S T G Raghukanth

In this article, a predictive model for ground motion characteristics is developed using the artificial neural network (ANN) technique. This model is developed to predict peak ground acceleration (PGA), peak ground velocity (PGV), peak ground displacement (PGD), spectral acceleration at 0.2 and 1 s. The input parameters of the model are moment magnitude (Mw), closest distance to rupture plane (Rcd), shear wave velocity in the region (Vs30), and focal mechanism (F). The updated NGA-West2 database released by Pacific Engineering Research Center (PEER) is employed to develop the model. A total of 13,678 ground motion records are used to develop the model. The ANN architecture considered in the study has four input nodes in the input layer, three neurons in the hidden layer, and three output nodes in the output layer. The ANN is trained by a hybrid technique combining genetic algorithm and Levenberg–Marquardt technique. The results of the study are found to be comparable with the existing relation in the global database. The model developed can be further used to estimate seismic hazard.

Sri Kedarnath temple, located in the Central Seismic Gap in Himalayan region in India is an ancient Hindu temple, which represents India’s rich history of culture, religion, science and technology. The temple structure is composed of concentric enclosures with the innermost sanctum sanctorum roofed with corbelled dome and tapering tower. Multi-leaf, dry stack stone masonry constructions with strong irregularities in plan and elevation along with the uncertainties related to the material and geometric characteristics of the temple pose various challenges to systematic seismic verification. Role of diaphragms to resist the lateral load is another area of concern. In addition, the selection of a unique seismic analysis strategy was a challenge. On the other hand, the effectiveness of non-linear dynamic analysis depends largely on selection of ground motion records, apart from the challenge of high computational demand. The current study addresses these issues by analyzing the outcomes of a comprehensive parametric study. Non-linear static and dynamic analyses have been performed on the 3D numerical model by considering the uncertainties in material parameters such as Elastic modulus of stone, Friction angle and compressive strength of stone joints. Synthetic ground motion records with PGA varying from 0.2 g to 1.0 g were used for TH analysis. The study reveals that the tapering tower above the sanctum is susceptible to collapse due to sliding shear failure.

The topography of a region significantly affects the ground motions generated during an earthquake. Due to the computational complexities involved, complete three-dimensional analysis of the effect of topography is not studied in detail. The present study tries to understand this effect by simulating ground motions on three-dimensional ridge and valley using a finite element methodology. Here, first the simulated results from the developed model are compared with the existing analytical expressions for wave propagation in two dimensions (2D). Then, a study is performed with topography profile considered as Gaussian with different source locations. The comparison between the cases is performed based on peak ground displacement (PGD) and peak ground velocity (PGV) amplification ratios. It is noted that the location and amplitude of amplification are related to the relative position and depth of source with respect to topography. The study also attempts to develop a relationship between the amplification ratios and topographic gradient. The developed models can be used as a basis to estimate the possible amplification that can occur at a site due to an earthquake.

Indo-Gangetic (IG) Basin, formed between the Indian shield and Himalayas, is the largest sedimentary basin in India. The region also constitutes many metropolitan cities, including the capital city New Delhi. The seismic risk in the region is attributed due to the proximity to seismically active Himalayan faults, the possible seismic wave amplification due to huge sedimentary layers, and the vulnerability due to urban agglomerations. However, the region lacks a dense set of recorded data curbing the direct assessment of ground motion intensities. Hence, seismic hazard needs to be estimated based on synthetic ground motions for possible scenario earthquakes. These ground motion simulations require a proper understanding of the spatial variation of material properties, viz., density and wave velocities in the region of interest. Hence, the present work focuses on developing the 3D regional velocity model for ground motion simulations in IG Basin spanning between longitude 74.5–82.5E and latitude 24.5–32.5N. The spatially varying material properties of the region are derived by suitably interpreting the available velocity models constrained according to geological features reported in the literature. The 3D velocity model derived from the study is first employed in a finite element platform to obtain low-frequency ground motion. These ground motions rich in low-frequency content are further combined with the high-frequency ground motion simulated using the Zeng-scattering method on a hybrid broadband ground motion generation platform. Hence, the simulated time histories comprise of energy in the period between 0 and 10 s, thus complying to engineering interest. The 3D velocity model developed from the study is validated using the strong motion data available for an event in the Main Boundary Thrust. The simulated time histories are observed to match the phase and energy content of recorded data. The model is further employed to simulate time histories for Mw 8.5 hypothetical earthquake in the Himalayas. The obtained response spectra are compared with the IS1893 code recommendations.

The present paper analyzes the effect of Himalayan topography on the characteristics of ground motions due to large earthquake events. A spectral finite element model is developed to simulate ground motions for two past Himalayan earthquakes viz. 2005 Chamoli earthquake and 2011 Sikkim-Nepal border earthquake. The simulations are carried out twice; in the first set of ground motion simulations, Himalayan topography is considered while a flat terrain is considered in the second set. It is observed that the recorded displacements and velocities for the two earthquakes show a good comparison with the simulations carried out considering Himalayan topography as compared to results for flat terrain. This observation affirms that ground motions on mountains are higher compared to those on flat terrains. Also, as the Himalayan region is a complicated cluster of several ridges and valleys, there is a need to define amplification factors specifically for this Himalayan region.