| Earthquake prediction is a recognized worldwide problem,especially the short-term earthquake prediction is difficult to break through up to now.One area that may hold promise in advancing the science of short-term prediction is the study of earthquake precursors.However,the understanding of seismic phenomena,especially precursory phenomena,is not clear at present,and there is still no clear answer as to what kind of precursory anomaly is the repeatable and reliable short-term precursors.Rock friction experiments and numerical simulation experiments are always effective means to study the mechanism of earthquakes and precursors.Although a large number of rock friction experiments show that the fault with experience pre-slip or precursor slip process before stick-slip instability,accompanied by the increase of acoustic events and the change of physical parameters such as voltage,the actual observation results are difficult to be consistent with the rock friction experiments and seismic nucleation theory in the laboratory.Since 2001,with the discovery of episodic tremor and slip events in the subduction zone round the Pacific Ocean and the associated non-volcanic tremor signals,the study of slow earthquakes has become a striking direction.What's more,so-called abnormal signals such as“precursor wave”,“deformation wave”and“stress wave”by seismologists may be low frequency seismic waves generated by the slow slip of the fault.When considering slow earthquake events,there are at least four types of earthquake:(A)foreshock-main shock,(B)slow earthquake foreshock-main shock,(C)foreshock-slow earthquake main shock and(D)slow earthquake foreshock-slow earthquake main shock.The neglect of slow earthquake events may result in underreporting of(B)events and false report of(C)events.Therefore,the study of slow earthquakes and the observation and analysis of suspected slow slip signals are of great significance to earthquake prediction.It is generally believed that due to the observation limit of seismology(due to the increase of noise at periods exceed 200 seconds)and geodesy(due to weak deformation signals from events less than Mw 6.0),there are event gaps in slow earthquake events with duration ranging from about 200 seconds to 1 day.Since a complete seismic record should contain three-component translational signals and three-component rotational signals,when both the translational and rotational(tilt)responses of the seismometer are considered,the lowest effective frequency can be extended to the outside of the seismometer's pass-band record,and the out of the pass-band signals in broadband seismometer cannot be simply discarded.At the same time,due to the huge amount of seismic data,the traditional method of identifying anomalies manually cannot accurately describe the spatio-temporal characteristics of anomalies.With the development of seismic detection technology,especially the application of artificial intelligence technology in microseismic events detection in recent years,it is possible to search and explore the existence of such low-frequency signals in continuous waveform data.Because the abnormal signals recorded in the field may be due to the near-field effect generated by the movement of the fault itself,or it may be the propagation effect of linear or nonlinear seismic waves excited by the movement of the fault,in this thesis the spring-slide block model and the newly developing crystal dislocation model Frenkel-Kontorova(FK)model are both used to study the slip evolution of macroscopic faults,especially the experimental conditions and influencing factors for slow slip.Then the evolution characteristics of nonlinear seismic wave propagation is simulated under the conditions of nonlinear and dispersion effects.Finally the deep autoencoder algorithm is used to make detailed spatial distribution statistics of the low frequency pulse-like signals in the past five months before the Wenchuan earthquake.Combining the distribution characteristics of the horizontal components of the rotational motions,the possible generation,propagation and receiving modes of low-frequency pulse signals near the Longmenshan fault are discussed.The main results of this thesis are summarized below:(1)According to the numerical simulation of the spring-slide block model,the evolution characteristics of the velocity pulse duration and the slip amplitude in the stick-slip motions are calculated:The velocity pulse duration Tslip and amplitude Vmaxof the velocity pulse decrease with the system stiffness k and the loading rate VL,especially the period decreases sharply at low loading rate,and changes slowly when the loading rate reaches 10-6 m/s.It is speculated that when the fault is in the slow slip stage,a small change of loading rate can produce a large periodic change.Combined with the conclusions of rock friction experiments,the pulse duration Tslip is inversely proportional to the system stiffness k,the loading rate VL and the effective normal stress?,and the pulse amplitude Vmax is inversely proportional to the system stiffness k,the loading rate VL,and directly proportional to the effective normal stress?.(2)From the theoretical solution of FK model,it can be concluded that the slip duration time T is directly proportional to the asperity space b and Poisson ratio v,and inversely proportional to the effective normal stress?.The numerical results show that the rupture velocity is closely related to the initial stress condition.The larger the stress gradient zone is,the faster the rupture velocity reaches.When the gradient zone reaches a certain width,the rupture velocity can exceed the shear wave velocity.The ratio of shear stress to normal stress is an important factor affecting slow rupture,sub-Rayleigh rupture and supershear rupture of the faults.(3)The one-dimensional FK model is applied to describe the main rupture process of the Wenchuan earthquake,and the calculated slip distribution is in good agreement with the actual source rupture inversion results.From the view of strain energy,the mechanism of the pulse-like anomalies recorded at Guza station before the Wenchuan earthquake is also analyzed.It is shown that if the fault moves slowly for about 20 minutes,the strain variation of 10-8?10-7 can be produced near the seismogenic zone,which is consistent with the strain recorded by Guza station.At the same time,by setting a lower initial stress ratio ?S-/?N,the slow rupture motion which similar to P wave can be simulated,and the propagation velocity is about4km/day.(4)In the numerical simulation of one-dimensional nonlinear seismic wave,when the nonlinear term and the dispersion term are both considered simultaneously,the propagation characteristics of linear wave can be obtained when the soliton is used as the source wavelet:the waveform shape and amplitude are remain unchanged in the propagation process,and the velocity of the wave is slightly less than that of the linear background medium.When the nonlinear of rock is further increased,it shows the characteristics of elastic-plastic wave propagation.The elastic-plastic wave does not propagate in regular spherical motions in the space.When it propagates to the elastic region,the same source signals cannot be found on different stations,and the synchronization signals may not be observed on different components of the same station.(5)The seismometer has a translational response,but the tilt response(rotational response)should also be considered.When the tilt response of the seismometer is considered,the frequency response function of the tilt is a low-pass filter,while the response of the translational signal is a band-pass filter.It is possible to preserve the low frequency signals which outside of the seismometer's pass-band record under the combined action of the two filters.So the seismic record considering the rotational response may widen the low frequency observation limit of seismology.(6)The deep autoencoder algorithm is used to calculate the spatical distribution of the low frequency pulse-like anomalies in Sichuan Province in the past five months before the Wenchuan earthquake.The results show that the stations with large number of anomalies are mainly distributed along the strike and on the east side of the Longmenshan fault zone,and are basically located in the area where the EW and NS component of the peak rotational motions are strong.(7)There are geological conditions for slow earthquake events in the Longmenshan fault zone,such as fluid,high pore pressure,high temperature,high Poisson ratio,etc.Slow slip is prone to occur in the range of friction stability parameter a-b?0 in the brittle-plastic transition zone.When the region is disturbed,the low-frequency pulse signal is excited and performs some elastoplastic characteristics under the nonlinear and dispersive terms.One single slow rupture event can be evolved into one wave,two waves or even multiple waves,spread in the form of non-spheric diffusion,and is easily recorded in the form of tilt(rotation)by seismometers or inclinometers. |
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