Evaluation of site effects on ground motions based on equivalent linear site response analysis

Subhadra Nampally, Simanchal Padhy, S. Trupti, P. Prabhakar Prasad, and T. Seshunarayana

We characterized local sites to evaluate the site-specific seismic hazard of the Chennai city in South India. The results are based on analysis of synthetic ground motions for a target event of magnitude 6.0 at different sites in the city. We investigated the influence of model parameters on the characteristics of ground motion through simulations and found that stress drop largely influences both the amplitude and frequency of ground motion. To minimize its influence, we estimated stress drop after finite bandwidth correction. Using the estimates of shear wave velocity averaged over the top 30 m of soil (VS30), along with the synthetic ground motion database obtained, equivalent linear 1-D site response analysis that approximates the nonlinear soil behavior was performed Fundamental natural frequency, Peak Ground Acceleration (PGA) at surface and rock levels, response spectrum at surface level for different damping coefficients, and amplification factors are presented at different sites of the city. Liquefaction study was done based on the VS30 and PGA values obtained. The major findings suggest that the northeast part of the city is characterized by (i) low VS30 values (< 200 m/s) associated with alluvial deposits, (ii) relatively high PGA value, at the surface, of about 0.24 g, and (iii) factor of safety and liquefaction below unity at three sites (no. 12, no. 37, and no. 70; Figure 1). Thus, this part of the city is expected to experience damage for the expected M6 target event. For details, please refer the article referred below.

Reference

Subhadra et al. (2018). Evaluation of site effects on ground motions based on equivalent linear site response analysis and liquefaction potential in Chennai, south India. Journal of Seismology, 22(4), 1075-1093.

Figure 1. Factor of safety (FS) against liquefaction over the city at (a) surface level and (b) at 8 m depth. Note the low values of FS both at the surface and at depth for few sites (no. 12, no. 37 and no. 70) in the northern part of the city.