Abstract: By using more than 360 digital wave data of 52 earthquakes recorded by Liaoning Digital Seismological Network between November 11th 1999 and January 10^th 2000, we use the PISA software to study the corner frequency ratio of 27 foreshocks before the M_S5.4 and 25 aftershocks before the M_S5.1 (the largest aftershocks after the main one). The result shows that before the M_S5.4 and M_S5.1 of Xiuyan earthquakes, the corner frequency ratio f_cp/f_cs of longitudinal wave and transverse wave all have obvious dropping anomaly before the 2 fairly large earthquakes. This anomaly will be the reference of future earthquake forecast and the basis of foreshock judgment.

Key words: Earthquake wave spectrum, Corner frequency ratio, Foreshock, Digital seismological network