In this paper, the role of basin shape in the site-city interaction (SCI) effects on the ground motion characteristics is documented. The effects of city type and city density on the free-field motion are also documented. The seismic responses of various 2D basin models with different shapes as well as city type and city density were simulated using a fourth-order accurate staggered-grid viscoelastic SH-wave finite-difference algorithm. The analysis of simulated responses of various site-city models, ground motion perturbations and spectral amplifications revealed that basin shape plays an important role in SCI effects on the ground motion. Further, a considerable SCI effect was obtained at the double resonance. The city density seems to be a more prominent parameter of SCI, which largely reduces the ground motion amplitude in the city as compared to the height of buildings. It was also inferred that the existence of different types of buildings may result in a decrease in coherency of building response.
This paper consists of two parts. The first part of the paper is concerned with developing a 3D staggered grid time-domain finite-difference (FD) code with fourth-order spatial accuracy for simulating the responses of viscoelastic geological models with a continuous variable grid size in all the directions. The 3D FD code is written in Fortran 90. In the developed code, the realistic damping in the timedomain simulation is incorporated based on the generalized Maxwell body rheological model proposed by Emmerich and Korn (GMB-EK) Geophysics 52,1252-1264, 1987 with improvements made by Kristeck and Moczo Bull Seism Soc Am 93, [2273][2274][2275][2276][2277][2278][2279][2280] 2003. The accuracy of implementation of the realistic damping is validated by comparing the numerically computed phase velocity, quality factors and spatial spectral damping with the same based on GMB-EK model and Futterman's relations. It is also concluded that the grid spacing ratio up to 5.0 can be used unharmed. The second part of the paper presents the application of the developed code to simulate the basement focusing effects on ground motion characteristics in an unbounded medium. The combined effects of sediment damping and the focusing caused by the hemi-spherical (HS) and hemicylindrical (HC) synclinal basement topography (SBT) on the ground motion characteristics are studied. The simulated responses and the computed snapshots revealed the SBT focusing and defocusing phenomenon, intense mode conversion and diffractions of the incident waves. A good match of the spectral amplifications caused by the HS and HC basement topography models at their focus with the same computed analytically also reveals the accuracy of the developed FD code. The response of elastic HS and HC basement topography models revealed an increase of spectral amplification with an increase of frequency. Further, an increase of rate of frequency-dependent amplification towards the focus was inferred. The average spectral amplification (ASA) caused by the elastic HS basement topography model at the focus is more than the square of the same caused by the elastic HC basement topography model, and this ratio of ASA is increasing with an increase of sediment damping.
The anomalous damage patterns developed by the focusing of seismic waves due to deep and shallow seated synclinal basement topography were reported during the Northridge earthquake of 1994 and the Nisqually earthquake of 2001, respectively. This paper presents the role of sediment velocity, depth and geometry of the basement topography in basement focusing effects on ground motion characteristics. An increase of amplitude of the mode converted and the diffracted waves with an increase of impedance contrast and curvature of the basement topography was inferred. It is concluded that the basement focusing effect is unaffected by the change of focal length due to the change of either sediment velocity or radius of curvature for a fixed chord length in the absence of sediment damping. Further, under a favourable condition, the focusing of multiples of the transmitted waves may cause much larger amplitude amplification than that caused by the focusing of the transmitted wave itself. Extensive spatial variations in ground motion level were obtained with the change of depth and chord length of the basement topography. A comparative analysis of the responses of semi-spherical basement topography (3D) and semi-cylindrical basement topography (2D) strongly suggests that 1D or 2D response of an area underlain by a 3D depression should not be used in predicting the ground motion.
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