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Garhwal Himalaya; maximum credible earthquake; site amplification; spectral acceleration; attenuation relation.

In this paper, we present a seismic hazard scenario for the Garhwal region of the north-western Himalayan range, in terms of the horizontal Peak Ground Acceleration. The scenario earthquake of moment magnitude 𝑀_w 8.5 has a 10% exceedance probability over the next 50 years. These estimates, the first for the region, were calculated through a stepwise process based on:

• An estimation of the Maximum Credible Earthquake from the seismicity of the region and Global Seismic Hazard Assessment Program considerations, and
• four seismotectonic parameters abstracted from near field weak-motion data recorded at five stations installed in Chamoli District of the Garhwal region in the aftermath of the 1999 Chamoli earthquake. The latter include:
• The frequency dependent power law for the shear wave quality factor, 𝑄_𝑆
• the site amplification at each station using horizontal-to-vertical-spectral ratio and generalized inversion technique,
• source parameters of various events recorded by the array and application of the resulting relations between the scalar seismic moment 𝑀₀ (dyne-cm) and moment magnitude Mw and the corner frequency, 𝑓_𝑐 (Hz) and moment magnitude 𝑀_w to simulate spectral acceleration due to higher magnitude events corresponding to the estimated Maximum Credible Earthquake, and
• regional and site specific local spectral attenuation relations at different geometrically central frequencies in the low, moderate and high frequency bands.

Sankar Kumar Nath¹ Khemraj Shukla¹ Madhav Vyas²

1. Department of Geology and Geophysics, Indian Institute of Technology, Kharagpur 721 302, India.
2. Schlumberger Limited, Houston, Texas 77077, USA.

Manuscript received

11 September 2007

Manuscript revised

4 January 2008

Accepted

27 March 2008