Granitic Layer Research Articles

On the Crustal Sources of Magnetic Anomalies in the Middle Urals

The 800×500 sq km area of the the Middle Urals, Russia, lying between 56 to 60° northern latitude and 54 to 66° eastern longitude, was studied in terms of the deep crustal structure. The study was based on three-dimensional modeling of the sources of magnetic anomalies in three layers of the Earth’s crust. The studied area covers the folded region of the Urals and adjacent structures of the East European Platform and the West Siberian Plate. The sources modeled in the near-surface layer down to a depth of 5 km make it possible to clarify the position of magnetized massifs, mainly consisting of mafic-ultrabasic rocks, and to trace their connection with mafic-ultrabasic belts in the granite layer and root blocks in the lower basaltic layer of the Earth’s crust. When the sources in the upper crustal layer are compared with those in the lower crustal layer, it is apparent that many belts on the platform have deep-seated roots and are located above the basalt layer protrusions, while most of the massifs in the Urals do not have deep-seated roots. The springs located under the western slope of the Urals allow reliably determining the depth to the basement of the ancient platform and the location of the eastern border of the East European Platform in the lower layer of the Earth’s crust. There were found extended zones of subsidence of the roof of the lower magnetized layer, which probably mark the boundaries of various terrains forming the paleo-island arc sector of the Ural fold system. The most extensive subsidence of the roof of the lower layer occurs to the west of the Tyumen-Chudinovsky fault and is perhaps the eastern deep-seated dividing line between the Ural fold system and the West Siberian Plate.

3D shear wave velocity imaging of the subsurface structure of granite rocks in the arid climate of Pan de Azúcar, Chile, revealed by Bayesian inversion of HVSR curves

Abstract. Seismic methods are emerging as efficient tools for imaging the subsurface to investigate the weathering zone. The structure of the weathering zone can be identified by differing shear wave velocities as various weathering processes will alter the properties of rocks. Currently, 3D subsurface modelling of the weathering zone is gaining increasing importance as results allow the identification of the weathering imprint in the subsurface not only from top to bottom but also in three dimensions. We investigated the 3D weathering structure of monzogranite bedrock near the Pan de Azúcar National Park (Atacama Desert, northern Chile), where the weathering is weak due to the arid climate conditions. We set up an array measurement that records seismic ambient noise, which we used to extract the horizontal-to-vertical spectral ratio (HVSR) curves. The curves were then used to invert for 1D shear wave velocity (Vs) models, which we then used to compile a pseudo-3D model of the subsurface structure in our study area. To invert the 1D Vs model, we applied a transdimensional hierarchical Bayesian inversion scheme, allowing us to invert the HVSR curve with minimal prior information. The resulting 3D model allowed us to image the granite gradient from the surface down to ca. 50 m depth and confirmed the presence of dikes of mafic composition intruding the granite. We identified three main zones of fractured granite, altered granite, and the granite bedrock in addition to the mafic dikes with relatively higher Vs. The fractured granite layer was identified with Vs of 1.4 km s−1 at 30–40 m depth, while the granite bedrock was delineated with Vs of 2.5 km s−1 and a depth range between 10 and 50 m depth. We compared the resulting subsurface structure to other sites in the Chilean coastal cordillera located in various climatic conditions and found that the weathering depth and structure at a given location depend on a complex interaction between surface processes such as precipitation rate, tectonic uplift and fracturing, and erosion. Moreover, these local geological features such as the intrusion of mafic dikes can create significant spatial variations to the weathering structure and therefore emphasize the importance of 3D imaging of the weathering structure. The imaged structure of the subsurface in Pan de Azúcar provides the unique opportunity to image the heterogeneities of a rock preconditioned for weathering but one that has never experienced extensive weathering given the absence of precipitation.

ГЕОХИМИЧЕСКИЕ ОСОБЕННОСТИ ГАББРОИДОВ И ГРАНИТОИДОВ НЯТЫГРАНСКОГО КОМПЛЕКСА (БУРЕИНСКИЙ МАССИВ)

Gneissic gabbros, gabbrodiorites, granodiorites, and granites of the Late Proterozoic Nyatygransky Complex, exposed by erosion in the eastern part of the Bureya massif, form small intrusive bodies within the boundaries of the Melgin metalliferous trough, as well as around it. Analysis of chemical and trace-element compositions of rocks from the Nyatygransky Complex revealed their geochemical features. The calculated agpaitic index values for gabbroids are very low and range from 0.11 to 0.33. Granodiorites have slightly higher index values ranging from 0.5 to 0.38, and only gneiss-granites are plumasitic with agpaitic index values between 0.45 and 0.75. Both gabbroids and granitoids are assigned to calc-alkaline rocks by their agpaitic index. Gneissic gabbros, dominated by dark-colored minerals (labradorite, hornblende), show enhanced alkalinity not common to gabbros (4%); in them, the magnesium content prevails over the iron content. They are characterized by high alumina content (15–18%), reaching 20.38%. Geochemically, they are interpreted as I-type igneous rocks. Gneissic granitoids with a slight predominance of the iron content over the magnesium content are less aluminous (Al2O3 12–15%). They are interpreted as S-type (sedimentary) and I-type (igneous) granitoids, formed in sedimentary and granitic layers of the earth’s crust. According to petrochemical diagrams, the gabbroids and granitoids of the Nyatigransky Complex belong to the volcanic arcs of the active continental margin.

Mechanical Performance of Concrete Segment Lining Structure of Shield Tunneling in Different Strata

There are many problems in the development of urban space in China. Among them, urban tunnels generally pass through many sections with very complicated geological conditions, and the construction will encounter great difficulties, so the mechanical behavior of shield segments in different complex strata is worth discussing. In this paper, the axial force, bending moment and pore water pressure of shield tunnel segments in the soft and hard uneven stratum, clay stratum and fully weathered granite stratum of overlying buildings are studied by establishing a rectangular element mechanical model based on the field test method. The analysis shows that the mechanical properties of shield tunnels in different strata are quite different, but their mechanical properties change stages are the same. The earth pressure on the left and right sides of the test ring is asymmetric in the soft and hard uneven stratum, and the vault pressure is much greater than the vault bottom pressure. The distribution of earth pressure in each position of the segment ring in clay stratum is relatively balanced, and the earth pressure on both sides is relatively small; in the fully weathered granite layer of the overlying building, the segment ring of the test ring is subjected to greater additional stress, and the internal force of the segment is much greater than that without the overlying building. Exploring the similarities and differences of segment stress in these three complex strata can provide an important basis for the design and construction of shield segments in complex strata.

Geophysical research on the interplays between soil–rock variability and hydrogeological structures: a case study

Understanding the spatial variations in soil–rock profiles is crucial for identifying weathered rock layers and mapping geological structures, particularly in the complex feldspar-rich granitic terrain of Penang Island, Malaysia. This study employs a combination of electrical resistivity and seismic refraction tomographic techniques, along with borehole lithological data, in three distinct areas on the island. Its goal is to provide insights into surficial and subsurface soil–rock characteristics and water-bearing structures within the North Penang Pluton (including a part of the Sungai Ara axis) for sustainable groundwater development, addressing the needs of the region's growing population. The research identifies three distinct geological units at the near-surface: residual soils, highly to moderately weathered/fractured granitic layers and fresh bedrock, each with its hydrogeological properties. Promisingly, the weathered layers offer significant groundwater development potential, with deep weathered and potentially fractured zones exceeding depths of 30 m in the study area. These findings have substantial implications for sustainable groundwater development, especially in tropical hard-rock terrains. In addition, the study underscores the limitations of small geophysical spacings when probing deeper groundwater resources. This knowledge is essential for informed decision-making in water resource management and infrastructure development, addressing the ongoing global challenge of ensuring access to clean water resources amid population growth.

Microseismicity of the Persian Gulf and the Zagros Mountain Massif According to Bottom Seismological Observations

Bottom seismological observations in the Persian Gulf over a short period made it possible to detect weak earthquakes with magnitudes (ML = –0.2–2.9), whose sources are located in the mantle under its water area and under the Zagros mountain range. On the section built across the coastline of the Persian Gulf, under the Zagros Mountains, the projections of the sources of the detected earthquakes form seismic focal layers, steeply dipping into the mantle to the northeast to depths of 120–180 km. The spatial distribution of strong and medium earthquakes, obtained from the updated earthquake catalog of the US Geological Survey and the ISC International Seismological Catalog, does not contradict the distribution of sources of micro- and weak earthquakes, but complements it, forming a separate seismic focal layer. According to the data obtained as a result of bottom seismological observations, the entire thickness of the earth’s crust of the region and the upper mantle, and not only the upper layers of the crust, as presented in a number of publications, are seismically active. It is possible that collision processes and accompanying phenomena (mantle seismicity and destruction of the granitic layer of the earth’s crust) are associated with the presumed presence and rotation of the earth’s surface with the center of rotation in the area of about. Cyprus.

Borehole Instability in Decomposed Granite Seabed for Rock-Socketed Monopiles during “Drive-Drill-Drive” Construction Process: A Case Study

Monopiles are commonly used in the construction of offshore wind turbines. However, implementing drive-drill-drive construction techniques in decomposed granite seabed may lead to borehole instability during the window period between drilling and pile driving, resulting in significant project losses. This study provides a comprehensive understanding and approach to address the causes of borehole instability in rock-socketed monopiles in decomposed granite seabed. Using the Pinghai Bay offshore wind farm project in Fujian, China as an example, the details of drive-drill-drive and reverse-circulation drilling techniques employed in monopile construction were introduced. An improved sampling method was utilized to obtain decomposed granite samples, and a series of in situ and laboratory tests were conducted to analyze the physical and mechanical properties of marine-decomposed granite. By examining three cases of monopile construction, the factors contributing to borehole instability during rock-socketed monopile construction in decomposed granite seabed were identified, and corresponding recommendations were proposed. The results indicated that construction technology and unfavorable geological characteristics of decomposed granite are the primary causes of borehole instability. Collapses occurred mainly in highly and moderately decomposed granite layers. Employing smaller boreholes can reduce the likelihood and impact of borehole instability.

Pre-Reinforcement Mechanism and Effect Analysis of Surface Infiltration Grouting in Shallow Buried Section of Long-Span Tunnel

In order to solve the problem that the hole-forming rate of boreholes is low and it is difficult to reach the designed length when supporting a long pipe shed in loose stratum in a shallow buried section of a long-span tunnel, it is necessary to pre-reinforce the loose stratum in order to improve the strength and integrity of the surrounding rock. Relying on the grouting project of the shallow buried section at the exit of Botanggou tunnel, it is assumed that the grouting material is Newtonian fluid and the steel floral tube shows cylindrical infiltration and diffusion. Through the analysis of the structural characteristics of the injected stratum, the conceptual model of infiltration grouting is established. Twelve groups of test slurry were prepared with ordinary Portland cement and ultra-fine cement, and through the analysis of the slurry parameters of each group, ordinary Portland cement slurry was selected with a water–cement ratio of 1:1 plus 3% water glass to strengthen the gravel layer, and ultra-fine cement slurry with a water–cement ratio of 1:1 plus 3% water glass and 0.3% polycarboxylate superplasticizer to strengthen the fully and strongly weathered porphyritic granite layer. Through the on-site single-hole grouting test and combining with the empirical formula, the maximum diffusion radius of single-hole infiltration grouting is calculated, and the sliding width of the sidewall is deduced using Terzaghi theory. To ensure the grouting effect, the 5 m expansion of the excavation profile is taken as the grouting range. Grouting construction adopts the overall order of periphery and then interior, and three-sequence opening and grouting are adopted in the same row of grouting holes, which can effectively prevent grouting running and grouting. For the strata treated by surface grouting, the construction of the long pipe shed is smooth and reaches the designed length, and there is no large deformation of the surrounding rock when excavated using the CD method. The treatment effect is analyzed by the P-Q-t control method, excavation observation method, and deformation monitoring method. The results show that the injected stratum is fully infiltrated and gelled, forms an obvious grouting stone body, the integrity and strength of surrounding rock are obviously improved, and the convergence values of the tunnel surface, vault subsidence, and clearance do not exceed the alarm value of 60 mm. The research results provide some awareness and understanding of the grouting pre-reinforcement of loose stratum in a shallow buried section of a long-span tunnel in the future.

RADIOLOGICAL IMPACT OF USING DECORATIVE GRANITE AS AN ATTENUATOR OF IONIZING RADIATION.

Granite is a widely available rock, which can be used as a shielding material, for bulk in the form of the aggregate in concrete. It has the weakness that it is more radioactive than many other rocks, which can be used in concrete. This paper looks at its properties as a shielding material and the activity level. Thus, the concentrations of 226Ra, 232Th and 40K, in granite were measured using a high pure germanium detector (HPGe). They were ranged from (15±4 to 49±5) Bq kg-1 for 226Ra, (22±4 to 78±4Bqkg-1) for 232Th and (791±13 to 1231±15Bqkg-1) for 40K. Radiological indices of radium equivalent concentration (Raeq), external (Hex), internal (Hin) and annual effective dose were less than worldwide recommended limits. The results emphasized, the granite samples had no radiation hazard. Nevertheless, the mass attenuation coefficients of granite samples were measured for the gamma rays of energy range 122-1408keV. The mass attenuation coefficients of the studied granite samples were ranged from 0.05 to 0.15cm2g-1. In addition, the average the half-value layer of granite was varied from 1.8cm for 122keV to 5.2cm for 1408keV. The results are that the attenuation characteristics are typical and match the values given by NIST for 'concrete' and that the activity levels of the samples examined are acceptable. Thus the granite may be used as an attenuator for ionizing radiation.

Effect of hydrogeological structure on geogenic fluoride contamination of groundwater in granitic rock belt in Tanzania

• Effect of geophysical structure on fluoride contamination have been researched. • Results suggest weathering promote fluoride leaching. • Intake from the fractured zone reveals fluoride depend on the fault direction. • Intake from the weathered zone increases fluoride with well yield potential. In the East African Rift Valley, geogenic fluoride contamination of groundwater is particularly noticeable as a drinking water problem. In the Tabora region, in the central part of Tanzania, which consists of two basins, Lake Tanganyika Basin (LTB) and an Internal Drainage Basin (IDB), within a semi-arid region, fluoride-contaminated groundwater that exceeds the World Health Organization standards is pumped from many wells in the fractured and weathered zones of the granitic layer underlying these basins. Fluoride contamination of groundwater is heterogeneously distributed in the granitic layer, posing an obstacle to groundwater development. In this study, a relationship between the fluoride concentration and the hydrogeological structure was investigated using geophysical survey data to achieve efficient groundwater development in geogenic contaminated areas. Based on the results, the following two mechanisms have been determined regarding fluoride contamination in the Tabora region. 1) With respect to the weathered zones in both the LTB and IDB, the greater the extent of weathering, the more readily fluoride leaches into groundwater. This suggests that fluoride leaching due to the chemical weathering of minerals containing fluoride increases with the weathering of rocks in the field. 2) With respect to the fractured zones in the LTB, a large well yield potential and a deeper water strike depth to the water vein result in lower fluoride concentrations. The suggested flow mechanism for the LTB is that water recharge in a groundwater depression area with a low fluoride concentration flow to substantial depth and over long distances through fault systems, which form preferential flow paths. This implies that the fractured zone along the fault system in the LTB is a high-priority reservoir for the development of safe water resources in the Tabora region. The hydrogeological mechanisms discussed in this study provide a basis for establishing estimation methods for the prediction of safe groundwater resources before a borehole is drilled. However, more reliable data needs to be collected to clarify the effects of hydrogeology on groundwater fluoride contamination for the fractured zone in the IDB.

Deep structure of South Caspian depression and connection of seismic-geological conditions with oil-gas bearing content

The geological model of South Caspian depression is specified, the seismic-geologic and geodynamic conditions of deep layers studied, a basis for the exploration of oil-gas structures in a new aspect developed as well. A complex analysis of geological, geophysical and seismological data obtained recently has been conducted, the distribution of the depth of sedimentary, granite and basalt layers, as well as the maps of the borders dividing them – Mokhorovich (Mokho-M), Konrad (K) and base surface (F) developed. It was defined that the depth of M surface fluctuates within 40–55 km, K – 20–35 km and the base surface 5–25 km. The history of the geological development of South Caspian depression has been studied, magmatic processes (intrusion formation), seismic-geodynamic activity (earthquakes), geotectonic movements (horizontal, vertical and circle), as well as the oil-gas perspectives of the deep layers of the region estimated. It is shown that the thickness of the granite layer in South Caspian depression decreased from 5–25 km to 4–5 km, and disappears in the deepest (1025 km) part of the Caspian Sea. Corresponding changes in the seismic activity on the map of earthquake epicenter and the distribution of epicenters by the depth were observed in the circle scheme within the intervals mentioned above, and the violent earthquakes with М≥6–8 coincide with 7–20 km diapason.

New technology of magnetotelluric polaron exploration-taking Huashan piedmont basement fault as an example

Detecting the Huayin Piedmont faults and basement conditions in the Weihe Basin through magnetotelluric polaron exploration technology found that the piedmont faults in the research area are quite developed, with two stepped normal faults with larger angles, along with many small faults; From the terrestrial polarization sub-bands, it is reflected that there are many fractures in this area, and the formation energy fluctuates greatly, indicating that the energy accumulation and formation energy of the formation are discontinuous, and there is a complete anomaly of the upper and lower formation energy at a certain depth. The comparative analysis of drilling and core data believes that it is the interface between the sedimentary rock layer and the underlying Qinling piedmont basement: the overlying stratum is the Tertiary sedimentary rock, and the lower basement is the Qinling Archaean granite layer; the fault structure of the Qinling piedmont is further understood and the basement rock layer and depth also proved the accuracy and reliability of the magnetotelluric polaron exploration technology.

Analysis of Ground Settlement Caused by Double-line TBM Tunnelling Under Existing Building

TBM tunnelling is less used in the subway construction in prosperous city due to the limitation of the engineering geological conditions. The studies on the influence of the TBM construction on the existing buildings are also limited. Therefore, based on the engineering case of tunnel crossing existing building in the section of Haiboqiao ~ Xiaocunzhuang station of Qingdao Metro Line 1, the numerical model that simulates the construction process of TBM tunnelling in slightly weathered granite layer is established by three-dimensional finite difference software FLAC3D to analyze the influence of TBM tunnelling. The comparisons between ground deformations obtained by FLAC3D and field monitoring data in different construction stages of double-line tunnel have been made firstly to validate the numerical model. Then the ground settlement characteristics, differential settlement and the stress distribution of the existing building and the stress of segment structure have been analyzed. During TBM tunnelling under existing building, the settlement of the building as a whole tends to increase, with the maximum measured settlement of 5.45 mm and the maximum differential settlement of 1.58 mm, which meets the control standard of relevant codes. Ground settlement groove along the transverse and vertical sections occurs near the building and tunnels, and the settlement becomes smaller with the farther the distance from the building and tunnels. The settlement curve on the cross section changes dynamically and is approximately V-shaped, and its width is about 5 ~ 6 times diameter of the tunnel. For the same cross section, the range of the settlement groove after tunnelling right line increases obviously compared with that after tunnelling left line (first construction), the settlement values also increase, and the symmetrical axis of the settlement curve is shifted to the right. For the segment structure, the maximum principal tensile stress occurs inside the arch bottom of the tunnel, while the maximum principal compressive stress occurs near the arch top and arch waist, with obvious stress concentration. For the existing building, the maximum principle tensile stress occurs at the door opening of the wall on the first floor and the maximum principal compressive stress is at the corner of wall. They are less than the tensile strength and compressive strength of the segment structure and building respectively, and have enough safety margins. This paper can provide important practical reference for the deformation control and protection design of surrounding buildings for relative construction.

Chemical and isotopic (H, O, S, and Sr) analyses of groundwaters in a non-volcanic region, Okayama prefecture, Japan: Implications for geothermal exploration

The chemical and D, O, S, and Sr isotopic composition of 26 thermal waters (>25 °C) and 25 cold waters from springs and wells in Okayama Prefecture were investigated to examine the geochemical characteristics of the geothermal resources in a non-volcanic region. No clear geochemical difference was noted between the thermal and cold waters. Some of these waters were formed by mixing of fossil seawater and meteoric water; however, most with low salt concentrations are of meteoric origin and are presumed to have flowed out with a residence time of several months. The waters studied were classified as Ca2+−HCO3− and Na+−HCO3− types, with 87Sr/86Sr values close to those of the granite base. These thermal and cold waters were formed by an infiltration of meteoric water into the granite layer that subsequently reacted with carbonate and clay minerals. Considering the main chemical components of the waters, the estimated maximum underground temperature was approximately 100 °C. Therefore, the utilization of these geothermal resources for geothermal heat pumps was judged to be effective even though calcium carbonate scale could precipitate in the heat exchanger.

About the mechanism of the granite-free zones formation in the Black Sea

One of the important unsolved problems related to the evolution of living conditions on Earth is the mechanism of the rapid transformation of the Black Sea from a shallow lake-type sea into a deep-water basin, the earth's crust in the central part of which does not have a granite layer. There is no explanation as to how “granite-free depressions” were formed at the bottom of the sea, which are currently covered by sediment. Investigations of these processes were started in the middle of the last century by scientists-geologists of the Institute of Oceanology of the Russian Academy of Sciences and its South. In this article, the authors propose a mechanism for the destruction of the earth's crust and the formation of depressions in the inner seas during the Messinian crisis.

Identification of potential liquefaction in Kabonena

The research was conducted in Kabonena village, which aims to see the subsurface structure and the liquefaction potential. The research was conducted using the geoelectric resistivity method with the Wenner system. The parameters used are geological data, hydrogeology and formation values to obtain between the resistivity value and the lithology of the study area. Furthermore, by considering the geological, hydrogeological conditions and formation factor values, the specific resistance values for each layer are interpreted. The results obtained show that the layers with specific resistance values of 39.16 - 97.9 Ωm are dominated by water-saturated sand/gravel. The resistivity value above 97.9 - 200 Ωm is the layer of molasses and alluvium deposits/coastal deposits, and the resistance value above 200 Ωm is the layer of crushed granite and granodiorite. The existence of a subsurface structure like this, so that in Kelurahan Kabonena it is called a liquefaction event. The location has a layer of saturated air that points to the east with a depth of less than 24 meters.

High-resolution 3D seismic imaging and refined velocity model building improve the image of a deep geothermal reservoir in the Upper Rhine Graben

Over the past 35 years, geothermal projects have been developed in the Upper Rhine Graben (URG) to exploit deep geothermal energy. Underneath approximately 2 km of sedimentary deposits, the deep target consists of a granitic basement, which is highly fractured and hydrothermally altered. Therefore, it has high potential as a geothermal reservoir. Despite dense 2D seismic data coverage originally acquired for oil exploration (for a target two-way traveltime between 300 and 700 ms), the faults at the top of the granitic basement (between 1400 and 4000 ms) are poorly imaged, and their locations remain uncertain. To gain a better understanding of this large-scale faulting and to ensure the viability of future geothermal projects, a 3D seismic survey was acquired in the French part of the URG during the summer of 2018. This paper describes how an integrated project, combining seismic data processing, high-end imaging, and enhanced interpretation, was conducted to improve the understanding of this complex basin for geothermal purposes. By revealing the deep granite layer and its complex associated fault network, the insight from this project can help accurately locate future production wells.

Thermochronology of the Angara–Vitim Granitoid Batholith, Transbaikalia, Russia

We reconstructed the history of the Angara–Vitim batholith (AVB), characterized by the formation of huge volumes of granitic magma using thermochronological analysis (40Ar/39Ar, U/Pb) and math testing selected age milestones on the basis of the model crystallization differentiation and dynamics of heat and mass transfer in a magmatic chamber. The consolidation time of the granite melt is estimated to be in the range of 320–290 Ma. Events with age are distinguished based on the presence of isotope dating clusters T1 = 245 ± 1 Ma; T2 = 212 ± 1 Ma; T3 = 156 ± 1 Ma; T4 = 125 ± 2 Ma. The thermal history of samples corresponding to the modern erosion level immediately imposes strict restrictions on the history of consolidation and cooling for rocks corresponding to the deep (>20 km) levels of the magmatic chamber. Calculations show that the lifetime of the residual melt at the deep levels of the magmatic chamber of the AVB can reach 100 Ma. Events with age 245 ± 1, 212 ± 1, 156 ± 1, 125 ± 2 Ma reflect the gradual transformation of the “semi-frozen granite layer” and the discrete nature of its tectonic exposure to the upper level of the Earth’s crust under the influence of tangential elastic deformations caused by the pulsating manifestation of intraplate mantle magmatism within the Siberian platform and its folded frame. The final stage of AVB tectonic exposure to the Earth’s surface (as a solid body) occurred from 60 Ma to the present time, reflecting the process of origin and development of the Baikal rift system.

Surface Rayleigh-Lamb waves in stratified substrate

Propagation of Rayleigh - Lamb waves has shortage of theoretical studies and demands in deeper analyses. Understanding the process of propagation of Rayleigh-Lamb waves depending on the geometrical and acoustical properties of the individual layers within the Earth’s crust is of great importance for the NDT of Earth exploration and development of seismic protection of buildings and structures. Propagation of these waves in the Earth’s crust upper layers is studied by analytical methods. A mathematical model for analyzing Rayleigh -Lamb waves propagating in stratified media is worked out. The analytical solution is constructed using Cauchy complex formalism and the method called Modified Transfer Matrix (MTM). As an example a multilayer system modeling the Earth’s crust, consisting of sedimentary rocks, a layer of granite, basalt and mantle, is considered. Equation for dispersion relations is derived. Fundamental modes of the dispersion curves for a stratified media are obtained.

A Comprehensive Analysis of Attenuation Characteristics Using Strong Ground Motion Records for the Central Seismic Gap Himalayan Region, India

ABSTRACT This study aims to estimate attenuation characteristics of the central Himalayan region of India concerning various strong-motion parameters such as Kappa value (κ) and site effects. We have tried to elaborate on the regional structural heterogeneities and their implications towards the seismic hazard assessment of the study region. A total of 81 earthquakes recorded at 50 stations situated in the central Himalayan region of India are used for the purpose. The particular focus is kept on Kappa value, which shows variability from 0.03 s to 0.095 s, inferring the higher values obtained in plains with deep sediment accumulations proving high-frequency energy dissipation and stiff-soil/rocky sites exhibit comparatively limited attenuation accordingly. To substantiate these results various attenuation parameters such as coda wave quality factor (Qc), intrinsic attenuation parameter (Qi), and scattering attenuation parameter (Qs), have been estimated for two regions in the central seismic gap Himalayan region of India employing the single backscattering model and Wennerberg formulation. The estimated values of Qc, Qi, and Qs are found to be highly dependent on frequency in the frequency range 1.5–24 Hz for both the regions. The average frequency-dependent relationships () estimated for both regions are and , respectively. The low value of Q0 shows that the region is highly heterogeneous while the higher value of η indicates higher seismicity in the area. It is also found that intrinsic attenuation is predominant over the scattering attenuation, envisaging the behavior of the wave attenuation through the absorption within the granitic layer at shallow depths. At lower frequencies, Qc values are found close to Qs values, which is in agreement with the theoretical measurements suggesting the presence of complex crustal heterogeneities beneath the region affecting the propagation of seismic waves experiencing considerable decay of energy through scattering. To confirm the aggregate attenuation on the stations, the site characteristics are also determined for examining the behavior of the amplification as the ground motion is comprised of the combined effect of the source, path, and site. The sites are amplified at a predominant frequency ( fpeak ) in between 1.5 to 10 Hz for the central Himalayan region. The different attenuation and amplification parameters like kappa, Q, and site effects can be utilized for detailed seismic hazard analysis (based on ground motion prediction equations) of the area as this region is of great importance from a socio-economic point of view.