The full range of 72h-forced, 72 superharmonic resonance periods, is detected in time-series of all 866 earthquakes of (robust averages of) Mw5.6+ from USGS, EMSC, and GFZ, 2015-2019 catalogs. The resonance is in the 55’–15 days long-periodic band (0.303 mHz–0.771605 μHz) at 99–67% confidence. Moreover, omitting of the 21 overrepresenting events has improved the result. The signal is clear, strong, and stable – demonstrating beyond doubt that Mw6.2+ seismicity arises due to long-periodic resonance. Remarkably, the natural mode’s cluster was detected too; it averaged 60.1’, while the overall strongest resonance period was also 59.9’, at 2.3 var%, or to within the 1Hz sampling rate – revealing that the 72 h forcer is the modulator of the Earth’s natural period via synchronization. The dominance property of the forcer also follows from detection of its many other fractional multiples: 14/5, 3/2, 5/12, 5/36, etc. After Schumann resonance discovery in the short band (extremely long band of the EM Spectrum), this is the second report ever of a full resonance bundle in any global data, and the first ever in tectonic earthquakes occurrences. The Mw6.2+ seismotectonics arises via resonance-rupture response of tectonic plates and regions to the resonant phase or its fractional multiples. Fundamental questions of geophysics including earthquake prediction can be solved if the Earth is taken to be a multi-oscillator nonlinear system. As an immediate benefit, the find enables a reliable partial seismic anti-forecasting (prediction of seismic quiescence), months ahead globally. This discovery of mechanically induced extreme-band energy on Earth invalidates the main (heat-transfer) geophysical hypothesis and thus should drastically diminish the role of chemistry in geosciences, specifically of geochemistry.
Elastic property‐porosity relationships are derived directly from microtomographic images. This is illustrated for a suite of four samples of Fontainebleau sandstone with porosities ranging from 7.5% to 22%. A finite‐element method is used to derive the elastic properties of digitized images. By estimating and minimizing several sources of numerical error, very accurate predictions of properties are derived in excellent agreement with experimental measurements over a wide range of the porosity. We consider the elastic properties of the digitized images under dry, water‐saturated, and oil‐saturated conditions. The observed change in the elastic properties due to fluid substitution is in excellent agreement with the exact Gassmann's equations. This shows both the accuracy and the feasibility of combining microtomographic images with elastic calculations to accurately predict petrophysical properties of individual rock morphologies. We compare the numerical predictions to various empirical, effective medium and rigorous approximations used to relate the elastic properties of rocks to porosity under different saturation conditions.
Echosounding, high-resolution shallow seismic data were collected along track lines spaced at 20km interval across the western continental margin of India. A detailed analysis of the underway data revealed the occurrence of methane-bearing gas-charged sediments and pockmarks in the shelf and gas hydrate horizons along the slope and rise. The presence of the zones of incoherent reflections accompanied by lack of acoustic penetration and the presence of discontinuous high-intensity reflections in the form of acoustic masking are seen extending from the seafloor down to 5-10m. Some are dome-shaped extending to the seafloor as observed off Coondapur and south of Mumbai. Others are inverted U-shaped acoustic masking with rounded edges discernible off Karwar, Mormugao and Mumbai. Significant contributions of gas from the slope sediments to the upper strata and the overlying waters are indicated by buried/exposed pockmarks and the prominent plumes are represented by strong echoes in the sediments. These pockmarks occur as conical and dish-shaped incisions often truncating the strata above the sub-bottom reflector. Distinct gas plumes rise from a few meters to about 70m above the sea floor. The existence of an acoustic turbid zone beneath the pockmarks and a continuous supply of gas from below towards the pockmarks indicate the sediments as gas-charged. The presence of bottom simulating reflectors (BSRs) in slope-rise areas off Goa suggest the presence of gas hydrates in water depths>500-2200m. Over the Laccadive ridge complex, BSR lies at about 460 milliseconds below the seabed where the water depth is found to be nearly 2000m. At places, it is found that chaotic and / or scattered hyperbolic reflections occur below BSR suggesting the presence of gas-charged sediments or free gas while the distinct blanking zones as well as acoustic voids appear above the BSR. It is concluded that the venting of gas causes the presence of pockmarks and seeps. Formation of gas hydrates in the deeper waters are probably caused by the rapid sedimentation and organic carbon-rich sediments coupled with an environment of high pressure and low temperature.
Magnetotelluric (MT) reconnaissance survey has been carried out over the geothermal region of Bakreshwar, West Bengal to study the resistivity, strike direction and dimensionality of the subsurface conductivity structure. This paper is an effort to get most from the already acquired data before planning for more data. The results of sequential analysis of polar diagrams, Swift's strike, rotation angle, tipper magnitude and skew have been presented for three MT soundings: (1) 500 m NE, (2) 1.2 km SW and (3) 9 km away in NE direction from the hot spring respectively. 1-D models have been prepared by very fast simulated annealing (VFSA), a nonlinear inversion technique, for both transverse electric (TE) and transverse magnetic (TM) modes and their resolutions at different depths have been compared. The nearest sounding to the hot spring shows two low resistive zones of about 100 ohm-m and 15 ohm-m at a depth of about 8 km and 12 km respectively in TM mode. The thicknesses of these two low resistive zones are 2.3 km and 3.4 km respectively. These zones, however, are local in nature as they are not seen even in the sounding 1.2 km away on the other side of the hot spring. It is seen that the TM mode gives better resolution at shallower depths than TE mode. Tipper and skew parameters indicate that near surface structure is 1-D or 2-D in nature but for larger depths it is 3-D in nature.
Frequency-domain shot-record migration can produce higher quality images than Kirchhoff migration but typically at a greater cost. The computational cost of shot-record migration is the product of the number of shots in the survey and the expense of each individual migration. Many attempts to reduce this cost have focused on the speed of the individual migrations, trying to achieve a better trade-off between accuracy and speed. Another approach is to reduce the number of migrations. We investigate the simultaneous migration of shot records using frequency-domain shot-record migration algorithms. The difficulty with this approach is the production of so-called cross terms between unrelated shot and receiver wavefields, which generate unwanted artifacts or noise in the final image. To reduce these artifacts and obtain an image comparable in quality to the single-shot-per-migration result, we have introduced a process called phase encoding which shifts or disperses these cross terms. The process of phase encoding thus allows one to trade signal-to-noise ratio for the speed of migrating the entire survey. Several encoding functions and two application strategies have been tested. The first strategy, combining multiple shots per migration and using each shot only once, provides a reduction in computation directly related to the number of shots combined. The second strategy, performing multiple migrations of all the shots in the survey, provides a means to reduce the cross-term noise through stacking the resulting images. The additional noise in both strategies may be tolerated if it is no stronger than the inherent seismic noise in the migrated image, and if the final image is achieved with less cost.
An integrated crustal density model along Nagaur-Jhalawar geotransect across the Aravalli fold belt is constructed based on the modelling of gravity data using the constraints from deep seismic reflection profiling results and near-surface geology. The observed gravity field along the transect shows high Bouguer and Free-air anomalies over the fold belt and lows on its flanks over the Marwar and Vindhyan basins. Results of 21/2 dimensional gravity modelling indicate that the gravity high in the central part of the profile is partly due to a prismatic shaped high density body (3.09 g/cm 3 ) in the lower crust extending from 18 km upto 45 km and partly due to the exposed high density metasediments of Delhi and Aravalli fold belt. The lows on the flanks have been attributed mainly to the presence of low density granites and sediments. The resultant crustal density model shows high density basement and lower crust underlying the adjoining Marwar basin in the west and Vindhyan basin on the east of fold belt. The large-scale thrusting of rocks along shear and fault zones from east to west might have resulted from continental collision during Proterozoic period.
A method is described for calculating the seismic response of an arbitrarily shaped interface below a homogeneous medium by the Kirchhoff integral in the time domain. It is shown, by comparison with other numerical techniques, that this method yields accurate results for reflections. The errors in calculating diffractions are tolerable if the distance of the receiver from the shadow boundary of reflection is not too large; this is usually the case in horizontal seismic profiling. The method has been applied to model qualitatively some typical features in record sections of the deep seismic reflection profile DEKORP2-S. This profile is characterized by numerous strongly curved events that are concentrated mainly in two areas of the profile. These signals can be addressed as diffractions from an interpretation of the travel times. Dynamic calculations, however, show that the surprisingly high amplitudes cannot be explained by diffracting elements like fault edges or small-scale inhomogeneities; instead, one has to assume cylindrically or spherically curved reflectors with a radius of at least 4 km. Some possible geological explanations for these structures, like diapiric intrusions or antiformal stacks, are discussed in view of the tectonic evolution of the Central European Variscides.
The three-layer Maxwell half-space model of the earth and a disk-load approximation of the Weichselian deglaciation history of Fennoscandia are used to calculate glacio-isostatic adjustment for this region. The calculations include the effects of deglaciation-induced geoid perturbations and eustatic sea-level rise and regard (1) lithosphere thickness, (2) asthenosphere viscosity and (3) ice thickness as the free model parameters. Numerical values of parameters (1}-(3) are estimated by calculating the past land uplift and present land-uplift rate observed in central Sweden (glaciation centre) and the past land uplift and past land tilt observed in southern Finland (glaciation margin). The uniqueness of the estimates and their sensitivity to uncertainties in (4) subasthenosphere viscosity, (5) ice cross-section and (6) deglaciation time are also assessed. The principal result of the investigation is that it suggests an upper bound of 80 km on the thickness of the Fennoscandian lithosphere.
