No abstract

The characteristic site response spectra of soft sedimentary rocks in the Garhwal Himalaya and a few localities in the adjoining Ganga plain and Himachal Himalaya have been studied through short period passive seismic source experiment using ambient noise data. Along with this, stand-alone temporal gravity data in the Doon valley was also acquired. It has been observed that there exist two extreme frequency bands in the gravity and seismic response spectra of alluvial soft sediments in the Doon valley and the adjoining regions, when it subjected to micro-seismic and gravity-tidal oscillation. The concept of damped harmonic oscillator was used to study the fundamental long period modes in the density driven diffusive fluid flow in the valley alluvial pore spaces, and observed that the flow was continued from minutes to a few days. In this case, apart from the diurnal component, the observed fundamental modes are in the bands from 0.42 to 1.83 days. The characteristic frequencies of seismic response spectra for the fundamental modes of thick soft sediments were also studied using the spectral ratio (HVSR) method of Nakamura. In the Ganga basin, a moderate resonant frequency of 3 Hz is sufficient for thick soft sediment to cause relatively large vertical amplitude, which suggest possibility for sub-surface secondary seismic effects like liquefaction. However, the hard rock terrains in the lesser Himalaya, where only a veneer of soft sediment is present, show relatively high frequency values of 15 to 18 Hz even to produce an H/V amplitude ratio of 1 and 1.7 and hence could be considered as relatively stable.

The possibility of a major earthquake like 2015 Gorkha–Nepal or even greater is anticipated in the Garhwal–Kumaun region in the Central Seismic Gap of the NW Himalaya. The interseismic strain-rate from GPS derived crustal velocities show multifaceted strain-rate pattern in the region and are classified into four different strain-rate zones. Besides compressional, we identified two NE–SW orienting low strain rate (~ 20 nstrain/a) zones; namely, the Ramganga-Baijro and the Nainital-Almora, where large earthquakes can occur. These zones have surface locking widths of ~ 72 and ~ 75 km respectively from the Frontal to the Outer Lesser Himalaya, where no significant surface rupture and associated large earthquakes were observed for the last 100 years. However, strain reducing extensional deformation zone that appears sandwiched between the low strain-rate zones pose uncertainties on the occurences of large earthquakes in the locked zone. Nevertheless, such zone acts as a conduit to transfer strain from the compressional zone (> 100 nstrain/a) to the deforming frontal active fault systems. We also observed a curvilinear surface strain-rate pattern in the Chamoli cluster and explained how asymmetric crustal accommodation processes at the northwest and the southeast edges of the Almora Klippe, cause clockwise rotational couple on the upper crust moving over the MHT.