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 NE 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 Oldham's (1898) approximation. We have re-evaluated 365 accounts of the earthquake and quantified 287 on a simplified version of the "MSK 1981" Intensity scale. The re-appraised 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 Oldham's 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. Introduction The earthquake of 12 June 1897 in the Shillong Plateau in north-eastern India is the largest intraplate event in the last two centuries to have occurred in the Indian subcontinent. Although contemporary geodetic data have been used recently to infer the rupture parameters of the earthquake (Bilham & England, 2001), much of what is know about the intensity distribution of the event comes from Oldham's detailed report and his subsequent estimates of aftershock activity (Oldham, 1899; 1901; 1904; 1920). Over the years his report and isoseismal maps have been used to assess the seismic hazard of this part of India, to evaluate the magnitude of the event, and to constrain the location and dimensions of the associated rupture zone (Figure 1). In view of the importance of the 1897 earthquake, a review of the intensities assigned by Oldham is warranted, especially since we may now incorporate data from Tibet and from published data not utilized by Oldham. This paper is concerned solely with the reappraisal of the intensity distribution of the Assam earthquake, drawing attention to potential pitfalls associated with indiscriminate use of intensity data. The earthquake The earthquake occurred beneath the Shillong Plateau in what was then the district of Assam, in northeast India. For many years the earthquake was considered to have occurred on a gentle north-dipping rupture beneath the Shillong Plateau, linked structurally to the style of Himalayan thrust faulting (Seeber et al., 1981, Molnar 1987; Molnar and Pandey, 1989, Gahalaut and Chander, 1992). This interpretation has recently shown to be untenable. According to recent analyses of geodetic data acquired in 1860, 1897 and 1936, the earthquake occurred on a steep SSE-dipping reverse fault beneath the northern edge of the central Shillong Plateau (Bilham and England, 2001). Rupture is inferred to have extended from 9 km to more than 30 km depth on a 110-km-long, SSE steeply-dipping reverse fault