Stress dissipation and seismic potential in the central seismic gap of the north-west Himalaya

Abstract

• One-third of the earthquakes in CSG show stress drop <1 bar. • High stress drop events are absent in the upper 10 km of the crust. • Source parameter values are lower in Kumaon than CSG. • Kangra and Kumaon segments can be tipped to host a future great earthquake. The central seismic gap (CSG) of the Himalaya continues to pose the ever more intriguing question of whether it has the potential to host a devastating earthquake. To address the issue, we examine the distribution and dissipation of post-seismic stress in the northwest segment of the CSG. This study computes the source parameters of 47 local earthquake events (3.5 ≤ M L ≤ 5.5) in the Garhwal-Kumaon Himalaya during the period 2016–2018 using P-wave spectral analysis. In addition, we compile the stress drop results for 326 earthquake events from numerous studies carried out across the seismic gap, to estimate the source parameter variation. The estimated seismic moments (10 12 ≤ M o ≤ 10 16 N-m), source radii (0.3 ≤ r ≤ 1.3 km) and stress drops (0.1 ≤ σ ≤ 40.6 bar) for Kumaon are observed to be significantly lower than the overall values for the CSG (10 10 ≤ Mo ≤ 10 20 N-m; 0.1 ≤ r ≤ 13.2 km; 0.01 ≤ σ ≤ 77 bar), showing incomplete dissipation of the accumulated stress. About one-third of the earthquakes in the CSG show very low stress drop (σ < 1 bar). The high stress drop events are predominant along the Main Himalayan Thrust at mid-crustal depths, with the upper ∼ 10 km of the brittle crust rarely hosting any big event. Based on the analysis, the scaling relation M o f c 3.677 = 6E + 15 N-m/s 3 has been proposed for the Kumaon region. Other scaling relations have been developed between important source parameters (M L , M w , M o , and σ) that may serve as valuable inputs for assessing the earthquake hazard in the region. The estimated b-value for the Kumaon Himalaya estimated from the local earthquake catalog is 0.64 ± 0.08, which is low compared to the Garhwal and rest of the NW Himalaya. The overall b-value of the CSG region computed from the ISC catalog is 0.62 ± 0.06. From the stress drop and b-value variation in the CSG, the Kangra and the Kumaon segments are identified to be potentially hazardous zones for a future great earthquake.