magnitude proxies currently employed by earthquake early warning systems in California are the predominant and the characteristic periods. These proxies, obtained using simple expressions that are valid for noise-free monochromatic signals, yield erroneous result. The log-average period, τ_log, introduced in this study, is calculated directly from the actual velocity spectrum of the first few seconds of the seismic record. Using data from South California and Japan consisting of 440 earthquakes whose magnitudes range between 3 and 7.3, it is demonstrated that τ_log is better correlated with the catalog magnitude than the predominant period and provides better magnitude assessment than the characteristic period for small magnitudes (M<4). The average prediction error is reduced with increasing the input interval up to 6 s. It appears that a single linear scaling describes the relation between log(τ_log) and the catalog magnitude for the entire magnitude range studied here.