EARTHQUAKE RUPTURE VARIABILITY ALONG THE CENTRAL SEISMIC GAP (77°- 82°E) OF THE HIMALAYAN FRONTAL THRUST (HFT), FROM WESTERN TO CENTRAL HIMALAYAS (Invited Presentation)

The Himalayas and their active tectonic plate boundary, the Himalayan Frontal Thrust (HFT), along with the down-dip detachment fault, the Main Himalayan Thrust (MHT), constitute one of the most seismically active collisional plate boundaries. Currently, 30% of India’s 1.4 billion population lives in the Himalayan foreland and is at risk of an earthquake. Extensive paleoseismic investigations along Himalayas, have improved our understanding of rupture variability, repeat-interval, and slip distribution. There are examples of historical earthquakes where the mere presence of surface ruptures, their magnitudes, and extents remain points of contention. Two notable examples are the AD 1505 Central Himalaya earthquake and the AD 1803 Garhwal earthquake, both pre-instrumental earthquakes that apparently occurred along the Central Seismic Gap (CSG). Therefore, we focused our work on the CSG segment, the area bounded to the west by the Delhi Haridwar Ridge (DHR) structural asperity and to the east by the Faizabad Ridge (FR). To assess the rupture extents and variability, we excavated two trenches approximately 80 km apart near Teliwara (78.431°E, 29.787°N) and Goujani (79.104°E, 29.395°N), and refined the segment-wide surface rupture extents of the AD 1505 and AD 1803 earthquakes within the CSG Segment through paleoseismic observations, geochronology, and integration of previously published earthquake chronology data (since 2006). Following the 'stringing pearls' approach and correlation of the Probability Density Functions of the overlapping earthquake ages at fifteen sites along the HFT, we suggest that the CSG segment has experienced three surface-rupturing paleo-earthquakes (E1-E3). E1 occurred between AD 1340 and AD 1448, potentially confined to the western part of the CSG segment. E2 occurred between AD 1438 and AD 1589, potentially rupturing about 460 km of the HFT, but remained within the segment, from Lal Dang east of DHR, to Koliabas within the FR. No evidence of E2 has been found beyond FR segment boundary. E3 occurred between AD 1741 and AD 1894 and potentially remained within the segment. This work enabled us to explore the variability in the rupture pattern within the CSG segment and underscores the role of geology and paleoseismology in impacting millions of people living in the foreland of the Himalayas.