Loose soft soils amplify earthquake vibrations through seismic wave resonance
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Peer-reviewed literature establishes that loose soft soil deposits significantly amplify ground shaking and alter seismic wave propagation during earthquakes.
The variations in seismic response between deep soft soil sites with different shear wave velocities were not fully understood. This study focuses on the seismic response of deep soft soil sites in the lower reaches of the Yangtze River, China. A nonlinear dynamic finite element model was developed for two representative deep soft soil sites with borehole profiles and the shear wave velocity tested by the single borehole method. Two nonlinear cyclic constitutive models are used and thus compared through the site seismic response. To accurately calibrate the nonlinear cyclic model parameters, resonant column tests were conducted on 21 soil samples collected from the two boreholes. The results show that the peak ground acceleration (PGA) under low-frequency (Liuan) input motion was higher for soft soil sites compared to that under medium- and high-frequency (Kobe and Nahanni) input motions. The PGA amplification factor for deep soft soil sites under different input motions can be approximated by an exponential function. The peak ground acceleration tends to be lower as the equivalent shear wave velocity (Vse) decreases. The shapes of the spectral acceleration were similar for the two sites, despite a substantial difference in the Vse between them. Additionally, a crossover point was observed in the spectral acceleration for the two sites. The period corresponding to this crossover point increased with increasing intensity of input motions, indicating that the sites became softer with higher intensity and thus generally exhibited a longer characteristic period of the spectral acceleration. This paper also highlights the significance of selecting nonlinear constitutive models and the precise calibration of model parameters in the seismic response analysis of deep soft soil sites, providing a scientific basis for future similar site analyses.
A key issue for the estimation of ground shaking is the proper selection of input motions at the seismic bedrock. At the same time, the effect of the input motion scaling strategy on structural response is typically studied disregarding the presence of the soil deposit. In this work, different soft soil deposits are selected by varying the shear wave velocity profiles and the depth to the seismic bedrock, modelling the soil behaviour through a nonlinear constitutive model implemented into a fully coupled FE code. Seven input motions are retrieved for several selection strategies in conjunction with two seismic intensity levels. Hence, more than 300 one-dimensional ground response analyses are performed. The results of the analysed cases, which are presented in terms of spectral response at ground surface and amplification factors, indicate that: (i) the use of an advanced elasto-plastic soil constitutive model accounts for nonlinear ground response effects, including higher site amplification in the mid-period range and deamplification of the peak ground accelerations; (ii) the different scaling strategies lead to comparable mean values of the amplification factors, and (iii) the variability of the amplification factors is significantly reduced when the scaling strategy seeks the compatibility with the target spectrum over a specified period range. The research will aid the prediction of local seismic site response over large areas, particularly in the absence of the fundamental period of a structure and facilitate its use in general recommendation for quantifying and reducing uncertainty.
Basin presence is believed to affect the ground surface response due to earthquakes, particularly in areas around the basin edge. Previous studies showed that 1-D and 2-D wave propagation analyses resulted in significant differences in amplification at the basin edge. However, the link between 1-D and 2-D responses has not been studied for engineering practices. In practical application, seismic studies were commonly performed using 1-D analysis, for example, to develop a city micro-zonation map. Based on practical considerations, it is necessary to estimate the scaling factor for the 1-D analysis by considering the basin presence, particularly for one containing soft soil. There are three stages carried out in this study. The first stage: collecting data on some basin geometries for the 2-D modeling references and then defining selected site class and input motions. The second stage: modeling 1-D and 2-D wave propagation using D-MOD and Fast Lagrangian Analysis of Continua (FLAC), respectively. The third stage: comparing spectral acceleration resulting from the 1-D and 2-D analyses to obtain the scaling factors. This research studied and reported the relationship between PGA values varied as 0.2 g, 0.3 g, 0.4 g, and 0.5 g, basin geometry (e.g., the angle was set to 5°, 10°, 15°, 30°, and 45°, with depth and width variations of 0.0125, 0.025, 0.05, 0.075, 0.1, 0.2, and 0.4, while the basin width was adjusted to 500 m, 1 km, 2 km, and 4 km), and the spectral acceleration in several observation points on the ground surface. Based on this evaluation, a series of scaling factors are proposed. These factors can be used for spectral acceleration from available hazard maps, commonly developed based on 1-D analysis. The application example of this scaling factor is presented in this study, using the Bandung Basin case.
Soil characteristics play a pivotal role in seismic risk assessment and earthquake-resistant design, with softer soils exhibiting significant ground motion amplification that elevates structural vulnerability. Construction on soft soil sites presents engineering challenges, as seismic wave propagation significantly alters signal characteristics between the source and the site. This study focuses on two critical engineering parameters—Peak Ground Acceleration (PGA) and Peak Ground Velocity (PGV)—developing site-specific predictive models for soft soil conditions (Vs30 = 180–360 m/s). Utilizing 4,328 records from the PEER NGA-West2 database and 2,462 records from the Engineering Strong Motion (ESM) database, we derive magnitude- and distance-dependent attenuation relationships through nonlinear regression analysis. The models incorporate key variables: moment magnitude (Mw), epicentral distance (EpiD), and shear-wave velocity (Vs30). The PGA model exhibits accelerated attenuation at short distances compared to conventional models (e.g., Boore et al. 1993), while converging with Campbell (1981) at larger distances. For PGV, the formulation effectively captures intermediate-frequency amplification, addressing systematic underprediction by Joyner & Boore (1988) and overprediction by volcanic-region models (e.g., Tusa & Langer 2016). These advancements provide engineers with optimized tools for soft soil seismic design, directly addressing liquefaction risks, resonance effects, and structural performance.
Abstract A comprehensive seismic site characterization of Igbogene in the Niger Delta region of Nigeria was conducted to evaluate the site response and liquefaction potential. Field investigations included borehole drilling, Standard Penetration Testing (SPT), and Multi-Channel Analysis of Surface Waves (MASW) testing to obtain soil samples and characterize subsurface properties. Laboratory tests determined soil classification, index properties, density, and geotechnical parameters per ASTM standards. Mathematical correlations were developed through regression modelling to estimate SPT N-values, shear wave velocity (Vs), and liquefaction and safety factor factor based on soil depth, composition, moisture content, and effective stress. The correlations demonstrated strong agreement with measured field and laboratory data, outperforming existing empirical models. Subsurface conditions comprised predominantly loose to dense silty sands and soft compressible clays within the upper 30 m. Shear wave velocity profiles matched closely with the developed velocity correlation, averaging 5.2% error. Site response analyses and simplified liquefaction evaluations were conducted to assess seismic hazards. The results showed amplification factors ranging from 1.5 to over 3 for the loose sandy soils, demonstrating their ability to significantly amplify ground shaking. Liquefaction potential maps were generated based on spatial variations in geologic conditions. This comprehensive site charac
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