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.

The seismic activity in the capital city of Delhi is a matter of concern in the design and safety of numerous infrastructure facilities such as buildings, pipelines, railway lines and heritage structures. The city is continuously exposed to several small and large earthquakes due to local and Himalayan earthquake sources. Determination of ground motion in this region is an important problem for engineers to estimate hazard due to future damaging earthquakes. This article discusses the application of the spectral finite element method to simulate ground motion time histories in and around Delhi for regional earthquakes. The regional ground motions are obtained using the SPECFEM3D Globe package considering the effect of topography, bathymetry and three-dimensional variations of material properties and ellipticity of the Earth. The method is demonstrated for two local earthquakes near Delhi by comparing the peak amplitudes, arrival times and duration of the simulated time histories with the available strong motion records. Further, a hypothetical large earthquake is assumed in the epicentral region and ground motions are simulated. The statistics of the obtained peak ground displacements and the contours of permanent ground residual displacements are presented for the epicentral region.

In the present study, strong motions are estimated at 17 stations in Southern Peninsular India (SPI) for the 7 February 1900 Coimbatore earthquake (Mw 6) using the empirical Green's function (EGF) method. The broadband recordings of three small earthquakes of ML 3.5, 2.9 and 3.0 respectively, are taken as EGFs to simulate ground motion. The slip distribution of the main event is considered as a von Karman random field. The stress drops of the three small events estimated from finite fault stochastic seismological model lie between 130 and 140 bars. The peak ground acceleration (PGA) values, an ensemble of acceleration time histories and response spectra, are estimated at all the 17 stations using corresponding EGFs, and the mean response spectra are reported. Another estimate of PGA is also obtained using the stochastic seismological model. The estimated PGA values from the two methods are compared to check the consistency of the results. It is observed that the mean PGA values are within the bounds of the maximum and minimum PGA values obtained from the EGF method, while the differences at some stations can be attributed to the local site conditions. The ground motions simulated in the present study can be used to perform nonlinear dynamic analysis for future and existing structures in the SPI region for any event of magnitude Mw 6.

In this paper, the seismic susceptibility of Imphal city with respect to ten synthetically generated samples of the historic 1869 Cachar (Mw 7.5) earthquake that occurred in the Kopili fault is presented based on the finite-fault seismological model in conjunction with nonlinear site response analyses. For all the synthetic sample earthquake events, the mean and standard deviation of surface level spectral ground acceleration at peak ground acceleration (PGA) and natural periods of 0.3 and 1 s have been reported in the form of contour maps. These contour maps can serve as guidelines for engineers and planners to identify vulnerable areas for possible seismic disaster mitigation of Imphal city.

The spatial distribution of seismic site coefficients at Guwahati is presented in this article. Estimated site coefficients consider different site types in the city and several level of ground shaking consider average of probable seismic event. The evaluation of site coefficients is based on the standard penetration test data at 105 boreholes distributed over the city. Equivalent linear one dimensional site response analyses are performed to predict the site amplification and to determine the spectral acceleration response. Due to the unavailability of recorded ground motion data for the region, synthetic ground motions simulated at the bedrock level for the city corresponding to several combinations of magnitude (Mw) and source-to-site distance (R) are used as input excitation. Simulated synthetic ground motion data with varying levels of excitation have been used to take care of the uncertainties in the input ground motions. The analyses depict the modification of the seismic ground motion due to the presence of soil overlying the bedrock.

This study investigates seismic wave amplification effects in the Indo-Gangetic (IG) basin for possible large earthquakes in the region using spectral-element simulations. The Indo-Gangetic basin is a large and deep sedimentary basin that covers the northern part of India, in which several mega-cities are located, including the capital city of New Delhi. The seismicity in the region due to presence of many active tectonic faults is an important matter of concern for engineers. The damage caused in a future large earthquake could affect a huge population and hinder the development of numerous large-scale industrial establishments. Due to local soil conditions and the structural complexity of the sedimentary basin, seismic wave amplification is expected. However, the absence of seismic data for large earthquakes and limited knowledge of the structure of the basin poses challenge in estimating shaking amplifications. Therefore, we model the 3D structure of the basin using Spectral Finite Element method (Specfem3D) including the topography of the Himalayan mountains, and compute synthetic seismograms for a suite of simulated rupture scenarios. First, we use two past earthquakes in the basin to calibrate our 3D model by comparing the simulated ground motions with the recorded data. Later, we consider realizations of potential future large earthquake (Mw 7.1), by generating different kinematic rupture models. We simulate earthquake scenarios for different source parameters to quantify the statistics of expected ground shaking levels. We then infer seismic wave amplification as a function of both frequency and basin depth for complex seismic sources. Our results indicate a maximum amplification of 16 in Peak Ground Velocity (PGV) and 19–35 in Spectral Accelerations (Sa) at long periods. The results presented in this study may be useful for engineers to predict ground motions for future large earthquakes in absence of any available seismicity data.

In the absence of an array of strong motion records, numerical and empirical methods are used to estimate the ground motion during 25th April 2015 Nepal earthquake. Spectral finite element method is used to simulate low frequency displacements. First, the simulated ground displacement is compared with the recorded data at Kathmandu. The good agreement between the comparisons validates the input source and medium parameters. The spatial variation of ground displacement is depicted through peak ground displacement and Ground residual displacement (GRD) contours near the epicentral region. The maximum GRD is of the order of 0.6 m in east–west, 1.8 m in north–south and 0.6 m in vertical (Z) direction respectively. Stochastic finite fault seismological model is used to simulate acceleration time histories. First, the seismological model is calibrated for the region with the available strong ground motion records at Kathmandu. The estimated stress drop for main-event and aftershocks lie in between 50 and 95 bars. Acceleration time histories are simulated at several stations near the epicentral region. Peak ground acceleration (PGA) and spectral acceleration (Sa) contour maps are provided. The estimated PGA near the epicentral region varies from 0.3 to 0.05 g. Another estimate of PGA for the main event is obtained from damage reports. The estimated PGA from simulations and damage reports are observed to be consistent with each other. The average amplification in the Indo-Gangetic plain is estimated to be in the order of 2–6. The simulated results from the study can be used as the basis for the possible ground motion behaviour for a future earthquake of comparable magnitude in the Himalayan region.

Success of earthquake resistant design practices critically depends on how accurately the future ground motion can be determined at a desired site. But very limited recorded data are available about ground motion in India for engineers to rely upon. To identify the needs of engineers, under such circumstances, in estimating ground motion time histories, this article presents a detailed review of literature on modeling and synthesis of strong ground motion data. In particular, modeling of seismic sources and earth medium, analytical and empirical Green's functions approaches for ground motion simulation, stochastic models for strong motion and ground motion relations are covered. These models can be used to generate realistic near-field and far-field ground motion in regions lacking strong motion data. Numerical examples are shown for illustration by taking Kutch earthquake-2001 as a case study. © Printed in India.