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Generation of seismic hazard maps for Assam region and incorporation of the site effects

Bandyopadhyay Srijit, Parulekar Yogita M., Sengupta Aniruddha

Acta Geophysica

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2022

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Vol. 70, no 5

1957--1977

EN

Probabilistic seismic hazard assessment (PSHA), including a site-specific amplification study, is crucial to evaluate site-specific spectra of soil sites to better understand the behavioural patterns of the soils under earthquake excitation. This paper represents the results of the PSHA for Assam state, located in the highest seismic zone of India ever delineated to date. In that sense, the study area is divided into ten areal zones concerning seismicity source modelling to represent the seismo-genesis of the Assam state in detail. The earthquake recurrence parameters of each zone are obtained from Gutenberg–Richter (G–R) recurrence relation with updated homogenized and de-clustered earthquake catalogue from 1735 up to 2021. Earthquakes with magnitude greater than 4 (M>4) are considered using eight attenuation relationships for continental active shallow crust region, subduction zone and intraplate region. Hazard curves are obtained using a logic tree structure thus minimizing the epistemic uncertainty. The peak ground acceleration (PGA) value obtained at the rock outcrop of the Assam state for 10, 5, 2, and 0.5% probability of exceedance in 50 years with return periods such as 475, 975, 2475, and 9975 years lies between 0.24 and 0.34 g, 0.3 and 0.44 g, 0.42 and 0.59 g, and 0.56 and 0.91 g, respectively. The estimated PGA value at rock outcrop level is comparatively higher than that reported in the codal provisions. Site-specific response spectra at bedrock level (Vs=1100 m/s) for major cities (Jorhat, Tezpur, Silchar, Dibrugarh, Guwahati, Nagaon) of Assam state have been proposed for different earthquake return periods of 475,2475, and 9975 years. Finally, site amplification study is performed for Guwahati city and surface level 5% damped response spectra with PGA of 0.696 and 0.924 g are obtained for earthquake a return period of 2475 and 9975 years, respectively.

2

Hazard estimation of Kashmir Basin, NW Himalaya using probabilistic seismic hazard assessment

Yousuf Maqbool, Bukhari Kaiser

Acta Geophysica

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2020

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Vol. 68, no. 5

1295--1316

EN

This study presents hazard estimation of Kashmir Basin, NW Himalaya using regional ground motion relations, representing one of the most seismically active region in the Himalayan belt. Fault-level seismic recurrence parameters are determined from an updated earthquake catalogue spanning from 25 to 2018 AD for all possible seismic sources. The estimated hazard maps are presented for three ground motion parameters (PGA, short and long period spectral acceleration) for 50, 100, 500 and 2500 years return periods. Moreover, uniform hazard response spectrums and hazard curves are presented for all ten districts of the basin. The southern section of the basin consisting districts of Budgam, Shopian, Pulwama and Kulgam show higher hazard levels due to presence of numerous seismogenic structures in close vicinity. Our results highlight that the imposed seismic hazard in Kashmir basin is highly underestimated which need to be redressed by modifying the current provisional design standards.

3

Site-specific Probabilistic Seismic Hazard Map of Himachal Pradesh, India. Part I. Site-specific Ground Motion Relations

Muthuganeisan P., Raghukanth S. T. G.

Acta Geophysica

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2016

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Vol. 64, no. 2

336--361

EN

This article presents four regional site-specific ground motion relations developed for the state of Himachal Pradesh in northwest Himalaya, situated in a seismically active region. These relations are developed from synthetic free surface ground motion databases obtained from a calibrated stochastic seismological model considering the characteristic properties of this specific region. The adopted methodology incorporates the site effects characterised through active MASW tests conducted in 22 important cities. The estimated ground motion levels from the developed relations are found to be in reasonable agreement with the recorded data.

4

Site-specific Probabilistic Seismic Hazard Map of Himachal Pradesh, India. Part II. Hazard Estimation

Muthuganeisan P., Raghukanth S. T. G.

Acta Geophysica

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2016

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Vol. 64, no. 4

853--884

EN

This article presents site-specific probable seismic hazard of the Himachal Pradesh province, situated in a seismically active region of northwest Himalaya, using the ground motion relations presented in a companion article. Seismic recurrence parameters for all the documented probable sources are established from an updated earthquake catalogue. The contour maps of probable spectral acceleration at 0, 0.2, and 1 s (5% damping) are presented for 475 and 2475 years return periods. Also, the hazard curves and uniform hazard response spectrums are presented for all the important cities in this province. Results indicate that the present codal provision underestimates the seismic hazard at cities of Bilaspur, Shimla, Hamirpur, Chamba, Mandi, and Solan. In addition, regions near Bilaspur and Chamba exhibit higher hazard levels than what is reported in literature