In this study, we have analysed the spatial variation of b-values (from frequency-magnitude distribution (FMD)) in the western Himalayas as an indicator to demarcate the potential zones of earthquake occurrences. This is done under the acceptance of interpretation that decrease of b-values is correlated with a stress increase in the epicentral region of an approaching earthquake event. In addition to this, the spatial association of the earthquake epicenters with the major thrusts in the region using weights of evidence method, to identify potential zones of earthquake occurrences have also been analysed. Both analyses were carried out using a historical earthquake (Mw> 4) database of the1900-2015 period. Finally, based on the spatial variation of b-values and ‘contrasts’ derived from weights of evidence method (thrust associations), the derived map information was geospatially combined to prepare a “spatial earthquake potential” map of the western Himalayas. This map demarcates the western Himalayas into 3 zones - high, medium and low potential for future earthquake occurrences.

No abstract

The Garhwal–Kumaun region of the Himalaya encompassing the state of Uttarakhand, India, has experienced several earthquakes in the past. Damage due to earthquakes is controlled by local site conditions, primarily resonance frequency and wave amplification from the ground. We present local site parameters with their site geology for 37 sites using ambient noise data. Horizontal to vertical spectral ratio technique is used to estimate the spectral ratio curves. Based on the type of curve, sites are classified into four classes, viz. clear peak, broad peak, double and multi‐peak, and flat H/V curve. Sites seen with clear or broad peaks are located on either soil or weathered rocks, thus indicating large impedance contrast and sharp discontinuity with large velocity contrast. Multiple peaks are observed in either soil or boulder bed and reveal large impedance contrast, probably representing shallow and thick strata. Sites with flat curves are found on weathered/phyllite/granite gneiss/granite schist rock types within highly dissected hilly areas. Fourteen sites have a peak frequency >6 Hz with a dominance of broad and clear peaks in the Lesser and Higher Himalayan regions. On the contrary, foothills and part of Siwalik sites exhibited a peak frequency between 1.14 and 4.94 Hz. The results demonstrate that sites with thick soil cover and boulder bed areas, that is, Doon valley and foothills, show low‐frequency peaks and hard rock or shallow bedrock sites, that is, Lesser and Higher Himalaya exhibit a higher frequency range. The estimated H/V amplitude and peak frequency values have shown a good correlation with site geology and geomorphology.