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  • Author or Editor: D. Shanker x
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Two swarms of small earthquakes occurred on 22-24 October, 1995 and 13-18 September, 1996 about 15 km to the West-South-West (WSW) of the 1991 Uttarkashi earthquake (mb = 6.6) in Himalayas. The later swarm has migrated 5 km to the WSW of the former. Analyses of their seismicity rates, width of apertures and migration rates show that these swarms are triggered by a disturbance caused by the occurrence of the Uttarkashi earthquake thus triggered by this event. The disturbance, having slow propagation, rate represents evidence of creep of the earth material transferring stresses to the WSW direction. Occurrence  of creep (stable-slip) motion is supported by the inferred south-west (SW) orientation of compressive stresses in  Uttarkashi earthquake and presence of the north-west (NW) trending shear zones in the region. Ongoing convergence between India and Tibet would have provided the necessary tectonic forces transverse to the NW-SE trend of the Garhwal Himalayas indicating the future seismic activity of the region.

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The occurrence of Quaternary thrusting in Himalayas and its geodynamics constraints in Southern Tibet is modelled using stress simulation analysis. 2D non-linear elastic and homogeneous wedge models, representing cross-sections of the Himalayas and Tibet are used. Simulated stresses for a set of boundary conditions, representing building up of Himalayas and Southern Tibet, reveals the region of thrust failure gradually recedes away from the wedge towards the base (lower boundary) with a decrease in the strength of the base. Thus, the result favours the preposition that a strong and a weak basal (Main Himalayan Thrust; MHT) respectively, below Himalayas and Southern Tibet is responsible for presence and restricting the extension of Quaternary thrusting in these regions. A decrease in strength of MHT from the Himalayas to Tibet is also supported by observational evidence and thermal modelling, imply partial melting along MHT.

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This article reviews the development of the study of relationship between the earth's rotation and the spatio-temporal distribution of global seismicity by stationary model of seismicity rates and annual seismic energy released. Observed variations of seismic energy release in shallow, intermediate and deep focus earthquakes and their frequency distribution confirms this correlation. The peaks of these parameters are controlled by the earth rotation. There exists a phase relationship among earth's rotation rate minima and maxima with the maxima (peaks) of the above parameters as well as Thrust (T) and the strike slip (S) dominating periods of global seismicity. In order to compare our results with observations, the space-time dependence of the frequency of earthquakes and annual energy release has been established. The results are in very good agreement with previous studies and further enhance our knowledge for global seismicity distribution.

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Movement and abstraction of groundwater in the geological formations are dependent on the hydro-geological parameters of the aquifers. The purpose of any aquifer test is to determine the hydro-geological parameters. Among the basic parameters are the specific storage, permeability and leakage coefficients. The hydro-geological parameters are hidden in the field test data and their identification is possible using the available physically plausible models suitable for the prevailing field circumstances. In this context, a generalized theoretical solution for the effect of partial penetration superimposed over the full penetration on draw-down in a large-diameter well in artesian aquifer discharging at a constant rate has been presented for non-dimensional quantities describing the variable geometries of wells. The well-function curves are developed by varying the percentage amount of drilling and the percentage amount of casing lowered which then control to vary the percentage amount of open-hole or screened interval for the three categories: when the diameter of the cased interval in which the water level changes is greater than, equal to, and less than the diameter of the open interval. The skin effect and the effect of leakage are neglected. A comparison of results with the published works has also been presented. The present study is useful in such areas where wells are located either in harder or in collapsible loose formations; and a decision is required that, at the planning, construction, or development stage, as to what extent the amount of drilling be reduced, and/or an additional amount of casing be lowered within the aquifer. Also this reduces the cost of well construction and development in a specific situation.

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Time dependent seismicity investigation in six seismogenic sources of Nepal and its adjoining areas in the Central Himalaya reveal that there is intermediate time clustering of the moderate size shallow earthquake in each seismogenic source. The inter-event times, between the successive shallow mainshocks, of the magnitude equal to or larger than certain cut-off magnitudes for each of these sources are used for long-term earthquake hazard prediction corresponding to individual sources of the region. For the hazard estimation, the following relations have been established here as: log T t = 0.46 M min +0.07 M p +0.02 log m 0 −2.38, and M f = 0.78 M min −0.25 M p −0.04 log m 0 + 4.32, where T t is the inter-event time measured in years; M min is the moment magnitude of the smallest mainshock considered; M p is the magnitude of preceding main shock, M f is the magnitude of the following mainshock and m 0 is the moment rate in each source per year. The value of σ = 0.22 and multi-correlation coefficient, R = 0.62 for the first equation and σ = 0.30 and R = 0.59 for the second equation are estimated.Based on these relations and using the magnitude and time of occurrence of the last main shocks in each seismogenic source, time dependent conditional probabilities of the next shallow main shocks during the next 10, 20 and 30 years as well as the magnitude of the expected main shocks are forecast.

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Seismically Kutch peninsula is very active. The distribution of seismicity in Peninsular Shield region from 1902 to 2001 show 12 earthquakes of M ≥ 6. The energy ratio from Kutch basin to Deccan trap is 20:1 and from trap to rest of the shield is 5:1. The last one hundred years seismicity data show Kutch basin is seismically more active than Deccan trap and the rest of the Peninsular Shield. The maximum magnitude of earthquake in the Kutch region is 7.7. The generations of large earthquakes in the region are difficult to explain, as plate boundary does not exist. In order to understand the physical processes that are taking place in the region to generate such large events the detailed analyses of geophysical and geological data have been examined in the light of development of rift, subsidence of basin, vertical tectonics and recent geophysical findings. In such regions, petrologic model can provide better explanation for release of fluid that generates large earthquakes, sprouting of sands, liquefaction, and large number of aftershocks activities and direction of stresses for aftershock sequences. The presence of magma in the Kutch upper mantle could be derived from various geological (subsidence of basin, development of rift faults) and geophysical observations (high heat flow over Cambay region, prominent positive Bouguer gravity anomalies and low shear velocity in the upper mantle). The inspection of seismological data shows all the medium size to large earthquake have occurred in shear zone of large gravity gradients or along the four major faults of the region. In view of geological and geophysical observations, petrologic model is proposed for generation of earthquakes in the region. The number of aftershocks and direction of stresses in the focal region of aftershocks would depend on the direction of movement of fluid incursion in the focal region after the occurrence of the main events. The recent Bhuj earthquake also shows more than 3000 aftershocks from Jan 29 to April 15, 2001. The expanding swarm activity in the focal region and the direction of stresses derived from first motion data of aftershocks for focal depths 2 to 8 km, 8 to 25 km, and 25 to 38 km supports the proposed model. Also, shear wave tomography studies in this region have revealed low shear wave velocity in the upper mantle of Cambay from shallow depth to 200 km depth showing high temperature zone. The analyses reveal the presence of conducting fluid in the focal zone, which is the main cause for generation of medium size to large earthquake in the region.

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The study of seismic activity at some stage in 1963 to 2006 in the Western Nepal Himalaya and its adjoining regions (28–31°N and 79–82.3°E), reveal that seismicity is non-uniform in space and time. The analyses of fault-plane solutions of twenty-four earthquakes inferred that the Western part of Nepal Himalayan frontal arc is in compressed state in which seismic activity is dominated by thrust faulting. Based on orientation of P-axes, compressive stress directed north-south to northeast-southwest approximately perpendicular to the prevailing stress along the major trend of the Himalaya. Thrust faulting coupled with shallow dip of nodal planes reflects that the Indian continental lithosphere is under-thrusting at a shallow angle. This information suggests crustal shortening in north-south direction in which earthquakes are generated due to northward compression. In the adjoining Tibet parts earthquake activity is due to normal faulting with east-west extension. These might be due to the presence of a relatively strong Main Himalayan Thrust, the plate boundary fault below the Himalayas, would have favored the occurrence of thrusting. While, a weak Main Himalayan Thrust below Tibet along with initiation of the Main Central Thrust can explain South Tibetan Detachment (geodynamic process) and associated stress field in Western Nepal Himalaya and its adjoining regions.

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