MACRO SEISMICITY AND SOURCE PARAMETERS OF THE GREAT KANGRA EARTHQUAKE OF 1905

Abstract

The morning of April 4, 1905 can not be forgotten in the Indian history, when a devastating earthquake occurred in Himachal Himalaya. This earthquake took a toll of about 20,000 lives, felt over an area of 416,000 square-km with epicenter located in the vicinity of Kangra. Macroseismic effects of this earthquake were studied and documented by Middlemiss (1910) and an isoseismal map was constructed on RF scale. In the present study from the re-evaluation of the documented macroseismic effects, the intensity to 400 localities has been assigned and an isoseismal map on the MM scale has been constructed. Attenuation of MM intensity with epicentral distance has been studied and relation of the form I (R) = a + bR + clog(R) is fitted to the intensity data. Four attenuation relations have been proposed for the region using different intensity data sets. From the study of the macroseismic data, locations of two bericenters, macroseismic epicenter, focal depth, moment magnitude, source dimensions and orientations of the source of the Kangra earthquake have been estimated. The isoseismal map of the Kangra earthquake on the MM scale shows that the entire area enclosed by isoseismal VIII has experienced same level of shaking. The focal depth of the earthquake estimated from intensity data is 23 km, and shows that the earthquake occurred at shallow depth in Sub Himalaya. From the linear relationship between moment magnitude and intensity for eight Himalayan earthquakes the estimated moment magnitude is 8. The bericenters and macroseismic epicenter computed from intensity data are located within the radius of 6 km from Kangra. Based on the computed orientation of the source it is inferred that the rupture started at a depth of about 23 km and propagated in the east 30° south direction over a length of about 150 km. A consistent value of 0.001 is obtained for the absorptions coefficient which agrees with that of Eastern United States obtained using Lg waves. An important outcome of the study is that the Kangra earthquake seems to have produced strong Lg waves that might have caused damages at large distances such as observed in the region around Dehradun. iii