Variation of Vp/Vs ratios for granitic layers and weekly average number of earthquakes in NE India

Vp/Vs ratios for the granitic layers in Shillong Plateau and the partially overlapping Tezpur seismic area, have been calculated from Wadati diagrams drawn on the basis of seismic phase data. From Shillong plateau the average of 23 readings for some months of 1979 for Vp/Vs is found to be 1.71. For Tezpur area the average of 29 readings of Vp/Vs for 1991-92 is found to be 1.73, but for 1995-96, the average Vp/Vs from 49 readings is found to be 1.68. The overall average from 78 readings for this area is 1.70. The low Vp/Vs for 1995-97 seems to be precursory. For the granitic layer, taking Vp = 5.92 kms/s and Vp/Vs = 1.70. We get Vs=3.48 km/s for NE India. Values of Vp/Vs ratios and number of shocks per day are plotted against time and are shown to undergo sudden lowering before many M³4.2 earthquakes. Vp/Vs is generally lowered below 1.60 in such cases. In Shillong plateau the number of shocks per day for 1979 is found to be three times the number in the adjoining Tezpur area, for 1991-97.

Effects of fracture connectivity on Rayleigh wave velocity dispersion

<p>The use of passive seismic techniques to monitor geothermal reservoirs allows to assess the risks associated with their exploitation and stimulation. One key characteristic of geothermal reservoirs is the degree of fracture connectivity and its evolution. The reason for this is that changes in the interconnectivity of the prevailing fractures affect the permeability and, thus, the productivity of the system. An increasing number of studies indicates that the Rayleigh wave velocity can be sensitive to changes in the mechanical and hydraulic properties of geothermal reservoirs. In this work, we explore the effects of fracture connectivity on Rayleigh wave velocity dispersion accounting for wave-induced fluid pressure diffusion effects. To this end, we consider a 1D layered model consisting of a surficial sandstone formation overlying a fractured and water-saturated granitic layer, which, in turn, is underlain by a compact granitic half-space. For the stochastic fracture network prevailing in the upper granitic layer, we consider varying levels of fracture connectivity, ranging from entirely unconnected to fully interconnected. We use an upscaling approach based on Biot’s poroelasticity theory to determine the effective properties associated with these scenarios. This procedure allows to obtain the frequency-dependent seismic body wave velocities accounting for fluid pressure diffusion effects. Finally, using these parameters, we compute the corresponding Rayleigh wave velocity dispersion. Our results show that Rayleigh wave phase and group velocities exhibit a significant sensitivity to the degree of fracture connectivity, which is mainly due to a reduction of the stiffening effect of the fluid residing in connected fractures in response to wave-induced fluid pressure diffusion. This suggests that time-lapse observations of Rayleigh wave velocity changes, which so far are commonly associated with changes in the fracture density, could also be related to changes in the interconnectivity of pre-existing fractures.</p>

A study of the Earth's crust near Gauribidanur in Southern India

abstract A hypothetical two-layered model has been evolved for the Earth's crust near Gauribidanur. The model is found to be consistent with the local earthquake and rockburst data obtained at Gauribidanur seismographic array. Sixty-seven well-recorded seismic events have been studied for this purpose. First arrival conditions for some of the phases in a “near-source” (epicentral distance Δ ≦ 10°) seismogram have been derived and subsequently made use of in establishing the nature of the first arriving phases pertaining to the available data. Thickness of the top granitic layer and the depth of Moho below this layer are found to be about 16 km and 19 km, respectively. Observed velocities are 5.67, 6.51 and 7.98 km/sec for P phases, and 3.46, 3.96 and 4.61 km/sec for the corresponding S phases. Values of the crustal parameters given by this study have been used in estimating the relative differences in travel times corresponding to various observed phases. These travel times may improve the precision with which earthquakes in this region could be located. Typical geographical and geological features of the region are also briefly described.

Reasons for absence of a 'granitic' layer in basins of the South Caspian and Black Sea type

A profile of deep seismic sounding across the South Caspian depression is described. The peculiarity of the structure of the depression is the reduced thickness of the Paleozoic and Mesozoic folded complexes, which is indicative of its geanticlinal development at that time. These complexes have seismic velocities in the range usual for granites. They were not discovered in earlier seismic studies in the Black Sea and Caspian Sea because the complexes are too thin, and the methods used did not allow discovery of detail. One peculiarity of these depressions is the so-called 'sedimentary-basaltic' crust. This arises when strongly metamorphosed rocks have been overlain by thick, poorly consolidated sediment, after a long period of submergence.

Structure of the earth's crust of epi-continental seas: Caspian, Black, and Mediterranean

Comparison of crustal structure of the deep basins of the Caspian, Black, and Mediterranean Seas indicates the similarities of the main characteristics: great Bouger anomalies, the absence of a 'granitic' layer, a thick sedimentary layer with a relatively smooth interface, aseismicity. However, each of the above-mentioned basins (or even different parts of one deep basin) has its peculiarities, which indicates a very complicated tectonic development of the entire Caspian–Mediterranean zone.

The transition zone between the Eurasian continent and the Arctic Ocean

Geophysical and bathymetric results have been used for the study of the transitional zone between the Eurasian continent and the Arctic Ocean, which is being considered by the authors as a junction area of continental margins with the adjacent seaward structures, and is dependent of the actual type of the crust found beneath the ocean.The deep Arctic Ocean comprises different types of crust: basins have an oceanic crust with increased sedimentary thickness; the Lomonosov and Mendeleev Ridges have crusts close to subcontinental; and the crust of the Gakkel Ridge is typically mid-oceanic. At least four types of transition zones may be distinguished: (1) 'normal' transition zone, where the 'granitic' layer is wedging out and the crust consists of a thinned sedimentary layer and 'basalt'; this is the junction of the continent and the oceanic basin; (2) a zone where the continental crust thins to the oceanic crust, the 'granitic' layer is absent, and the 'basalt' layer is underlain by the 7.5 km/s layer, rather than the normal mantle; this is represented by the junction of the continent with the rift zone of the median ridge that juts out into the shelf and extends under the continent; (3) a zone where the continental crust thins, the 'granitic' layer is wedging out, but the crust does not thin to the oceanic crust, mainly because of an increase in thickness of sediments; this zone is a junction of the continent with a basin with subcontinental crust; (4) a zone where the continental crust thins, but does not reach oceanic thickness; this is a junction of the continent with the oceanic crust. In the transition zones of the first and third types structural downwarping compensated by the sediments has been developed.The development of the transition zone of the Arctic was intermittent in the geological past, which we see expressed by the asymmetric development of the Greenland–Canadian and Eurasian sectors. These examples of different structures of transition zones are not unique. The transition zone between the Asiatic continent and the Indian Ocean appears to be most similar in its complexity to the transition zone of the Arctic Ocean. However the 'normal' type of transition zone characteristic of much of the continents of Australia, South America, Africa, and parts of other continents frequently occurs here.