Vishal Chauhan

Variable anelastic attenuation and site effect in estimating source parameters of various major earthquakes including M w 7.8 Nepal and M w 7.5 Hindu kush earthquake by using far-field strong-motion data

Strong-motion records of recent Gorkha Nepal earthquake (Mw 7.8), its strong aftershocks and seismic events of Hindu kush region have been analysed for estimation of source parameters. The Mw 7.8 Gorkha Nepal earthquake of 25 April 2015 and its six aftershocks of magnitude range 5.3–7.3 are recorded at Multi-Parametric Geophysical Observatory, Ghuttu, Garhwal Himalaya (India) >600xa0km west from the epicentre of main shock of Gorkha earthquake. The acceleration data of eight earthquakes occurred in the Hindu kush region also recorded at this observatory which is located >1000xa0km east from the epicentre of Mw 7.5 Hindu kush earthquake on 26 October 2015. The shear wave spectra of acceleration record are corrected for the possible effects of anelastic attenuation at both source and recording site as well as for site amplification. The strong-motion data of six local earthquakes are used to estimate the site amplification and the shear wave quality factor (Qβ) at recording site. The frequency-dependent Qβ(f)xa0=xa0124f0.98 is computed at Ghuttu station by using inversion technique. The corrected spectrum is compared with theoretical spectrum obtained from Brune’s circular model for the horizontal components using grid search algorithm. Computed seismic moment, stress drop and source radius of the earthquakes used in this work range 8.20xa0×xa01016–5.72xa0×xa01020xa0Nm, 7.1–50.6 bars and 3.55–36.70xa0km, respectively. The results match with the available values obtained by other agencies.

Fractal dimension variability in ULF magnetic field with reference to local earthquakes at MPGO, Ghuttu

ABSTRACT Ultra-low frequency (ULF) geomagnetic data recorded during 1 January 2010 to 31 December 2010 at multi-parametric geophysical observatory (30.53°N, 78.74°E) in Garhwal Himalaya region of Uttarakhand, India, are analyzed. From the temporal variation of polarization ratio, the presence of seismo-magnetic disturbances superposed upon background geomagnetic variations are inferred. Considering earthquake process as a self-organized critical system based on flicker noise characteristics, fractal dimension for each day is estimated using two methods namely power spectral (FFT) method and Higuchi method. Variability in fractal dimension is studied in the background of local earthquakes (M ≥ 3.5) within a zone of radius 150 km from observing station multi-parametric geophysical observatory (MPGO), Ghuttu. Fractal dimension variability indicates that average fractal dimension for first half of the year is increased as compared to average fractal dimension of second half of the year and there is gradual increase in the fractal dimension before earthquakes. It is also observed that during the first half of the year, there is seismic activity within zone of 150 Km radius centred at around MPGO, Ghuttu. There are no earthquakes during the second half of the year. Gradual increase in the fractal dimension before earthquakes, observed elsewhere in the world, is considered precursory signature of seismo-electromagnetic field emissions.

Co-seismic ionospheric GPS-TEC disturbances from different source characteristic earthquakes in the Himalaya and the adjoining regions

Abstract Co-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 21.15min after the earthquake origin time and continued till 22.78min 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 23min of the earthquake occurrence. But, it continued around 14min 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 25.58min later and continued around 17.92min 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.

Ionospheric response to the total solar eclipse in India on 22 July, 2009

Since The variations of Total Electron Content (TEC) and amplitude of the fixed frequency VLF transmitter signal (fu2009=u200919.8u2009kHz, NWC, Australia) are studied at Agra (Geographic Lat. 27.2°N, Long. 78°E), India during the total solar eclipse of 22 July, 2009 which was longest seen in India ever since 18 August, 1968. The equipment used for the study are a dual frequency GPS receiver (GSV 4004V) and a Soft PAL (Software based phase and amplitude logger) receiver. The data for a period of fifteen days (±7 days from the date of the event) are analysed and it is found that the TEC decreased by about 30% from normal days during the total solar eclipse, and the amplitude of the VLF signal also decreased likewise. The period of the data analysis is characterised by a low level of geomagnetic activity, hence the decrease in TEC and amplitude of the VLF signal is unlikely to be influenced by geomagnetic disturbances. The results are interpreted in terms of depression in electron densities at all ionospheric heights a...