Abstract: Geomagnetic low-point displacement method and amplitude-phase method are frequently used for short term earthquake forecasting, both of which are based on distortions in diurnal variation of the geomagnetic vertical component. Low-point displacement method is able to extract the duration of anti-phase anomalous distortion, but this method is always with higher false rate and the reliability of each anomaly cannot be determined. In order to deal with this problem, the amplitude-phase method is used to 159 anomalies occurring during 2008 to 2018, and the transient difference area S_x and error C standing for the extent of anti-phase distortions are extracted as a standard to validate the low-point displacement anomalies before earthquakes. The results show that the criteria suggested in this paper makes the accuracy of low-point displacement anomalies related to earthquakes rise from 39% to 70%. It demonstrates the effectiveness of amplitude-phase method and its actual application ability. The statistical results also exhibit the regional differences, in which the ranges and mean values of S_x and C differ before the earthquakes in Xinjiang, Tibet, Yunnan and Sichuan regions.

Key words: Geomagnetic diurnal-variation, Geomagnetic low-point displacement, Geomagnetic amplitude-phase method