Research On Displacement And Frequency Spectrum Characteristics Of Microtremors Before Strong Earthquakes

Many geophysical informations are included in the microtremors on theseismograms. In this paper, the microtremors were studied in further based on theprevious study of microtremors velocity frequency spectrum.The data of this paper were selected from broad-bandseismic stations of Sichuan,Qinghai and Gansu seismic networks before17days of Wenchuan earthquake withMs8.0at May12,2008. These data include the signals of microtremor. The samekind waveforms are selected before14days of Yushu earthquake with Ms7.1at April14,2010. Velocity records of seismic waves have been transformed to thedisplacement records, then Fourier transform are used to analyse these displacementrecords. We get the following results:(1) Before Wenchuan earthquake, the amplitude of microtremor displacement ofall stations belong to Sichuan seismic network increased inordinately. Which meanslong period waves had appeared. Before the earthquake happened, the peakextremum of Microtremor Fourier spectrum at0.1—0.3Hz is increased persistentlyin2-3days. Microtremor in EW, NS direction changed before the earthquake.(2) Before Yushu earthquake, the amplitudes of microtremor displacement allstations from Qinghai, Gansu and Sichuan seismic network are not increased, whichmeans long period waves had not appeared. Before the earthquake, the Fourierspectrum peak extremum of microtremor at0.1—0.3Hz is increased in1-2days. Onthe base of the analyzing results of three components, we found that the angle ofarctan (NS/EW) and the angle of arctan (UD/NS) changed direction inversely inYushu station.(3) Through comparing the microtremor change before Wenchuan and Yushuearthquakes, we found that different earthquake have different features on themicrotremor before earthquake. Before Wenchuan earthquake, microtremor mightindicate that the slow dislocation occurred before the faults ruptured. Fault zone isinfluenced by western Bayankala block extrusion. With the stress accumulating, thefault rupture might slightly move toward to eastward. Before the Yushu earthquake,we could not find the low-frequency waves. It possibly related to the earthquake type and inoculation process.At present, the research of microtremor is insufficient, the frequency contentsshould be studied in further. The characteristics at different frequency range and therelationship of microtremor and earthquake are useful for the earthquake prediction.