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Abstract

The Hindukush- Pamir Himalaya region is one of the most active orogenic regions of the world having complex and peculiar tectonics, experienced several large to great earthquakes in the past. The area 25⁰-40⁰N and 65⁰-85⁰E, have been considered for b-value mapping. A homogeneous and complete seismicity database with cut-off magnitude Ms≥ 4.0 has been prepared for the period 1853 to Oct. 10, 2005 from different existing sources. The a and b-values for the study region G-R relationship (Log N = a – b M) has been estimated. In order to map a and b-values the most complete seismicity database for Ms ≥ 4.0 during the period 1963 to Oct. 10, 2005 has been used. The a and b-value has been estimated by maximum likelihood method using spaced grids of 0.50 X 0.50 with 50 to 200 nearest earthquakes and considering spatial variation of Mc. The spatial variation of ‘a’ value ranges from a low of 5.20 to as high as 8.51 and b-value ranges from a low of 0.7 to as high as 1.5. The Low b- value regions infer as the growing stress regime releasing larger magnitude earthquakes while high b value indicates increased crustal heterogeneity and low stress buildup with continued stress release through numerous smaller magnitude earthquakes. The b value varies as a function of depth suggest a random distribution of low to high b-value showing the heterogamous crustal structure beneath it. However, in subduction zones which may indicates a phase transformation of material in subducting slab.