Manifestation of “flower structures” in geophysical models of the Central Tien Shan

Based on the analysis of deep geophysical (geoelectric and seismic) models of the Central Tien Shan, structures with the morphology resembling the crown of palm trees or the shape of a flower were identified. Geoelectric models are considered along a series of regional profiles (75º, 76º, 76º 30’). The length of the profiles intersecting all the main tectonic structures of the Tien Shan ranges from 75 to 250 km. Particular attention was paid to those zones of concentrated deformation, where the tectonic regime combines the conditions of shear and lateral compression (transpression zones). The structure of the collisional - accretionary wedge of the Atbashi zone in the distribution of electrical and velocity characteristics of the geological section is considered. Geoelectric models plotted along a series of regional profiles identify areas of increased electrical conductivity and show “flower structures”. The integral picture of the distribution and morphology of zones of increased electrical conductivity in the segments of the Earth’s crust of the Central Tien Shan may reflect a discretely localized manifestation of palm tree structures due to the evolution of transpressive suture zones during the Hercynian and Alpine tectogenesis.

Simplified Seismic Modelling of Fractured Rock: How Effective is the Localised Effective Medium Compared to Explicit Representation of Individual Fractures

Seismic waves can be an effective probe to retrieve fracture properties particularly when measurements are coupled with forward and inverse modelling. These seismic models then need an appropriate representation of the fracturing. The fractures can be modelled either explicitly, considering zero thickness frictional slip surfaces, or by considering an effective medium which incorporates the effect of the fractures into the properties of the medium, creating anisotropy in the wave velocities. In this work, we use a third approach which is a hybrid of the previous two. The area surrounding the predefined fracture is treated as an effective medium and the rest of the medium is made homogeneous and isotropic, creating a Localised Effective Medium (LEM). LEM can be as accurate as the explicit but more efficient in run-time. We have shown that the LEM model can closely match an explicit model in reproducing waveforms recorded in a laboratory experiment, for wave propagating parallel and perpendicular to the fractures. The LEM model performs close to the explicit model when the wavelength is much larger than the element size and larger than the fracture spacing. By the definition of the LEM model, we expect that as the LEM layer becomes coarser the model will start approaching the effective medium result. However, what are the limitations of the LEM and is there a balance between the stiffness, the frequency and the thickness, where the LEM performs close to an explicit model or approaches the effective medium model? To define the limits of the LEM we experiment varying fracture stiffness and source frequency. We then compare for each frequency and stiffness the explicit and effective medium with five models of LEM with different thickness. Finally, we conclude that the thick LEM layers with lower resolution perform the same as the thinner and finer resolution LEM layers for lower frequencies and higher fracture stiffness.

METHODS FOR CALCULATION OF FREQUENCY DEPENDENT RAYS DEPENDING ON THE MODEL PARAMETRIZATION

The paper presents algorithms for calculating frequency-dependent rays. Three variants for the parametrization of seismic models are considered: a grid model, a grid model with the boundaries of geological bodies specified on the grid, and a model specified by explicit functions. The paper presents an analysis of frequency-dependent ray tracing algorithms, determines the optimal algorithms for each model parameterization, as well as their comparison with the results of standard ray theory and finite-difference modeling.

Shape-function-based non-uniform Fourier transforms for seismic modeling with irregular grids

Multi-dimensional Fourier transform on an irregular grid is a useful tool for various seismic forward problems caused by complex media and wavefield distributions. Using a shape-function-based strategy, we develop four different algorithms for 1D and 2D non-uniform Fourier transforms, including two high-accuracy Fourier transforms (LSF-FT and QSF-FT) and two non-uniform fast Fourier transforms (LSF-NUFFT and QSF-NUFFT), respectively based on linear and quadratic shape functions. The main advantage of incorporating shape functions into the Fourier transform is that triangular elements can be used to mesh any complex wavefield distribution in the 2D case. These algorithms, therefore, can be used in conjunction with any irregular sampling strategies. The accuracy and efficiency of the four non-uniform Fourier transforms are investigated and compared by applying them in the frequency-domain seismic wave modeling. All algorithms are compared with exact solutions. Numerical tests show that the quadratic shape-function-based algorithms are more accurate than those based on linear shape function. Moreover, LSF-FT/QSF-FT exhibits higher accuracy but much slower calculation speed, while LSF-NUFFT/QSF-NUFFT is highly efficient but has lower accuracy at near-source points. In contrast, a combination of these algorithms by using QSF-FT at near-source points and LSF-NUFFT/QSF-NUFFT at others, achieves satisfactory efficiency and high accuracy at all points. Although our tests are restricted to seismic models, these improved non-uniform fast Fourier transform algorithms may also have potential applications in other geophysical problems, such as forward modeling in complex gravity and magnetic models.

An Innovative Approach to Accuracy of Co-Seismic Surface Displacement Detection Using Satellite GNSS Technology

This paper discusses a new method for determining co-seismic displacement using the Global Navigation Satellite System (GNSS) for the precise detection of positional changes at permanent stations after an earthquake. Positioning by the Precise Point Positioning (PPP) method is undertaken using data from the GNSS satellites and one designated station. A time series is processed by an anharmonic analysis before and after an earthquake and these one-day solutions increase the accuracy of measurements. The co-seismic static displacement can be precisely detected from the analysed time series before and after the earthquake, which can be used for the verification of seismic models. Reliability of the estimation of the size of the co-seismic offset is given by the mean square error (RMSE) of the shift. In this study, RMSE was determined by two approaches, initially from variances within PPP processing, and secondly when no positional change from the GNSS before or after the earthquake was assumed. The variance of the data in the time series gives a more realistic estimate of RMSE. This dual approach can affect seismological interpretation due to the need for the interpreting geophysicists to determine which case of co-seismic displacement is more probable for any given locality. The second approach has been shown to provide a more realistic co-seismic displacement accuracy in this study.

Extreme variability of Tibetan thermal lithosphere

<p>We present a thermal model for the lithosphere in Tibet and adjacent regions based on the new thermal isostasy method and our compilation of the Moho depth based on published seismic models. The predicted surface heat flow is in agreement with the few available, reliable borehole measurements. Cratonic-type cold and thick lithosphere (200-240 km) with a surface heat flow of 40-50 mW/m<sup>2</sup> typifies the Tarim craton, the north-western Yangtze craton, and most of the Lhasa Block that is possibly refrigerated by underthrusting Indian lithosphere. The thick lithosphere of the Lhasa block extends further north in its western and eastern segments than in its central section. We identify a North Tibet anomaly with a thin (<80 km) lithosphere and high surface heat flow (>80-100 mW/m<sup>2</sup>), possibly associated with the removal of lithospheric mantle and asthenospheric upwelling. Other parts of Tibet have an intermediate lithosphere thickness of 120-160 km and a surface heat flow of 45-60 mW/m<sup>2</sup>, with a patchy style in eastern Tibet. In the Qaidam deep sedimentary basin the lithosphere is about 100-120 km thick. The heterogeneous thermal lithosphere beneath Tibet suggests an interplay of several mechanisms as the driver of the observed uplift.</p>

Moho density contrast in Antarctica determined by satellite gravity and seismic models

SUMMARY As recovering the crust–mantle/Moho density contrast (MDC) significantly depends on the properties of the Earth's crust and upper mantle, varying from place to place, it is an oversimplification to define a constant standard value for it. It is especially challenging in Antarctica, where almost all the bedrock is covered with a thick layer of ice, and seismic data cannot provide a sufficient spatial resolution for geological and geophysical applications. As an alternative, we determine the MDC in Antarctica and its surrounding seas with a resolution of 1° × 1° by the Vening Meinesz-Moritz gravimetric-isostatic technique using the XGM2019e Earth Gravitational Model and Earth2014 topographic/bathymetric information along with CRUST1.0 and CRUST19 seismic crustal models. The numerical results show that our model, named HVMDC20, varies from 81 kg m−3 in the Pacific Antarctic mid-oceanic ridge to 579 kg m−3 in the Gamburtsev Mountain Range in the central continent with a general average of 403 kg m−3. To assess our computations, we compare our estimates with those of some other gravimetric as well as seismic models (KTH11, GEMMA12C, KTH15C and CRUST1.0), illustrating that our estimates agree fairly well with KTH15C and CRUST1.0 but rather poor with the other models. In addition, we compare the geological signatures with HVMDC20, showing how the main geological structures contribute to the MDC. Finally, we study the remaining glacial isostatic adjustment effect on gravity to figure out how much it affects the MDC recovery, yielding a correlation of the optimum spectral window (7≤ n ≤12) between XGM2019e and W12a GIA models of the order of ∼0.6 contributing within a negligible $ \pm 14$ kg m−3 to the MDC.

Methods of calculating the soil response spectrum for designing in seismically active area

The article is a continuation of the research of the effect of combining seismological and engineering-seismological methods based on real geological and geophysical data. A probabilistic seismic hazard analysis and numerical modeling of the soil response were performed for the site. Using the Monte Carlo method, 100 realizations of each of the 106 seismic models of the site were prepared to take into account the uncertainty and spread in shear wave velocities of the geological layers. Comparing and analyzing the results obtained, it was found that the generalized response spectrum of the soil relative to the surface, taking into account the parameters of a 30-meter thickness, provides for most of the considered spectral periods higher estimates of spectral accelerations than numerical modeling and can be used to preliminary estimate the amplitudes of the response spectrum of the soil profile before performing seismic microzoning.