The deadliest stable continental region earthquake occurred near Bhuj on 26 January 2001

A large destructive earthquake occurred on 26 January 2001 in the region of Kutch, Gujarat, in Western India, with magnitude Mw 7.7. The earthquake caused very heavy damage and a large number of casualties with more than 20,000 deaths. A preliminary study of ground deformation, damage pattern and aftershock distribution is presented.     

Aftershock Activity and Frequency-dependent Low Coda Qc in the Epicentral Region of the 1999 Chamoli Earthquake of Mw 6.4

— On 28 March, 1999 (19:05:10.09, UT) a significant earthquake of M _w 6.4 occurred in the Garhwal Himalaya (30.555°N, 79.424°E). One hundred and ten well-recorded aftershocks show a WNW-ESE trending northeasterly dipping seismic zone extending from a depth of 2 to 20?km. As the main shock hypocenter occurred at the northern end of this seismic zone and aftershocks extended updip, it is inferred that the main-shock rupture nucleated on the detachment plane at a depth of 15?km and then propagated updip along a NE-dipping thrust plane. Further, the epicentral distribution of aftershocks defines a marked concentration near a zone where main central thrust (MCT) takes a significant turn towards the north, which might be acting as an asperity in response to the NNE compression due to the underthrusting of Himalayan orogenic process prevalent in the entire region. Presence of high seismicity including five earthquakes of magnitude exceeding 6 and twelve earthquakes of magnitude exceeding 5 in the 20th century is presumed to have caused a higher level of shallow crustal heterogeneity in the Garhwal Himalaya, a site lying in the central gap zone of the Himalayan frontal arc. Attenuation property of the medium around the epicentral area of the 1999 Chamoli earthquake, covering a circular area of 61,500?km² with a radius of 140?km, is studied by estimating the coda Q _c from 48 local earthquakes of magnitudes varying from 2.5–4.8. These earthquakes were recorded at nine 24-bit REFTEK digital stations; two of which were equipped with three-component CMG40T broadband seismometers and others with three-component L4-3D short-period seismometers. The estimated Q _o values at different stations suggest on average a low value of the order of (30?±?0.8), indicating an attenuating crust beneath the entire region. The frequency-dependent relation indicates a relatively low Q _c at lower frequencies (1–3?Hz) that can be attributed to the loss of energy due to scattering on heterogeneities and/or the presence of faults and cracks. The large Q _c at higher frequencies may be related to the propagation of backscattered body waves through deeper parts of the lithosphere where less heterogeneities are expected. An important observation is that the region north of MCT (more rigid highly metamorphosed crystalline rocks) is less attenuative in comparison to the region south of MCT (less rigid slightly metamorphosed rocks (sedimentary wedge)). The acceleration decays to 50% at 20?km distance and to 7% at 100?km. Hence, even 1g acceleration at the source may not cause significant damage beyond 100?km in this region.     

Ionospheric effects of transient celestial X-ray and gamma-ray events

The paper presents the results on investigations of the ionospheric effects arising out of the transient celestial events at X-ray and gamma-ray energies such as that from X-ray novae and cosmic gamma-ray bursts. The VLF data recorded at Ahmedabad corresponding to the 164 kHz transmission from Tashkent are examined to see whether there are any observable effects time coincident with the transit of the X-ray novae Cen X-4 and Cet X-2 around their intensity maxima. Besides, the VLF data have also been examined in relation to the cosmic gamma-ray bursts detected by Vela, IMP-6 and OSO-7 satellites.Theoretical computations are carried out for estimating electron density enhancements using the available data on intensities and energy spectra for these events. Further, the observational resutls are explained in terms of these theoretical calculations.Presently at Indian Space Research Organization, Bangalore, India.Paper presented at the COSPAR Symposium on Fast Transients in X- and Gamma-Rays, held at Varna, Bulgaria, 29–31 May, 1975.     

Sensitivity analysis of seismic hazard for the northwestern portion of the state of Gujarat, India

We test the sensitivity of seismic hazard to three fault source models for the northwestern portion of Gujarat, India. The models incorporate different characteristic earthquake magnitudes on three faults with individual recurrence intervals of either 800 or 1600 years. These recurrence intervals imply that large earthquakes occur on one of these faults every 266–533 years, similar to the rate of historic large earthquakes in this region during the past two centuries and for earthquakes in intraplate environments like the New Madrid region in the central United States. If one assumes a recurrence interval of 800 years for large earthquakes on each of three local faults, the peak ground accelerations (PGA; horizontal) and 1-Hz spectral acceleration ground motions (5% damping) are greater than 1 g over a broad region for a 2% probability of exceedance in 50 years' hazard level. These probabilistic PGAs at this hazard level are similar to median deterministic ground motions. The PGAs for 10% in 50 years' hazard level are considerably lower, generally ranging between 0.2 g and 0.7 g across northwestern Gujarat. Ground motions calculated from our models that consider fault interevent times of 800 years are considerably higher than other published models even though they imply similar recurrence intervals. These higher ground motions are mainly caused by the application of intraplate attenuation relations, which account for less severe attenuation of seismic waves when compared to the crustal interplate relations used in these previous studies. For sites in Bhuj and Ahmedabad, magnitude (M) 7 3/4 earthquakes contribute most to the PGA and the 0.2- and 1-s spectral acceleration ground motion maps at the two considered hazard levels.     

An intense SFE and SSC event in geomagnetic H, Y and Z fields at the Indian chain of observatories

Changes in the three components of geomagnetic field are reported at the chain of ten geomagnetic observatories in India during an intense solar crochet that occurred at 1311 h 75° EMT on 15 June 1991 and the subsequent sudden commencement (SSC) of geomagnetic storm at 1518h on 17 June 1991. The solar flare effects (SFE) registered on the magnetograms appear to be an augmentation of the ionospheric current system existing at the start time of the flare. An equatorial enhancement in AH due to SFE is observed to be similar in nature to the latitudinal variation of SQ (H) at low latitude. AF registered the largest effect at 3.6° dip latitude at the fringe region of the electrojet. AZ had positive amplitudes at the equatorial stations and negative at stations north of Hyderabad. The SSC amplitude in the H component is fairly constant with latitude, whereas the Z component again showed larger positive excursions at stations within the electrojet belt. These results are discussed in terms of possible currents of internal and external origin. The changes in the Y field strongly support the idea that meridional current at an equatorial electrojet station flows in the ionospheric dynamo, E.Presently at: School of Physics, University of New South Wales, Sydney, Australia     

Coastal boulders as evidences of high-energy marine events from Diu Island,west coast of India: storm or palaeotsunami?

Natural Hazards - The coastal segment of Diu Island from west coast of India has been studied for its boulder deposits. The dimensions, morphology and characteristics of these boulders were studied...     

Understanding plot‐scale hydrology of Lesser Himalayan watershed—A field study and HYDRUS‐2D modelling approach

Soil moisture dynamics have a significant effect on overland flow generation. Catchment aspect is one of the major controlling factors of overland flow and soil moisture behaviour. A few experimental studies have been carried out in the uneven topography of the Himalayas. This study presents plot‐scale experiments using portable rainfall simulator at an altitude of 1,230 m above mean sea level and modelling of overland flow using observed datasets. Two plots were selected in 2 different aspects of Aglar watershed of Lesser Himalaya; the agro‐forested (AF) plot was positioned at the north aspect whereas the degraded (DE) plot was located at the south aspect of the hillslope. HS flumes and rain gauges were installed to measure the runoff at the outlet of the plot and the rainfall depth during rainfall simulation experiments. Moreover, 10 soil moisture sensors were installed at upslope and downslope locations of both the plots at 5, 15, 25, 35, and 45 cm depth from ground level to capture the soil moisture dynamics. The tests were conducted at intensities of 79.8 and 75 mm/hr in AF plot and 82.2 and 72 mm/hr in the DE plot during Test 1 and Test 2, respectively. The observed data indicate the presence of reinfiltration process only in the AF plot. The high water holding capacity and the presence of reinfiltration process results in less runoff volume in the AF plot compared with the DE plot. The Hortonian overland flow mechanism was found to be the dominant overland flow mechanism as only a few layers of top soil get saturated during all of the rainfall–runoff experiments. The runoff, rainfall, and soil moisture data were subsequently used to calibrate the parameters of HYDRUS‐2D overland flow module to simulate the runoff hydrograph and soil moisture. The components of hydrograph were evaluated in terms of peak discharge, runoff volume and time of concentration, the results were found to be within the satisfactory range. The goodness of fit of simulated hydrographs were more than 0.85 and 0.95 for AF and DE plot, respectively. The model produced satisfactory simulation results of soil moisture for all of the rainfall–runoff experiments. The HYDRUS‐2D overland flow module was found promising to simulate the runoff hydrograph and soil moisture in plot‐scale research.     

Drainage morphometry of Himalayan Glacio-fluvial basin,India: hydrologic and neotectonic implications

Morphometric analysis, being widely used to assess the drainage characteristics of the river basins, has been found to be a useful tool to delineate the glacial till covered overburden material as well as to identify areas prone to flash floods in present studies. A number of parameters including the stream frequency, drainage density and drainage texture suggest that the unconsolidated, unstratified and highly permeable glacially deposited overburden till material facilitates the infiltration of snowmelt and rainwater in the Pindari glacio-fluvial basin, Eastern Kumaun Himalaya, India. Likewise, other till overburden covered glacial and proglacial areas of Higher Himalayan regions have been contributing to the groundwater budget. The shape parameters further suggest that the sub-basins with higher form factor are more prone to flash floods. Besides this, the anomalies in the morphometric parameters have been found to be a useful tool to delineate zones of active tectonics in such areas.     

Low CodaQcin the Epicentral Region of the 2001 Bhuj Earthquake ofMw7.7

On 26 January, 2001 (03:46:55,UT) a devastating intraplate earthquake of M_w 7.7 occurred in a region about 5 km NW of Bhachau, Gujarat (23.42°N, 70.23°E). The epicentral distribution of aftershocks defines a marked concentration along an E-W trending and southerly dipping (45°) zone covering an area of (60 × 40) km². The presence of high seismicity including two earthquakes of magnitudes exceeding 7.7 in the 200 years is presumed to have caused a higher level of shallow crustal heterogeneity in the Kutch area; a site lying in the seismic zone V (zone of the highest seismicity for potentially M8 earthquakes) on the seismic zoning map of India. Attenuation property of the medium around the epicentral area of the Bhuj earthquake covering a circular area of 61,500 km² with a radius of 140 km is studied by estimating the coda-Q_c from 200 local earthquakes of magnitudes varying from 3.0–4.6. The estimated Q₀ values at locations in the aftershock zone (high seismicity) are found to be low in comparison to areas at a distance from it. This can be attributed to the fact that seismic waves are highly scattered for paths through the seismically active and fractured zone but they are well behaved outside the aftershock zone. Distribution of Q₀ values suggests that the local variation in Q₀ values is probably controlled by local geology. The estimated Q₀ values at different stations suggest a low value of Q=(102 ± 0.80)^*f^{(0.98 ± 0.02)} indicating an attenuative crust beneath the entire region. The frequency-dependent relation indicates a relatively low Q_c at lower frequencies (1–3 Hz) that can be attributed to the loss of energy due to scattering attenuation associated with heterogeneities and/or intrinsic attenuation due to fluid movement in the fault zone and fluid-filled cracks. The large Q_c at higher frequencies may be related to the propagation of backscattered body waves through deeper parts of the lithosphere where less heterogeneity is expected. Based on the attenuation curve estimated for Q₀=102, the ground acceleration at 240 km distance is 13% of 1 g i.e., 0.13 g agreeing well with the ground acceleration recorded by an accelerograph at Ahmedabad (0.11 g). Hence, it is inferred that the Q₀ value obtained from this study seems to be apt for prediction of ground motion for the region.     

Intense night-time equatorial radio scintillations by sporadicE layers

Almost saturated scintillations of radio beacons from geostationary satellites received at an equatorial station during night-time have been shown to occur even during complete absence of spreadF on the vertical incidence ionograms at the same location. These scintillation events were observed when the ionograms showed blanketing type of sporadicE layers simultaneously at different heights. It is suggested that strong equatorial radio wave scintillations during night-time are caused by multiple scattering between different levels of large plasma density gradients in theF or sometimes in theE regions of the ionosphere.