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Conventional well logs serve as effective tools for estimation of pore pressure (PP), tectonic strain and stress magnitudes in normal and thrust faulted parts of the Upper Assam basin. Several models have been developed for prediction of PP and stress magnitudes over the years. Acquiring tectonic strain in active area enhances the accuracy of in situ stress estimate in the Upper Assam. This paper discusses Eaton's sonic model for PP estimation and assesses stress magnitudes incorporating tectonic strain in poroelastic models. Eaton's exponent is varied from 0 to 3 to get best-fit results for PP prediction validated with available modular dynamic tester and mud weight data. PP is estimated for geological formations: Dhekiajuli, Girujan, Tipam, Barail, Kopili, Sylhet, Langpur to the granitic basement in normal and thrust faulted areas of the Upper Assam basin using Eaton's exponent of 1.0. The predicted tectonic strain using uniaxial model varies from −0.0044 to 0.0061. The computed tectonic strain using biaxial poroelastic model ranges from 0.000099 to 0.00077 along NE–SW and from −0.001 to −0.0019 along NW–SE directions for normal faulting regime, respectively. In thrust faulting regime, the tectonic strains are obtained as 0.00092 and 0.00053 along NE–SW and NW–SE directions, respectively. The vertical (S V), maximum (S H) and minimum (S h) horizontal stress gradients range 21.7–23.8, 14.9–23.1 and 12.0–21.4 MPa km−1, respectively. Moreover, we have proposed a multiple linear regression model for evaluation of S h from independent parameters, namely, PP, Poisson's ratio and S V from three wells and dependent parameter as leak-off test (LOT) data. This is further tested with other three wells in the study area. The predicted S h is found to have satisfactory correlation with observed LOT for these wells. [ABSTRACT FROM AUTHOR]

We analyse broad-band SS waveform data recorded by several networks in Europe with sources mainly in the west Pacific to study the underside reflections of teleseismic SS waves in the lithosphere and the upper mantle beneath eastern Asia and the NW Pacific ocean. SS bounce points sample a corridor from the Aleutian, Kamchatka and Japan subduction zones through the North China Craton and Central Asian Orogenic Belt to the Tibetan plateau. The corridor passes through different tectonic units such as subduction zones, an old continental shield, a fold belt and a high plateau. We investigate the seismic structure of the lithosphere and the mantle transition zone beneath the different geotectonic units along the profile and infer the correlation of geodynamic processes at different depths. We explore the short period frequency content in the SS waveform data and use moveout correction and common midpoint stack to acquire profiles with high lateral and depth resolution from the crust to the mantle transition zone. Clear SS precursors of the 410 and 660 km discontinuities show the effects of the interaction between the subducted oceanic lithosphere and the mantle transition zone beneath the NW Pacific subduction zones. A low-velocity layer has also been detected beneath the 410 km discontinuity and can be traced along the entire profile. Due to the improved resolution acquired by the method presented here we have been able to study the shallower structures such as the Moho and the lithosphere–asthenosphere boundary by SS precursors. The continental Moho can be clearly seen along this corridor. The depth variation agrees well with earlier receiver function results. We also see negative reflectors along the profile at varying depths, which can be interpreted as the lithosphere–asthenosphere boundary. [ABSTRACT FROM AUTHOR]