Now showing 1 - 9 of 9
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    Seismic recurrence parameters for India and adjoined regions
    (01-10-2022)
    Dhanya, J.
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    Sreejaya, K. P.
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    This article focuses on estimating the seismic recurrence parameters of India and adjoining regions based on a comprehensive catalogue assimilated from various sources. The study region encompasses latitude 0 ∘ N–40 ∘ N and longitude 65 ∘ E–100 ∘ E. The updated catalogue for the region contained 69519 events, including 28770 mainshocks. The updated catalogue was employed in the estimation of recurrence characteristics of the region. Here, zonal and spatial smoothening-based approaches were employed to estimate seismicity characteristics on a grid of 0.1∘× 0.1∘. The active regions like the Himalayas, North-Eastern India, Andaman, and Koyna-Warna regions were observed to have relatively lesser b values indicating the occurrence of larger magnitude events and higher values for activity rate N(4), indicating a more frequent occurrence of an earthquake. The reported values can be further used in seismic hazard estimations for the region
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    Prediction of Ground Motion Intensity Measures Using an Artificial Neural Network
    (01-06-2021)
    Sreejaya, K. P.
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    Basu, Jahnabi
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    Srinagesh, D.
    The present study aims at developing a prediction model for ground motion intensity measures using the artificial neural network (ANN) technique for active shallow crustal earthquakes in India. The database for the study consists of 659 ground motion records collected from 138 earthquakes recorded by various seismic networks in the study region. Owing to the lack of near-field data, we have added 116 records from seven earthquakes over a distance < 30 km and M > 6 from the NGA database. The developed model predicts 21 ground motion parameters (GMPs) in both horizontal and vertical directions, with input predictor variables of magnitude (M), hypocentral distance (R), site condition (S), and flag for the region (f). A multi-layer perceptron (MLP), with a total of 276 unknowns, constitutes the architecture of the model. The residuals associated with the GMPs are analyzed in detail to aid in hazard calculations. In addition, a comparison of the developed model with global relations is performed. Further, the model is demonstrated by performing seismic hazard analysis for GMPs for 2% and 10% probability of exceedance in 50 years. The ANN model is a first version and has to be improved as more strong motion data becomes available for the region. The developed ground motion model must be combined along with other global models in seismic hazard analysis.
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    Seismic hazard map of India and neighbouring regions
    (01-12-2022)
    Sreejaya, K. P.
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    Gupta, I. D.
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    Srinagesh, D.
    This article presents probabilistic seismic hazard analysis (PSHA) of India and adjoined region, carried out to develop a new national seismic hazard map for India. The hazard map is developed using fault oriented spatially smoothed seismicity approach. A catalog of earthquakes has been compiled for the region (Latitude 50N − 400 N and Longitude 650E − 1000E) from 2600BCE to 2019CE to estimate the seismicity parameters. Eighteen suitable ground motion prediction equations in the logic tree framework are used for the four major geological regions of the country. The hazard is estimated at rock sites (B-C boundary type) conditions in terms of peak ground acceleration (PGA), short-period (0.2 s), and long-period (1s) spectral acceleration maps and uniform hazard spectra, with 2% and 10% probabilities of exceedance in 50 years. Higher hazard values are observed in the Hindukush-Pamir regions and Northeast India, whereas central India and the southern peninsular regions are less prone to seismic threat. The proposed maps find their application in the seismic design of structures, risk assessment, and as an input for updating the existing code provisions.
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    Generation of broadband spectra from physics-based simulations using stochastic LSTM network
    (01-11-2023)
    Sreenath, Vemula
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    Sreejaya, K. P.
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    This study aims to develop a model that predicts high-frequency response spectra and damage-related ground motion parameters using low-frequency physics-based simulations (PBS) for horizontal and vertical components. The model is a stochastic dropout-based long short-term memory (LSTM) network, which accounts for spectra interdependencies and high-frequency spectra's stochastic nature. Adding anelastic distance as an input term significantly improved the model's performance. Models are developed for different cutoff periods (validity of PBS low-frequency spectra): 0.75 s, 1 s, 1.5 s, and 2 s, using the 2015 Nepal main and aftershocks and combining them with global near-source strong-motion data. The models are validated using near- and far-field predictions and a good agreement with the recorded data is observed. The mean squared error, mean absolute error, and coefficient of determination for the 0.75 s horizontal model is 0.174, 0.3171, and 0.9295, respectively. Additionally, the study generates spectra and time histories for the 2001 Mw 7.6 Bhuj earthquake, which had no recordings. The obtained spectral values agree well with global stable-continental region models, and the time histories could capture characteristics such as duration, amplitude, and arrival times.
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    A Vertical-to-horizontal spectral ratio model for India
    (01-01-2022)
    Podili, Bhargavi
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    Sreejaya, K. P.
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    Srinagesh, D.
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    This paper presents the results of a study on the characteristics of vertical-to-horizontal ratio (V/H) of 5% damped acceleration spectra for Indian ground motion records. Preliminary analysis indicates that at least 50% of the records exceed the Indian seismic design code considered norm of 2/3 at both the short and long period ranges. In addition, currently there are no V/H spectral ratio models available for India and the global models do not capture the trend across all the periods efficiently. Therefore, new ground motion models for the V/H spectral ratio are derived for two of the active tectonic regions of India: the Western Himalayas and the North-East India. Moment magnitude, focal depth, distance, site soil class, and focal mechanism are considered while deriving these models for a wide range of spectral periods (0.01–10s). The goodness of these models is verified through comparison with other globally available V/H models.
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    Hazard consistent vertical design spectra for active regions of India
    (01-10-2022)
    Sreejaya, K. P.
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    Podili, Bhargavi
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    According to the current Indian Standard earthquake-resistant design code (IS 1893, 2016), the vertical design spectrum is assumed as two-thirds of the horizontal spectrum. However, a previous investigation of the Indian ground motion records suggests that the vertical spectra exceeded the horizontal in many instances, which would render the previous consideration inadequate. Therefore, the current study aims at developing an appropriate recommendation for obtaining vertical design spectra, through development of hazard consistent vertical design spectra for the active regions of India. The vertical design spectrum is generated through scaling of a weighted average vertical-to-horizontal ratio (V/H) ground motion prediction equation (GMPE) to the horizontal component of the uniform hazard spectrum. The uniform hazard spectrum is obtained for the two active regions of India– the Western Himalayas and the North East India through probabilistic seismic hazard analysis and the weighted average V/H model is obtained by combining the V/H GMPE developed for India with several global models. Based on the proposed relations, a structural engineer can obtain site-specific vertical design spectra with ease and accuracy.
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    Hybrid Broadband Ground Motion Simulation for 2015 Mw 7.9 Nepal Earthquake
    (01-10-2022)
    Sreejaya, K. P.
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    The present study aims at simulating broadband ground motions in the epicentral region of the 2015Mw 7.9 Nepal event, using hybrid broadband technique. The spectral element method is used to simulate the low-frequency ground motion. The three-dimensional material property variation and the basin geometry at the Kathmandu basin are incorporated in the spectral element model. High frequency synthetics are simulated using scattering Green's function approach by consistently using the source and medium model of low frequency simulation. The simulated Low Frequency (LF) (0-0.3Hz) results are combined with high-frequency scatterograms to generate broadband ground motions (0-10Hz). The scattering parameters for broadband ground motion simulation are estimated from the recorded data of Nepal mainshock. These parameters are used further for simulating the ground motions over a grid of stations at 2km × 2km spacing at the epicentral region. The simulated results are shown as peak ground acceleration (PGA), peak ground velocity, and spectral acceleration contours plots. The maximum PGA in the horizontal and vertical directions are 0.35g and 0.32g in the epicentral region. Also, the acceleration time histories and corresponding response spectra are presented for some of the selected cities in the region where no records are available. These simulated outcomes are used for analyzing the validity of Indian seismic codal provisions at the near-field of large Himalayan earthquakes. The results show the significant underestimation of near-field hazard by the codal provisions at the Himalayan region.
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    Seismic wave propagation simulations in Indo-Gangetic basin using spectral element method
    (01-01-2023)
    Sreejaya, K. P.
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    Srinagesh, D.
    This study focuses on developing a 3-D computational model of the Indo-Gangetic basin (IG basin) using the spectral element method. The region includes the subcontinent's most densely populated areas. The basin is unique as it consists of geologically younger sedimentary layers along with several ridges and depressions in its domain. However, the proximity of great Himalayan earthquakes and the presence of thick sedimentary layers of the basin results in higher seismic hazards. The limited instrumentation of the domain poses challenges in understanding the response of the basin due to a seismic event. This motivated us to develop a computational model of the IG basin by incorporating the best-known geometry, material properties and fine resolution topography. In the lateral direction, the modelled part of IG basin spans over ∼6° × 4° (between longitude 80.5°-86.5°E and latitude 25°-29°N). The validation of the developed basin model is performed by simulating the ground motions for the 2015 Mw 7.9 Nepal main shock and five of its aftershocks. Both qualitative and quantitative comparison of the simulated time histories suggests that the developed model could accurately simulate ground motions over a frequency range of 0.02-0.5 Hz. The developed basin model is then used to understand the seismic wavefield characteristics during the 2015 Mw 7.9 Nepal main shock. The spatial variation of peak ground velocity (PGV), as well as amplification, are investigated at a 0.2° × 0.2° grid and selected cities in the basin. The contours of PGV amplification indicate a higher value of ∼8-10 in the horizontal direction and ∼2.5-3.5 in the vertical direction for sediment depth >4 km. A comprehensive comparison of the simulated PGVs and the ground motion prediction equations shows that, while the simulations agree with the prediction, they also show heterogeneity of ground-motion distribution that cannot be fully described by empirical prediction relations. Hence the results from this study are more reliable and find applications in seismic hazard assessment of the cities in the basin. Besides, the results can be used to guide the installation of future seismic stations in the region.
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    A 3D computational model for ground motion simulation in Peninsular India
    (2024-08-01)
    Sreejaya, K. P.
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    Due to the gradual and constant accumulation of seismic energy, Peninsular India (PI) is typically considered seismically stable with low to moderate seismicity. The seismic studies in Peninsular India always resorted to synthetic ground motion simulations, because of the limited instrumentation and hence lack of recorded data. In the absence of a well-defined medium model for PI, the usual practice is to use simple site proxies or one-dimensional velocity structures for ground motion simulations. However, the region consists of multi-scale geometric complexities, significant topography, and sedimentary basins and is surrounded by deep oceans. Thus, the radiated seismic wave field in the region is influenced by the medium properties and in the absence of a well-defined tomography model the reliable estimation of seismic hazard is a challenging problem in PI. Therefore, the seismic wave propagation in PI can be investigated using numerical simulation with reliable 3D computational model for PI, incorporating the knowledge of the underlying Earth structure. Hence, the present study attempts to develop a sophisticated three-dimensional (3D) medium model of Peninsular India for physics-based ground motion simulations for regional earthquakes. This is aided by the availability of one-dimensional (1D) velocity models and the crustal structure from the receiver function analysis which provides valuable insight into the variation of material properties in the region. In the present study, >100 s of 1D velocity profiles are collected from various literature, which is then grouped under 23 different geological regions identified in PI (as per GSI (2000)). The averaged material properties are assigned per each geological region and the information on sediment depths, basin geometry, topography, and bathymetry are incorporated. We use the spectral element method (SEM) to calibrate our 3D computational model by simulating synthetic seismograms and comparing them to recorded ground motions for two past earthquakes: the 2001 Mw 7.6 Bhuj earthquake and the 1997 Mw 5.8 Jabalpur earthquake. Further, the seismic waveforms at the near field of 2001 Mw 7.6 Bhuj event are simulated using a refined regional model. The spatial variability of associated seismic intensities and peak ground velocity (PGV) amplification are investigated. In addition, a study of the impact of model depth truncation and sphericity on ground motion is also conducted. The implemented medium model is the first of its kind for Peninsular India and can reliably be used in seismic wave propagation studies in the region. The simulated outcomes from the model are of engineering importance as these results can be used for seismic hazard assessment of the region.