Some Issues On Estimation Of Strong Ground Motion Field

The variation of ground motion at support point is very important for the seismic analysis of structures with large dimension such as long-span bridge, long-distance pipeline, large dam, and so on. The internal force of the structure with non-uniform seismic input may be much different from that with uniform seismic input. The time for each support point when seismic wave arrives is different. The excitation at each support point is incompletely coherent. The amplitude and frequency content of ground motion at each support point is also different. Multi-support excitation considering the spatial variation of ground motion is needed to describe these above differences. According to the statistical results, the correlation of ground motion within a distance of several hundreds is closely related to the distance and the frequency content of ground motion. In the frequency range below a certain frequency, ground motions at adjacent points are completely coherent and can be represented by deterministic wave theory; for higher frequencies, ground motions at adjacent points are incompletely coherent and highly random. The essential reason of the correlation in this spatial scale is because ground motions at each point are from the same source and very close propagation path. Ground motion in the near field is significantly affected by the source. The depth, size, orientation, mechanism and rupture process of the source affect the characteristics and spatial distribution of near-fault ground motion field.Based on the latest progress of the estimation of near-field strong ground motion, this dissertation addresses four links of the method in order to provide better technical support of the multi-support excitation to the seismic analysis of long-span structures.The importance to determine the global and local parameters of the source model considering the results from active fault exploration, local seismological and geophysical research was pointed out. Three issues were emphasized: the location, attitude and depth of the causative fault must be determined according to the results from active fault exploration; the determination of rupture length, width, and area should consider the local observation data sufficiently, either the rupture plane parameters or some other parameters; the fault segmentation could provide a reference for the location and size of asperity. The procedure that combines two kinds of estimation by truncated normal distribution and represents uncertainties was demonstrated. In order to avoid distortion in the spatial pattern of the estimated ground motion field, choosing mean source model by evaluating the response spectral residuals at trial points was proposed.Two issues in stochastical approach for synthesizing the high-frequency ground motion from point source were studied. Three improvements on the two issues were made, two for source spectral model and one for phase spectrum.The first one of the two improvements on source spectral model was connecting the rupture area to the frequency exponents in Masuda source spectral model while keeping the product of the two frequency exponents as 2.0. This improvement describes the decrease of the corner frequency with rupture development and maintains the estimated ground motion independent of the fault-discretization scheme. In the second improvement, the corner frequency from the total seismic moment was adjusted by the ratio of the sub-source moment and the average moment to reflect that asperity was the major scattering source of high-frequency wave. The scaling factor was also adjusted to keep the estimated ground motion and far-field received energy independent of the fault-discretization scheme. With the second improvement, the precision of stochastical approach was enhanced significantly. The observed data of the Northridge earthquake was used to test the effects of the two improvements.The phase spectrum of ground motion from a point source should be related to crustal parameters, propagation medium parameters, and the geometrical relation between the source and the observation point. The uncertainties in the estimated ground motion from adopting random phase spectrum should be reduced. A new idea to improve phase spectrum was proposed. In the proposal, random phase spectrum was replaced by the phase spectrum of the acceleration Fourier spectrum from the displacement caused by shear dislocation source in infinite homogeneous space, the complex spectrum was formed by combining the new phase spectrum and the estimated amplitude spectrum, and the acceleration time history was obtained by the inverse Fourier transform of the complex spectrum. The new approach was tested with different fault mechanisms and rupture starting points. The test results showed that the response spectrum of the synthesized acceleration time history using the above proposal can represent the mean level of ground motions with different random phase spectra. The observed data of the Northridge earthquake was also used to test the effect of this improvement. This improvement eliminates the distortion of the random phase spectrum on the spatial pattern of estimated ground motion field.The detailed computation of the displacement caused by shear dislocation source in infinite homogeneous space was introduced. Equations for variables such as point source orientation, dislocation vector, and normal vector were deduced in seismological universal coordinate system. Two issues, the simplification of numerical integration and the computation of sub-source rise time, were discussed. The results show that: when the sub-source size is smaller than 1km, the numerical integration could be simplified by multiplying the sub-source area to the displacement from point dislocation source; the sub-source rise time should be determined by the relation between magnitude and rise time rather than by sub-source dislocation.The consequences of filtration, numerical differentiation, and numerical integration in the superposition of ground motions at low and high frequencies were studied. Some reference principles were proposed: the displacement and velocity in high frequencies can be obtained by the integration of acceleration instead of by changing the ground motion indicator; When computing the velocity and acceleration from low-frequency displacement, low-pass filtration must be performed after numerical differentiation to avoid the oscillation; The final displacement and velocity could be obtained either by the integration of the superposed acceleration or by the direct superposition of the filtered displacement and velocity.Ground motion in the near field is quite complicated. This dissertation only improved some links of the estimation of strong ground motion field but didn't solve all problems. Issues that need to be researched further were proposed at the end of the dissertation.