The Study In Key Techniques Of Ground Motion Simulation In Sea Area

Due to the frequent occurrences of earthquakes in China sea,it is essential to improve the reliability of earthquake resistance estimation and the earthquake resistance of coastal and offshore structures in cities and island reefs.The long-period components in offshore seismic records are anomalously larger than in their mainland counterparts.Thus,the direct application of the research results,obtained on basis of mainland seismic records,to seismic design or risk analysis of coastal and offshore structures might underestimate the seismic risk or load,in particular for the long-period structure.However,the deployment of large-scale seafloor seismic monitoring networks is rare in China,which results in the huge shortage of the needed onshore and offshore seismic records.Thus,it is necessary to establish numerical method to get the synthetic records for enriching the available onshore and offshore seismic records' database.In order for accuracy,such simulation method should be able to take into account the effects of seawater,seafloor sediment and topography,oceanic crust of which the thickness is typically much smaller than in the mainland case.Moreover,according to marine geological survey,the wave propagation in oceanic crust showing significant anisotropy.Up to now,a complete numerical method,taking into account above mentioned effects and the anisotropic property of medium in oceanic crust,is still not available which is the main focus of this paper.On basis of Legendre spectral element method for wave simulation,a complete method for seismic wave-field simulation in complex marine environment has been established after the realization of time-domain viscoelastic rheological and the anisotropic constitution modelling,fluid-solid coupling and the needed Perfectly Matched Layer(PML)for infinite-domain truncation in Legendre spectral element method.The main research works can be summarized as follows:1.Optimized approximation for time-domain viscoelastic rheological constitution model: based on the property that viscoelastic time-domain constitution modelling can be approximately established with generalized standard linear solids in an arbitrary finite frequency band,the impact on errors of approximation for constitution is analyzed in details.Via clarifying that the fitting error is dependent on the bandwidth rather than the specific values of upper and lower limit of frequency interval,the feasibility of obtaining the universal instruction for choices of the number of standard linear solids is illustrated.Taking into account the two factors,the ratio of distance of wave propagation to the interested wavelength in simulation and the valueof standard linear solids' number,the table for optimal choices of standard linear solids' number in modelling the viscoelastic constitution with different Q values is established by using the criterion for measurement of time-frequency goodness-of-fit value between the synthetic and observed waveform,which is also widely adopted for quantitatively assessing the error of both forward wave simulation and wave inversion.Furthermore,a method of using different number standard linear solids in generalized standard linear solids for modelling the layered viscoelastic media with significantly varied Q values is proposed,which solves the problem in the waste of computation and the required storage or lowering the accuracy of synthetic waveform caused by using the same number of standard linear solids to approximate viscoelastic medium in wave simulation.2.The construction and stable realization of time-domain weak-form PML: In case of infinite-domain truncation with uniaxial PML,a novel method for its construction has been proposed via complex stretching the weak-form wave equation in infinite domain,the obtained time-domain uniaxial PML is in weak-form which can be directly discretized by spectral element method.It avoids the numerical instability or inaccuracy caused by independently complex stretching the field equation and boundary and/or interface condition,of which the stretched ones could be improperly matched.Uniaxial PML for truncation of the infinite-domain with wave equations of coupling perfect fluid and viscoelastic solid has also been derived.Due to the viscoelasticity,the obtained PML contained the inverse Fourier transformation of multi-pole rational fraction in frequency domain,of which the classic treatment are complex and most often found to be instable.Thus,an universal and simple treatment has also been provided,which greatly improve the computational efficiency of the PML for viscoelastic wave simulation.We successfully and stably implemented our derived PML in wave simulation with Legendre spectral element.3.The construction and stable realization of time-domain weak-form multi-axial PML and the complete method for seismic wave-field simulation in complex marine environment: the methods for deriving the weak-form uniaxial PML are extended to time-domain weak-form multi-axial PML,of which the interface is approximately perfect.It overcomes the existing difficulty to use multi-axial approximate PML for the truncation of infinite-domain consists of layered heterogenous anisotropic media.The computation storage of the newly derived multi-axial PML are smaller compared with the classical ones.Via the combination of the above listed work,the complete method for seismic wavefield simulation in complex marine environment has been established.