The short-term anomalies detected by broadband seismographs before the May 12 Wenchuan earthquake,Sichuan,China

We investigated broadband seismographic waveform data in 7~13 May 2008 from 40 seismic stations of the National Digital Seismograph Network of China(NDSN) after the 12 May 2008 Wenchuan earthquake,showing that obvious precursor was widely detected throughout China.The premonitory phenomena is that tremor amplitudes in the period band of 2~10 s have increased steadily within 1~3 days before the main shock rupture.Anomaly tremors can also be detected by gravimeters and tiltmeters.Tremor signals are widely detected in mainland China,but they are very strong in Southeastern China and weak in Western China.The possible slow slip of South China block before the great Wenchuan earthquake may be the cause of tremors,which is related to some frictional processes of the slow slip.