Interpretation of Intensity Attenuation Relation of 1905 Kangra Earthquake with Epicentral Distance and Magnitude in the Northwest Himalayan Region

Seismic intensity information of historical earthquake of NW Himalayas has been utilized for evaluating the attenuation of the intensity with the epicentral distance. Purposefully, the observed intensity of big historical earthquake may implicitly incorporate some site effects arising due to the near surface geology or deposited soil. The isoseismal map of the 1905 Kangra earthquake yields an area of perceptibility defined by the intensity VII (RF Scale), which is mainly elongated in the NNW-SSE direction. However, the higher isoseismals of intensity VIII and IX are more symmetrical and elongated in the SE direction. These isoseismals are more compressed in the northeast and northwest direction indicating fast attenuation characteristics, which may be either due to the presence of any transverse geotectonic feature or change in lithology. In the present study the regression relation has been derived to incorporate the attenuation of intensity with variable magnitudes at different epicentral distances. The derived attenuation relation is useful for assessing damage of a potential future earthquake (earthquake scenario-based planning purposes) for the Kangra region, Northwest Himalaya. The derived relation is mentioned below:

               I(R, Ms) = 2.856 + 1.31^* Ms - 0.0017 R^* - 0.9598 ^*ln(R)

The decrease in intensity with distance has been used to calculate the PGA using the relation of Atkinson and Sonley (2000). The calculated values are quite in agreement with the values obtained by others around its epicentral zone. Another derived relation is used to calculate the size of the Kangra earthquake (Mw = 8.4) using intensity attenuation characteristic.