A Methodology For Simulating Near-fault Broadband Ground Motion Of Large-scale Complex Sites And Its Application

Earthquake ground motions are determined by the source,propagation path and site conditions,which are highly complex to characterize.Furthermore,several major earthquakes in recent years have shown that near-fault sites are the most severe seismic zones.In the current context of frequent global earthquakes(e.g.,five major earthquakes of magnitude 7 or greater have occurred in China since 2008)and the construction of "resilient cities"(e.g.,Beijing has taken the lead in incorporating seismic resilience into general urban planning in 2018),the demands for accurate ground motion parameters close to fault regions with complex site conditions as well as multi-dimensional and multi-supported broadband inputs for seismic analysis of major engineering structures have become particularly urgent.In this paper,a FK/SEM/FEM modeling scheme for near-fault broadband ground motions is developed considering the contributions of source rupture,seismic wave propagation and site effects to ground motions,and then research on the near-fault seismic effect of large-scale complex sites and the whole process analysis from rupture to structural response is systematically carried out.The main research works are as follows:1.A frequency-wavenumber domain(FK)method for simulating broadband seismic wave propagation in crustal layers with a finite-fault source is proposed.First,the FK method based on the modified dynamic stiffness matrix is established with the aid of the solution idea of dynamic Green’s function in layered half-space,which can solve the broadband seismic wave propagation of dislocation point sources in semi-infinite space.Then,the principles of subsource division and source parameter setting of finite faults in seismology and geophysics are fully investigated,and a kinematic hybrid source model that can express both low-wavenumber determinism and high-wavenumber stochasticity is introduced.Eventually,a procedure for solving the finite-fault broadband seismic response based on point-source superposition is established,and its correctness is fully verified.2.A physics-based FK-SE hybrid method for simulating near-fault seismic wavefields in 3D complex sites is proposed.Following the concept of two-step domain reduction,the semi-analytical FK method is used to calculate the broadband seismic response in semi-infinite space,simulating the wavefield input from the source to the truncated boundaries;followed by a high-precision spectral element method(SEM)is used to finely solve the regional wavefield of the 3D complex site model,dealing with complex scattering and nonlinear problems.The formed hybrid method fully integrates the advantages of FK and SEM to realize efficient modeling of 3D broadband ground motion from finite faults to specific sites(including near-surface low-velocity soils),considering the overall physical mechanism.3.A FK-SE-FE hybrid method for seismic response simulation of engineering structures considering the "source-path-site" effect is proposed.Following the concept of three-step domain reduction,the seismic wavefield obtained by FK-SEM is used as multi-dimensional and multi-supported near-fault seismic input,and then the finite element method(FEM),which is more practical in engineering,is used to complete the fine modeling and elastic-plastic seismic analysis of engineering structures.The formed FK-SE-FE method builds a bridge from seismology to engineering,and realizes the physical modeling of the whole process from source rupture to structural response.In this regard,the time-history analysis of engineering structures based on earthquake scenarios can be performed,which is expected to provide a new idea and direction for the seismic evaluation of major projects.4.The presented methodology is widely applied to the problems of seismic field prediction,site seismic effects and seismic responses of site-structure systems under deterministic physical models,and the applicability of the established methods to full chain hazard assessment is comprehensively investigated.First,based on the FK method,the seismic response of a multi-scale(from crustal to geotechnical)site in Tianjin area due to dislocation point sources is studied.Meanwhile,3D near-fault ground motion simulation of the Menyuan earthquake with a magnitude of 6.9 due to a kinematic finite-fault source is completed.Second,based on the FK-SE two-step method,the near-fault seismic effects of 3D sedimentary basins are studied using the theoretical and actual sites as examples.Also,taking the 1679 Sanhe-Pinggu earthquake with a magnitude of 8.0 and the 2021 Yangbi earthquake with a magnitude of 6.4 as examples,the near-fault seismic fields in the actual regional-scale seismic zones are predicted by considering the topographic and geomorphic features.Finally,based on the FK-SE-FE three-step method,the seismic responses of engineering structures under earthquake scenarios are studied using benchmark flexural steel frame structures and Tianjin subway stations as examples.These examples reveal the comprehensive effects of focal characteristics,propagation paths and site conditions on earthquake disasters,and lead to meaningful and informative conclusions.The research results can promote the development of ground motion simulation based on physical mechanism in large-scale,broadband and whole process.They can provide direct theoretical support for the formation mechanism and development law of seismic hazards,and serve as a reference for the seismic protection of engineering structures in complex sites.These will contribute to the construction of "resilient cities" and tackle the bottleneck problems in the fields of near-fault site effects and multi-dimensional and multi-supported seismic inputs for major projects.