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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.

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A B S T R A C TCo-seismic ionospheric disturbances (CIDs) due to different source characteristic earthquakes are investigated through GPS-derived vertical total electron content (VTEC). We investigated VTEC changes related with Moderate, Strong, Major and Great earthquakes occurred at different tectonic settings in both Himalaya and non-Himalayan regions. The VTEC and its anomalies are computed using GPS data obtained from the local network of GPS stations in Himalaya with the IGS stations surrounding the epicentre region. Irrespective of the source characteristics, significant CIDs are observed during Major and Great earthquakes, namely; (i) the Mw 7.8, 25th April 2015 Gorkha earthquake, (ii) the Mw 7.6, 8th October 2005 Kashmir earthquake, and (iii) the Mw 8.6, 28th March 2005 Nias-Simeulue earthquake.The VTEC anomalies due to the Gorkha event are observed 21.15 min after the earthquake origin time and continued till 22.78 min with the amplitude range from −0.530 to 0.517 ( ± 0.11) TECU. CIDs associated with the Kashmir earthquake are noticed only at two GPS sites (NADI and PAN2) roughly after 23 min of the earthquake occurrence. But, it continued around 14 min with the amplitude range from −0.12 to 0.177 TECU ( ± 0.02 TECU = 1σ). However, for the case of Ocean-Island Arc Nias-Simeulue earthquake, anomalous CIDs were observed only at GPS site NTUS, where the disturbances started around 25.58 min later and continued around 17.92 min with amplitude range from −0.077 to 0.058 TECU ( ± 0.02 TECU = 1σ). We inferred that the CIDs originated because of earthquakes in Himalaya have relatively larger VTEC magnitudes (> 15 TECU) that propagates faster in reaching and registering in the ionospheric layers compared with the non-Himalayan earthquakes. Secondly, different lithospheric-ionospheric coupling mechanism is operating in transferring the seismic energy, which originated from the Continent-Continent and the Tsunami genic Ocean-Island Arc collision zone, to the ionosphere.