Abstract: We selected 12 regional earthquakes with 4<M<7 in China to analyze the frequency spectrum of near-field P-wave acceleration for the first 3 seconds, and found out that the low-frequency information in the near field, especially lower than 1Hz, is more stable. Through processing the first 3-second near-field P-wave acceleration using low frequency narrow bandwidth filters, and redesigning the traditional magnitude formula, we obtained four magnitude calculation formulas M₁、M₂、M₃、M₄ in low frequency band and the magnitude estimation in earthquake early warning M_n. We apply the method and results to 13 earthquakes from K-Net catalog with 4<M<7 in Japan, and the result shows that the method has a better degree of dispersion and is more reliable, comparing with P_d magnitude estimation method which is widely used now. The magnitude calculation formulas in this paper are more reliable in estimating near-field magnitude and can be applied to earthquake early warning system.

Key words: Near-field P-wave, Earthquake early warning, P-wave magnitude