Paleoseismic evidence limiting the lateral rupture extent of the 1697 and 1950 earthquakes in the Assam Seismic Gap: Insights into the role of segmentation and earthquake recurrence

Description

Determining the exact surface rupture lengths and recurrence periods of historical earthquakes is fundamental to characterizing the behaviour and seismic potential of active faults. To address this challenge, we present results from a paleoseismological investigation at three sites along the Himalayan Frontal Thrust in Niglok town, Arunachal Pradesh. Radiocarbon dating of 47 detrital charcoals, combined with optical and fallout isotope dating, constrains the most recent earthquake post-CE 1654. This corresponds with the historically documented CE 1697 Sadiya earthquake. The site has witnessed at least three seismic events since before 17635 BCE. A minimum co-seismic slip of 4.8±0.33 m is estimated during the last two surface-rupturing episodes recurred at ~1400-year intervals. The westward end of the 1697 rupture coincides with the transverse Kopili Fault. This defines a surface rupture length of ~285 km from Niglok to Nameri sites. The segment accumulated a slip deficit of ~5.7 m since the last rupture, sufficient to nucleate Mw 8.5 earthquake. In contrast, the 1950 CE event terminated to the west at the Marbang site located east of Niglok, while its eastern end is at Wakro, along the Mishmi Thrust. The study confirms lateral segmentation of the north-eastern Himalaya, with major transverse structures separating the segments. Each segment behaves seismically independently. Segment-specific recurrence intervals in the north-eastern Himalaya—1456±507 years (Darjeeling-Sikkim), 550±211 years (Bhutan), ~1400 years (Assam-Arunachal), and ~1800 years (Eastern Himalayan Syntaxis)—indicate random and non-characteristic seismic energy release, unlike the central Himalaya. This irregular stress-release regime warrants segment-dependent Seismic Hazard Assessment models in the Himalaya.

Institutions

• University of Delhi

  Delhi, New Delhi
• Wadia Institute of Himalayan Geology

  Uttarakhand, Dehradun

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