The Distribution of Intensity Associated With the Great 1897 Assam Earthquake and Bounds on the Extent of the Rupture Zone

The great 1897 Assam earthquake probably was associated with underthrusting of the Indian subcontinent beneath the Shillong Plateau, and as such it absorbed part of India's convergence with the Himalaya. Seeber and Armbruster (1981) concluded that slip occurred on a fault that extended north beneath the Himalaya and south beneath much of what is now Bangladesh and that the east-west dimension of the rupture was 550 km. A reassessment of the information given by Oldham (1899) leads me to favor Oldham's inference that the length of the rupture was only about 200 km. The southern edge of the rupture was more likely at the southern edge of the Shillong Plateau than south of it. The possibility that the rupture extended beneath the Himalaya seems to be permissible but not provable from Oldham's observations. Thus, it would be unwise to assume that the 1897 earthquake released the strain that had accumulated east and west of the Shillong Plateau since the last earthquakes in those areas. Although the risk of a large earthquake in the near future in the segment north of the Shillong Plateau is probably less than that in the segments of the range to the east or west, the 1897 earthquake may not have relieved the strain in that segment as well. Given the evidence that the 1897 earthquake was a very large earthquake, however, it seems unlikely that there will be a repeat of such an event in the next hundred years, or more.