Simulation Of Long Period Strong Ground Motion At Near-Field By Numerical Green Function Method

Strong ground motion apperceive the characteristics and laws of vibration beginning and developing near or on ground surface in earthquake, which deal with a series of the problems concerning the earthquake source, wave propagation path and local site condition. The influence of earthquake source on near-field ground motion is rather distinctly. It is difficult to deal with the influence appropriately in the research. The influence is also concerned especially in engineering earthquake-resistance. In order to meet the urgent need of Project, City Active Fault Detected and Earthquake Risk Estimation in China, after the research of random synthesis and simulation on the basis of stochastic finite fault, based on numerical Green function method, an simulation approach of long period strong ground motion in near field is developed, which complete the whole thought of strong ground motion simulation entirely. Strong ground motion records shows that the near-field ground motion is random in high frequency, and in low frequency, it is influenced by wave propagation path and local conditions. The numerical simulation is an effective approach in low frequency to deal with the 3D velocities structure of lithosphere and complex local site conditions. Limited by computation resources, the strong ground motion simulation is possible in long period of great than 1s . In order to improve the simulation efficiency and precision, we simplify the calculating model and approach reasonably, with witch all of the simulations can be finished in one Computation Center of Province. 1. Being divided into two steps, the calculation of numerical Green function is Simplified. Taking account of the influence of finite fault model, homogeneous medium around fault, velocities structure of overburden layers and local conditions, the 3D simulation of long-period strong ground motion at near-field can be divided into two steps. Firstly, based on kinematics dislocation model of finite fault, by the analytic method in infinite uniform elastic medium, the displacement time histories caused by all sub-sources can be synthesized on the bottom of overburden layer, which providing the input for finite element numerical simulation. Then, in asymmetry medium, with local artificial transmitting boundary, the explicit decoupled finite element simulation approach is applied to model the near-field strong ground motion. This simplified approach not only enlarges the computing region effectively, but also reduces the run time and storage in computer. 2. Defined the slip-time function of sub-sources on the finite fault systematically On a finite fault plane, the inhomogeneous characteristics can be expressed by the un-uniform distribution of average dislocation of sub-sources and the un-uniform of rupture expanding in temporal and spatial space. In this paper, the Brune model is used to character the slip-time function. For a certain sub-source on the fault, rising time T is related to medium stiffness μ, shear velocity Vs , the average dislocation D of sub-source and drop stress ?σ. The rupture delay can be calculated according to the distance from initial rupture sub-source and rupture velocity. Then slip-time function of every sub-source can be defined and the finite fault model can be established in terms of slip un-uniform distribution on temporal, spectral space. 3. Displacement field caused by earthquake moment tensor in infinite uniform medium In this paper, instead of simple couples, the earthquake moment tensor is introduced to describe fault attitude and the rupture mechanism, which also express the relationships among forces, slip direction and slip-time function of sub-sources. The dislocation could be equivalent to three-order earthquake moment tensor with 9 force couples and every couples expression include fault strike, dip angle, rake and so on. In infinite medium, according to the analytic formula of 3-order Green function, Green function of the 9 couples can be computed and added. The displacement caused by sub-source is the convolution of Green function and slip-time function in spatial and temporal space. And then the displacement field on the bottom ofoverburden layers is the displacement addition of all sub-sources. 4. Applicability and Reliability demonstration of the simplified in near field by several aspects (1) According to the source parameters and velocity structures in Mw6.7 Northridge earthquake, 1994, in infinite uniform elastic medium, Green function caused by one sub-source is calculated with analytic method and the simplified simulating approach. The results compatibility reveals that the two approaches are equivalent. (2) By the simplified approach, we simulate long period ground motions of four rock stations in Northridge earthquake. The simulation acceleration histories and velocity histories are coincident with their records in durations, amplitudes and envelopes. The large acceleration pulse and velocity pulse in the simulation show the hanging wall effect and rupture directivity effect, which are also accord to these records well. (3) In the paper, different dips and rupture mechanism of faults are taken into account in the simulation of long period strong ground motion in near-field, such as single lateral rupture and double lateral rupture of vertical strike fault, oblique normal fault, oblique thrust fault and oblique strike fault. The simulation characteristics of peak values distribution on the ground and the waveform display primary characteristics of hanging wall effect and rupture directivity effect. Other characteristics are also revealed in the simulation, such as the amplitude of parallel strike component is larger than that of normal strike component and vertical component. The vertical component and normal strike component have no distinct difference. Generally, the amplitudes caused by thrust fault are larger than caused by strike fault, and the amplitudes caused by normal fault are the smallest of all. (4) As an important part of finite element numerical simulation, long period strong ground motion is studied according to local site conditions effect. As an example, in Shidian Ms5.9 earthquake, 2000 in Yunnan Province, we set up 2D Basinmodel including its surrounding rock region. Taking the un-uniform displacement histories caused by earthquake source as input wave, basin effect is simulated by finite element method which contributed us to get the basin effect mechanism, long period ground motion character and soil amplification characteristic. As a conclusion, the approach developed in this paper is reasonable and applicable in simulating and predicting strong ground motion in near-field. Finally, after summary of the whole work, some ideas and works to be done more in the future are presented.