Reevaluated Intensities for the Great Assam Earthquake of 12 June 1897, Shillong, India

The great Assam Earthquake of 1897 (8 < M < 8.1) is the largest known Indian intraplate earthquake. It raised the northern edge of the Shillong Plateau by more than 10 m, resulting in the destruction of structures over much of the plateau and surrounding areas and causing widespread liquefaction and flooding in the Brahmaputra and Sylhet Floodplains. Shaking intensity data for the earthquake are crucial for estimating future earthquake hazards in northeast India and Bangladesh since similar earthquakes will no doubt recur. Yet despite the availability of numerous felt reports, no evaluation of isoseismal contours has been attempted since Oldhamʹs (1899) approximation. We have reevaluated 365 accounts of the earthquake and quantified 287 on a simplified version of the "MSK 1981" intensity scale. The reappraised isoseismals are consistent with the geodetic mechanism for the earthquake and are smaller, less regular, and less elliptical than those inferred by Oldham and suggest that Oldhamʹs intensities were unsuspectingly inflated by 1.5-3 intensity units. The revised intensity data provide new quantitative constraints on the attenuation of perceived intensity as a function of distance in northeastern India.