Every year, natural disasters significantly impact our lives, one of which is the most destructive earthquake hazard. Being a natural hazard, earthquakes may also be expressed using several physically connected parameters. These physical parameters can help us to estimate the cause and time of earthquakes with their potential towards a particular hazard scenario. One of these physical parameters is the b-value. When combined with other physical parameters like the fractal dimension, a b-value can provide excellent information about an earthquake hazard. Most of the time, significant temporal changes in the b-value of a region are found to be related to a significant earthquake event (Wyss, 1973; Schurr et al., 2014; Shi et al., 2018; Chen and Zhu, 2020). Hence, a detailed study of the b-value for a region can lead towards understanding future hazard prospects.
The Frequency-Magnitude relation represented by Gutenberg and Richter (1944) is as follows:
\({\text{log}}_{10}N=a-bM\) .. Eq. 1
Where N is the number of earthquakes having a magnitude greater than or equal to magnitude M, a and b are the constants. The constant, b, is a function of seismicity and frequency of the events, which is a crucial parameter to understand a particular region's stress patterns and hazard scenario.
Since the Himalayas encircles most of the Nepal region, it is one of the most tectonically active locations in the world and hence vulnerable to big earthquakes (Bilham and Wallace, 2005; Kumar S. et al., 2010; Stevens and Avouac, 2017). The Nepal Himalaya has been the subject of extensive research (Mohanty, 2023) because of its complex tectonic structure, which results from the collision of the Indian and Eurasian plates. Since Nepal is a part of this seismically active mountainous region in the world, this reason has lured us to study the geo-tectonics of the region. Several significant earthquakes have struck Nepal and its surroundings, causing a substantial loss of life and economic damage. The Gorkha earthquake (2015), with a magnitude of 7.8 Mw, was one of the recent primary and devastating earthquakes to hit this area. This significant hazard has almost killed 8,857 people, with a mammoth economic loss of around 10 billion USD in the history of Nepal.
In the past, significant studies have been carried out in Nepal in geophysics, geodynamics and seismology (Singh et al., 2017; Mohanty et al., 2016; Singh et al., 2015; Molnar, 1984). Schelling and Arita (1991) studied the thrust tectonics and crustal shortening along with the structure of the far-eastern Nepal Himalayas. They proposed that the average shortening rates across the far-eastern Nepal Himalayas and south of the Tibetan Plateau range between 7.4 mm to 15.3 mm per year since the Main Central Thrust (MCT) initiation. Pandey et al. (1995) studied the inter-seismic stress accumulation on the Nepal Himalayan crustal ramp. They concluded that the strain accumulation on a mid-crustal ramp, connecting a flat decollement under the Lesser and Sub Himalaya with a deeper decollement under the Higher Himalaya, probably acts as a geometric asperity where strain and stress build up during the interseismic period. He proposed that the large Himalayan earthquakes could nucleate there and probably activate the whole fiat-and-ramp system up to the blind thrusts of the Sub-Himalayas. Upreti (1999) studied the stratigraphy and tectonics of the Nepal Himalayas.
Bollinger et al. (2007) studied and reported the evidence of the seasonal modulations of the seismicity in Nepal Himalaya and found that the earthquakes along the shallow Main Himalayan Thrust are more numerous in winter than in summer, independent of magnitude range. Ader et al. (2012) studied the seismic hazard implications due to the convergence rate across the Nepal Himalayas and the inter-seismic coupling on the Main Himalayan Thrust. They found a large moment deficit due to the locking of the Main Himalayan Thrust (MHT) in the interseismic period, indicating a significant deficiency of seismic slip over that period or very infrequent sizeable slow slip events. Ram and Wang (2013) studied the probabilistic seismic hazard in Nepal and found a high-hazard scenario in the far-western and eastern sections, whereas a low hazard in southern Nepal. Adhikari and Paudyal (2014) studied the spatial variations of the seismicity in the Nepal Himalayan region. They found a non-uniform distribution of earthquakes with specific segments having intense seismic activity in the thrust zone of Central Himalaya, signifying complex geologic and tectonic settings. The study proposed that the recent seismic activities are mainly due to the shallow focus events. The eastern parts of the Central Himalaya region are most active, with high but scattered seismic activities. However, the western portion between the MCT and the MBT could be more seismic. Chaulagain et al. (2015) performed a seismic risk assessment and hazard mapping of Nepal and proposed a higher seismic hazard potential in the mid-western and eastern parts of Nepal.
In contrast, southern Nepal has the lowest seismic hazard. Singh (2016) studied the spatial variations of the seismic b-values across the N-W Nepal Himalayas and found that the Garhwal-Kumaun segment, which is the major part of the 500–800 km central seismic gap, represents the highest seismic potential with the lowest b-value, indicating high-stress concentration. Gualandi et al. (2017) studied the pre- and post-seismic deformation related to the 2015 Nepal earthquake. They found a mixed contribution from secular inter-seismic loading, seasonal variations driven by surface hydrology, and co-seismic and transient post-seismic deformation in this region. The regional variations of the stress level with the help of the b-value in the Himalayas were studied by Ramesh et al. (2018) after the 2015 Nepal earthquake, which found an increased b-value due to the asymmetric release of stress towards the eastern side of the Gorkha epicentre.
The western side of the Gorkha earthquake represented a low b-value, suggesting a future earthquake scenario. Kumar and Sharma (2019) studied the Nepal Himalayan region's seismicity by analyzing the area's spatial distributions. Zilio L. et al. (2019) studied the seismicity in the Himalayas, which is controlled by fault friction and geometry, and proposed that the Himalayan seismicity can be bimodal: blind earthquakes (up to Mw ~ 7.8) tend to cluster in the downdip part of the seismogenic zone, whereas infrequent great earthquakes (Mw > 8) propagate up to the Himalayan frontal thrust. Rajaure S. (2021) did the seismic hazard assessment of the Kathmandu Valley and its adjoining regions and found that the PGA (0.23 g) for a 200-year return period approximately agrees with the instrumentally recorded PGA (0.25 g) in the Kathmandu Valley.
It is becoming essential to study and monitor the seismicity and stress pattern in the Nepal Himalayan region to understand the future probabilistic hazard scenario. Considering the above-discussed facts, the present work addresses the distribution of the b-value across the different segments of Nepal to understand hazard scenarios better.