Research on Anomaly Changes of Microtremor before Strong Earthquake

Basing on the seismic continuous w aveforms from 12: 00 am on Apr. 28 to 1: 00 pm on May. 12 in 2008 recorded by Xuchang Station,w e quantitatively analyzed frequency domain and amplitude of microtremor variation before three M ≥7. 0 earthquakes,including Andrea Ivanov Islands M7. 0 earthquake on Apr. 2,2008, East coast of Honshu offshore M7. 1 earthquake on Apr. 8,2008 in Japan and Wenchuan MS8. 0 earthquake on Apr. 12,2008 in Sichuan. The conclusions w as obtained as follow ing: the microtremor anomalies w ere recorded by three three-component of seismometer before three earthquakes,and w as obvious in the vertical component. The 0. 2 ~ 0. 3 Hz spectrum amplitude value of Fourier spectral changed greatly,and the process had the characteristic that the earthquake occurred in the process of rapid increase-reach to maximum-decrease-decrease. The microtremor anomaly w as appeared in 2. 3,3. 3,2. 9 days before three earthquakes,w hich w ere impending anomaly.