A new characteristic parameter _max or peak displacement P_d. The comparison results show that for magnitude between 4.0-7.3, S_dτ has better correlation with magnitude and higher estimate accuracy than either τ^p_max or P_d. Hypocentral distance is not required when using S_dτ, but it can be used to further improve the accuracy of magnitude estimate. These results confirm that S_dτ can significantly improve the accuracy and timeliness of continuous magnitude estimation in an EEW system.