Study Of Seismic Wave Amplitude,Similarity And Coda Wave Strength With SEM

The seismic waves excited by earthquakes are propagated in the earth and measured by seismic sensor and converted to digital signal data.The signal data are transferred to Seismic Network Center real time.There,different signal detection methods are employed to process the seismic data.Seismic event bulletins are built after the seismic signal detections are associated and the events are located.Seismologist may investigate the earthquake source parameters,earth structure with these earthquake bulletin and seismic wave data.These three processes are corresponding to seismic wave signal propagation,detection and earthquake or earth structure deeply investigation.Some researches are conducted to investigate the attenuation of Pn and Lg during propagation,cross-correlation detection method,similarity changes dute to different factor and the teleseismic P coda waves excitation.The crust and upper mantle structure are more complex and inhomogeneous comparing with earth deep structure.The seismic signals recorded by regional stations,for example Pn and Lg,are mainly propagated in upper mantle and crust.So the decrease of the regional signal amplitude is more complex than teleseismic signal.Together with the two dimension line source response to three dimension point source response conversion,the synthetic seismograms calculated by two dimensions Spectral Element Method(SEM2D)are used to investigate the amplitude decrease of Pn and Lg as the epicenter increasing.The researches in this thesis especially focus on the influence of the structure inhomogeneity and the Moho topography on the signal amplitude of Pn and Lg.The effect of velocity gradient on Pn amplitude is studied in this thesis.The influence of surface topography on the attenuation of Lg is also investigated.The research results show:the Pn amplitude may be abnormal variation due to the upper mantle gradient,Moho topography and upper mantle inhomogeneity.The abnormal variation amplitude observations of Pn across Western Tien Shan are investigated and modeled.The amplitude of Lg may be affected by the inhomogeneity of crust,the thickness variation of crust due to the Moho topography and the velocity singularity under the Moho.Furthermore,the surface topography has heavy influence on the amplitude of Lg wave.Although the Lg amplitudes may also be affected by several factors,the amplitudes of Lg are more stabler than these of Pn.This make the magnitudes estimated using Lg waves are more stabler than which estimated by Pn waves.The seismic wave signals are detected in order to build earthquake bulletin after the record data are transferred to Station Network Center.The short-time-average through long-time-average trigger(STA/LTA)is usually used in the seismic signals detection.These signals are general excited by earthquakes all over the world.The epicenter and source mechanism are unknown for these earthquakes.However,cross-correlation detector may be a more effective method to detect the seismic signal excited by some "repeating earthquake" of which the location and the source mechanism may be known.This detection technology combine the signal detection and the event identify together.Cross-correlation detection based on single 3-component station may have high false-alarm rate.To overcome this problem a new method is introduced in this thesis to screen out false detections based on that the triggering time difference between different stations should be approximately same as that for the template event.The test result of the detection for a quarry explosions show that the new method can eliminate the false detection efficiently without the detection capability loss.The relation between the seismic signal similarity and the source mechanism disturbance and the distance between two epicenters are investigated also.The seismic waves generated by two natural earthquakes may be similar if the epicenters of the two earthquakes are same and the fault solutions disturbing within a limit of tens degree.The cross-correlation coefficient may decrease rapidly for explosion source as the no-explosion component increasing in the source mechanism.This character make the cross-correlation detection method may be more efficient for explosions than for natural earthquakes.The synthetic seismograms in three dimension model synthesized by three dimensions SEM are used to investigate the relation between the seismic wave signal similarity and the distance between the two epicenters.The experience relation between the cross-correlation coefficient and the distance between the epicenters is hold true within special distance for real inhomogeneous media.A hybrid method program of SEM3D and general ray theory is introduced in this thesis.The hybrid program is based on the representation theorems in seismology.The wave-field and stress-field in the source region are numerically calculated with the SEM3D.Then,the teleseismic P displacement seismograms are synthesized by integrating all the contribution of displacement and stress on an interfacing in the source region.Because of the SEM3D's wave-filed calculation in the source region,a complex model can be calculated.The complex model may be consisted of irregular interface(for example irregular topography)or inhomogeneous structure.So,the hybrid program is sufficient for the investigation into the influence of structure model in the source region on the teleseismic P seismograms.The influences on teleseismic P waves of several complex structures around the source are studied in this thesis.The strong Rg waves generated by shallow underground explosion can be scattered to form teleseismic P coda waves by complex structure around the source.The strength of teleseismic P coda wave comparing to the direct P wave depend on the source depth in model structure with simple Gauss mountain topography.But the coda wave amplitudes aren't correlated clearly with the source depth for underground explosions in structure with topography of Northern Korea Nuclear Test Site.The analysis of the observed teleseismic P coda wave of 2009 and 2013 Nuclear Test in Northern Korea also show that the strengths of teleseismic P wave with different take-off angle and azimuth are different.This indicates that it is difficult to estimated the relative depth of different explosions with different locations using the teleseimic P wave coda strength.