Detection And Research Of Earth's Free Oscillations Based The Continuous Observation

The sudden release of energy in a very large earthquake can set the Earth into vibration.These vibration can during tens of seconds to tens of minutes,which called the Earth's free oscillations.Earth's free oscillation have been used to constrain and provide into the internal structure of the earth and dynamics.The Earth's free oscillation can be divided into two categories.In spheroidal oscillations they are party radial and partly radial,and in toroidal oscillation they are pure tangential.Gravimeter,strain gauge,inclinometer,seismographs all can record the oscillation signal.The observatory network of the Crustal Deformation China Seismological Bureau was equipped with vertical pendulum tiltmeter,borehole tiltmeter,tunnel extension meters,multi-component borehole strainmeter,body strainmeter.All of these instruments can record the Earth's free oscillation.We detected the Earth's free oscillation excited by the Japan MW9.0 Earthquake using theses instruments'observations.In addition,comparing the different observed results which obtained by those instruments,we analyzed their capacity to observe different kinds of oscillation and the modes in different frequency band.Using signal enhancement methods?product spectrum over multi-group records?,even the gravest modes can be detected.The analog free oscillation signal be calculated using Mineos package.Comparing 15 seismograph observations of seismic network in Hubei with the analog signal,observations of these seismograph in low frequency band were analyzed.We got the following conclusion:?4?By analyzing the observations of vertical pendulum tiltmeter,borehole tiltmeter,tunnel extension meters,multi-component borehole strainmeter,body strainmeter,all of the spheroidal oscillation modes(₀S₃—₀S₃₀)and toroidal oscillation modes(₀T₃—₀T₂₀)excited by the Japan M_W9.0 Earthquake were detected.The results shows that the body strainmeter and the vertical pendulum tiltmeter has a better capacity for observing spheroidal oscillation.With regard to observing the toroidal oscillation,the multi-component borehole strainmeter got a best result.The vertical pendulum tiltmeter observed a few toroidal oscillation modes,and the tunnel extension meters can only observe some low-order modes.In addition,we made explanation to these differences from the observation principles.?5?Using signal enhancement methods?product spectrum over multi-group records?for the first time,all of the spheroidal oscillation modes(₀S₃-₀S₃₈),almost all of the toroidal oscillation modes(₀T₃-₀T₂₈)and 21 harmonic modes were observed.Compared with a single set of observations,product spectrum got a higher SNR.And even gravest modes were detected.The results shows that the tiltmeters has a special sensitivity to detect low-frequency Earth's free oscillation?1 mHz-5mHz?.?6?There is an excellent agreement between the observations and synthetics in 1.5mHz to 4.5 mHz band.In>4.5 mHz band,observed amplitude significantly lower than synthetics.It shows that the attenuation of the Earth's free oscillation signal in actual is higher than it in the PREM model in this band.In addition,Basing on all of the modes in 1.5 mHz to 4.5 mHz band,we analyzed the operation condition of the equipment.The results shows that most of the seismograph observation condition is good except few stations like YC,NZH,DJI.