PALEOSEISMOLOGY OF THE SHILLONG ANTICLINE, NORTHEAST INDIA: EVALUATING CONSTRAINTS FROM BROKEN CAVE DEPOSITS

Densely populated Northeast India is considered one of the six most seismically active regions in the world and has experienced 18 large earthquakes (M>7) over the last 100 years. Yet, a lack of observable surface ruptures in the region has hindered long-term paleoseismic reconstructions. To date, radiocarbon analysis of organic matter captured in liquefaction features has provided the primary means for dating past large earthquakes and determining earthquake recurrence intervals in northeast India over the last 1,500 years.

We are evaluating the potential of broken speleothems (secondary precipitates) in caves developed along the Shillong Anticline for reconstructing the temporal pattern of past major seismic events in northeast India on millennial and longer timescales. Tectonically broken speleothems have been successfully used in various regions worldwide to date past seismic events (e.g. Kagan et al., 2005, Geology; Panno et al., 2009, BSSA). Mawmluh Cave on the southern margin of the Shillong Anticline is an ideal location to test and develop this approach due to its close proximity to the active Dauki fault and the presence of multiple types of broken speleothems, which include actively growing and annually banded aragonite speleothems. The Dauki fault is an active blind thrust fault and was likely responsible for the magnitude 8.1 earthquake of 1897. In 2013, we collected broken speleothems, including toppled stalagmites, stalagmite stumps with rejuvenated growth, and fallen soda straw stalactites, from Mawmluh Cave, while avoiding speleothems broken by unconsolidated sediment failure and those in areas of the cave subject to flooding or likely human interference. Initial U-Th analyses show that Mawmluh speleothems have ~40-50 ppm U, low ²³²Th, and low initial ²³⁰Th based on near-zero age samples from active stalagmites. Accordingly, Mawmluh speleothems are geochemically highly favorable for precise and accurate U-series dating of samples as young as a few years, allowing us to compare speleothem breakage dates with known major seismic events in the region. If successful, this method will aid in assessing seismic hazards in the area, thereby supporting measures to mitigate seismic risk.