Evidence of surface rupture associated with paleoearthquakes in the Trans Yamuna Segment of Northwestern Sub-Himalaya, India - Focus on the Sirmurital Active Fault 

Abstract

Himalayan seismicity exhibits a bimodal mechanism with great earthquakes (+M8) rupturing the upper locked segment of the Main Himalayan Thrust (MHT) and blind earthquakes (up to Mw ~ 7.8) rupturing the down-dip section of the decollement. However, few imbricate and out-of-sequence active structures have been overlooked with limited research in the hinterland regions with respect to active fault mapping and paleoseismological trench excavations. The present study focusses on the western section of the Trans-Yamuna Active Fault (TYAF) in northwestern Sub-Himalaya, hosting the Sirmurital Active Fault (SAF). The SAF exhibits a distinctive south-side up trace, obliquely cross-cutting the Main Boundary Thrust (MBT). Satellite data, coupled with detailed field investigation confirms that the above fault dips 60˚ to the north, with a fault scarp of 40 m height. Minor strike-slip component is also confirmed with the presence of a subtle pressure ridge, and preliminary fluvial terrace mapping. Generation of a high-resolution Digital Elevation Model (DEM) from Cartosat-1 stereo pairs, and quantification with total station mapping of fault scarp further confirmed the terrace displacement. Subsequently, trench excavation (31 m in length) across the SAF at Sirmurital village provided compelling evidence of atleast two paleoearthquakes displacing and deforming Quaternary sediments along two identified fault strands. Soft-sediment deformation features complemented with injection features suggests its genetic link with paleoearthquakes. Radiocarbon dating analysis offers insights into the probable timing of the faulting events. Overall, the tectonic placement of the SAF provides a unique opportunity to document the occurrence of a normal fault in the hanging wall of a megathrust system and its potential to generate earthquakes in the highly populous mountainous belt of NW Sub-Himalaya. Therefore, the SAF along with the other associated faults, in the hinterland calls for detailed evaluation for a more comprehensive seismic hazard assessment.   Keywords: Out of sequence; Sirmurital Active Fault; Cartosat-1; Paleoearthquakes; Soft-sediment deformation; Seismic hazard