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The study interprets the effect of seasonal (pre-, co- and post-monsoon) hydrological loading due to the Ganga river basin over the seismicity and geodetic deformation in the Garhwal Himalayan region. Geodetic data from six cGPS sites are used, three sites out of six are established in the Himalayan region, one is located in the Ganga basin and two sites are located in the Indian shield. First, we estimated the average seasonal variation in GRACE-derived TWS of entire India for the period of 2002–2016. It is observed the seasonal effect over TWS (Terrestrial Water Storage) due to hydrological fluctuations in the Indo-Gangetic Plain (IGP). It has also been noticed that the geodetic velocities for both the North and East components are more in pre-monsoon than post-monsoonal season. Whereas the vertical component in cGPS sites of Himalaya (DEHR and MAND) agrees to the hypothesis of upliftment during post-monsoon and subsidence in pre-monsoon. TWS mass change is detected high during the co-monsoon (2.32 × 104 km3) and it is observed low −2.22 × 104 km3 (in pre-) and −8.13 × 103 km3 (in post-) monsoon seasons respectively. GPS measured horizontal displacement agrees with seasonal loading but it contains some anomalous behaviour, which may be the result of surface hydrological mass movement or the effect of the Gorkha earthquake, 2015, however an inverse relation between vertical geodetic displacement and loading have been observed. The average strain rate values over cGPS sites during 2016–2018, are more negative in post-monsoon than in pre-monsoon which means that plate movement is highly compressional. We have also inferred a significant role of loading over the seismicity and linear strain rate, which increases with decreasing hydrological loading in the Ganga river basin and vice versa. [ABSTRACT FROM AUTHOR]

[Display omitted] • Geodetic measurement at eighteen GNSS stations provides a maximum surface deformation of ∼13 mm/yr in Higher Himalaya. • An average compression rate of −0.07 ± 0.01 µ strain/yr is reported that gradually increases from Sub Himalaya to Higher Himalaya. • A high convergence rate of 16.7 ± 2.0 mm/yr and locking width of 100 ± 20 km of Main Himalayan Thrust is derived. • A large amount of slip deficit (∼3.5 m) since past 220 years indicating the region has potential to generate a megathrust earthquake (Mw ∼8). Himalayan mountain belt is one of the most rapidly deforming plate collision zones and is prone to produce megathrust earthquakes. In this study, a detailed measure of present-day deformation and strain-build up rate in the Kumaun-Garhwal region of Northwest Himalaya is carried out through eighteen GNSS stations extending from Gangetic Plains to Higher Himalaya during 2014–2019, to assess the seismic hazard potential of the region. The recorded GPS data at sites is analyzed along with nearby stable International GNSS stations for estimating daily basis precise positioning of the sites and further velocity solution is generated after applying the random noise estimated from GPS time series data. The acquired site velocity is used to calculate the principal strain rates and rotation rates using two independent approaches; least-square inversion approach based on scale factor and triangulation method. The derived strain rate reveals a mean rate of compression of ∼−0.07 ± 0.01 μ strain/yr towards northeast-southwest that gradually increases from the Outer Himalaya to Higher Himalaya for both approaches. We assume that this plate boundary compression deforms the Himalayan wedge across the Main Himalaya Thrust (MHT) and use Okada formulation for a rectangular dip-slip dislocation to calculate the plate convergence rate. Based on a regular grid search, we obtain a best fit convergence rate of 16.7 ± 2.0 mm/yr and locking depth of 17.4 ± 3.5 km. The strain budget analysis indicates that the Northwest Himalayan region has enough potential to produce at least one devastating earthquake of Mw ∼8 in the present scenarios. [ABSTRACT FROM AUTHOR]