Crustal thickness and velocity structure beneath Singapore’s seismic network

We estimated the crustal thickness and velocity structure beneath the five stations comprising the Republic of Singapore’s seismic network. Our data set was composed of 697 teleseismic receiver functions and 7 months of broad-band data that was cross-correlated to produce inter-station Green’s functions. Surface wave group velocities were extracted from the Green’s functions to obtain dispersion data for a path from central Sumatra to Singapore in order to provide a complimentary data set to the receiver functions. Crustal thickness was estimated via an H − k stacking technique, and high-resolution 1D P-wave velocity profiles were generated beneath each station by jointly inverting receiver function stacks and the group velocity data using a linearised time-domain inversion scheme. Crustal thickness beneath four stations was found to be between 28.0 km and 32.0 km, while one station in the northeast of Singapore indicates 24.0 km thick crust. This implies a significant crustal thinning beneath Singapore over the lateral extent of 50.0 km. Inversion results exhibit several crustal features that are observable in the derived models at all five stations, indicating that they exist across Singapore as a whole. There appears to be an upper-crustal high-velocity zone beneath Singapore, underlain by a velocity inversion. Station NTU shows slower near-surface velocities than the other stations, consistent with its situation above the sedimentary Jurong formation. These results expand the available global velocity data set, as well as being useful for assessing the seismic hazard in Singapore.     

Thermoelectric characteristics of semiconductor minerals in earth’s deep crust and their seismogenic significance

Geo-electric anomalies are generated during the process of stress accumulation and release associated with earthquakes. However, the mechanism of these anomalies remains equivocal. Based on the analysis of thermoelectric characteristics of semiconductor minerals of the earth’s deep crust such as graphite, ferrosilicon alloy, magnetite etc., we perform finite element analysis to evaluate the principles governing the thermoelectric power generated by minerals and rocks. The results show that graphite, ferrosilicon alloy and magnetite all exhibit Seebeck effect and can be superimposed. And the thermo-electric field can be enhanced with the activation temperature increases, the content of thermoelectric minerals increases, the size of aggregates increases, and the spacing of thermoelectric minerals grains decreases. Seismogenic processes would generate a similar thermal gradient. The natural semiconductor minerals in this thermal field show a thermoelectric effect, forming a thermoelectric field that interferes with the background electric field. This study indicates that thermoelectric effect may have an important influence on the formation of geoelectric field.     

Metallogeny of deep structures of the Earth’s crust in Southern Sikhote-Alin: Evidence from gravity data

The spatial relations between ore deposits in Sikhote-Alin and deep density inhomogeneities of the Earth??s crust down to a depth of 30 km have been examined. The ore areas and regions show a discrete correlation with the anomalies of the normalized density of the equivalent spherical sources of gravity anomalies at depths of 1 to 2, 4 to 5, 10?C12, and 24 km presumably provoked by magmatic bodies of different compositions. The depth of the magmatic bodies with the intermediate-to-basic composition of the initial magmas and the southeastward-decreasing vertical range of their correlation with the ore regions depend on their structural position. In the case of magmatic bodies of felsic and mixed compositions, the metallogenic specialization of the corresponding ore-magmatic systems is correlated with their inferred vertical range. Tin ore systems are characterized by a smaller vertical range (5?C10 km) of the correlation with density inhomogeneities, whereas tin-tungsten-gold ore systems are marked by a wider range (20?C25 km). Tin-lead-zinc systems occupy an intermediate position (12?C20 km). The ore-controlling role of the boundaries between the lithostructural complexes of the Earth??s crust and the central-type structures in the distribution of deep sources of ore mineralization is shown.     

Seismic velocity and Poisson’s ratio tomography of the crust beneath East Anatolia

Eastern Anatolia is a region in the early stages of continent–continent collision and so provides a unique opportunity to study the early development of continental plateau. Located within the Alpine–Himalayan fold-thrust fault belt, the Anatolian plateau is geologically very complex, with over half of the surface area covered with late Cenozoic volcanics of diverse composition. The plateau is also seismically active and is dissected by numerous seismogenic faults predominantly of strike-slip motion. In this study, we determine 3-D tomographic images of the crust beneath eastern Anatolia by inverting a large number of arrival time data of P- and S-waves. From the obtained P- and S-wave velocity models, we estimated the Poisson’s ratio structures for a more reliable interpretation of the obtained velocity anomalies. Our tomographic results are generally consistent with the major tectonic features of the region. High P- and S-wave velocity anomalies are recognized near the surface, while at deeper crustal layers, low seismic wave velocities are widely distributed. Poisson’s ratio exhibits significant structural heterogeneities compared to the imaged velocity structure. The seismic activity is intense along highly heterogeneous zones and is closely associated with pre-existing faults in the central and western parts of the study area. Results of the checkerboard resolution test indicate that the imaged anomalies are reliable features down to a depth of about 40 km. The low-velocity/high Poisson’s ratio zones in the middle to lower crust are consistent with many geophysical observations such as strong Sn attenuation, low Pn and Sn velocity, and the absence of mantle lid, implying the presence of partial melt in the uppermost mantle.     

The influence of tropical cyclones upon the Earth’s crust

Doklady Earth Sciences - This paper discusses the influence of tropical cyclones (TCs) upon the Earth’s crust beneath the ocean bottom, which is associated with rapid variations of pressure...     

The 3d relief models and structure of the Earth’s upper crust in the Amur region

The analysis of 3D relief models of the lower Amur region and several adjacent areas suggested that the structure of the region is related to the horizontal tectonic layering of the upper part of the Amur plate. When it was dislocated to the northwest at the terminal Cretaceous, some fragments of the upper layers were displaced not strictly synchronously but with some lag relative to the whole plate. This scenario was responsible for the formation of the main morphostructural elements of the region: river valleys, mountain ranges, and graben series. These inferences are supported by field observations and some geological data. The proposed hypothesis can also be applied for several other regions.     

Connection between variations of the stress-strain state of the Earth’s crust and seismic activity: The example of Southern California

A three-dimensional geomechanical model of Southern California, including mountain relief, fault tectonics, and characteristic internal borders, such as the roof of the consolidated crust and Moho surface, was created. The initial stress state of the model is determined by the gravitational force and horizontal tectonic movement, established on basis of GPS observations. Monitoring of variations in the stress state of the Earth’s crust and lithosphere, which are generated by seismic processes, has shown that the model enables us to predict an increase of seismic activity in a region and to mark the places in which average earthquakes can occur in the following two weeks.     

Susceptibility evaluation and mapping of China’s landslides based on multi-source data

Landslides are occurring more frequently in China under the conditions of extreme rainfall and changing climate, according to News reports. Landslide hazard assessment remains an international focus on disaster prevention and mitigation, and it is an important step for compiling and quantitatively characterizing landslide damages. This paper collected and analyzed the historical landslide events data of the past 60 years in China. Validated by the frequencies and distributions of landslides, nine key factors (lithology, convexity, slope gradient, slope aspect, elevation, soil property, vegetation coverage, flow, and fracture) are selected to construct landslide susceptibility (LS) empirical models by back-propagation artificial neural network method. By integrating landslide empirical models with surface multi-source geospatial and remote sensing data, this paper further performs a large-scale LS assessment throughout China. The resulting landslide hazard assessment map of China clearly illustrates the hot spots of the high landslide potential areas, mostly concentrated in the southwest. The study implements a complete framework of multi-source data collecting, processing, modeling, and synthesizing that fulfills the assessment of LS and provides a theoretical basis and practical guide for predicting and mitigating landslide disasters potentially throughout China.

     

The influence of the inclination of the accretor’s magnetic axis on the accretion-disk structure in intermediate polars

Typical changes of the accretion-disk structures in intermediate polars are studied as a function of the inclination of the accretor’s magnetic field. Thre-dimensional numerical modeling was performed for seven differentmagnetic-axis inclinations. The results showthat the system forms a magnetosphere region, and that column accretion occurs. The action of the magnetic field tilts the inner parts of the disk along the magnetic axis of the accretor. The magnetic-field inclination appreciably influences matter transfer in the disk and accretion processes. Generation of toroidal magnetic field, magnetic braking, and alignment of the dipole magnetic field result in oscillations of the accretion rate. A direct relationship between the field inclination and the oscillation amplitude is found, as well as an inverse relationship between the field inclination and the oscillation period.     

New data on the deep structure of the Northern Vent of the Great Tolbachik Fissure Eruption (1975–1976)

In order to restore the deep structure in the region of the Northern Vent (NV) of the Great Tolbachik Fissure Eruption (GTFE) (1975–1976), low-frequency microseismic sounding was applied. For this purpose accumulation of spectra of the microseismic field was performed in a wide frequency band in 29 points along a linear profile 14 km in length embedded transversely to the fissure eruption. A deep cross section of the Earth’s crust was constructed up to 20 km, reflecting the distribution of relative velocities of transverse seismic waves. The revealed structural heterogeneities were interpreted with consideration of previously known results of complex studies of the eruption. The existence of an abnormal structure at the depths of 2–3 and 7–8 km under the NV GTFE was confirmed, which could be low-depth magma chambers. Deep subvertical low-velocity structures were revealed and spatially registered, which probably feed the conduits of the eruption. It was demonstrated that the ways of possible magma supply to the peripheral chamber at the depth of 2–3 km could be various. For the first time for the zone of areal volcanism, variation of the character of magmatic intrusions was demonstrated at the transition from a crystalline basement to the near-surface depth: subvertical forms are replaced with a system of sills and interesting injections.     

Isotope-geochemical constraints on the formation of the early Earth’ crust

Analysis of isotope-geochemical data obtained for the early crustal complexes of the Earth provided constraints on the formation time, scales of development, and geochemical features of protocrust. Most informative were isotope-geochemical and geochemical data on the oldest zircons with ages up to 4.4 Ga, short-lived ¹⁴⁶Sm/¹⁴²Nd isotope system, and lead isotope composition of the oldest rocks of Greenland. The presence of positive ¹⁴²Nd anomaly in the rocks of West Greenland and negative anomaly in the amphibolites of the oldest Nuvvuagittuq greenstone belt of the Superior province (O’Neil et al., 2008) indicates the early differentiation of the Earth material into depleted mantle and enriched (basaltic) crust (Caro et al., 2006; Benett et al., 2007a, b; O’Neil et al., 2008). Pb-Pb isotopic systematics of the oldest crustal rocks from West Greenland and Labrador testifies that high μ enriched crust (²³⁸U/²⁰⁴Pb = 10.9) of basaltic composition already existed 3.9 Ga ago (Kamber et al., 2003). Based on isotope-geochemical and geochemical features of the oldest zircons in the Late Archean greenstone belts of the Yilgarn block (Western Australia), the crust of intermediate-felsic composition and water on the Earth’s surface already existed 4.4 Ga ago (Wilde et al., 2001).     

Precessional motions of structural blocks of the Earth’s crust

Instrumental observations revealed a new type of motion previously not described in the literature, the precessional motions in the structural blocks of the Earth’s crust. The precession-nutation motions are caused largely by a complex response of a structural block and the adjacent tectonic structure, acting as a discontinuous zone between blocks, to tidal deformation. Irregular precession with a period of about one day complicated by the half a day period nutation defines a complex loading pattern characteristic of the internal structure of faults adjacent to the block.

     

Underthrusting of Tarim beneath the Tien Shan and deep structure of their junction zone: Main results of seismic experiment along MANAS Profile Kashgar-Song-Köl

The results of reflection CMP seismic profiling of the Central Tien Shan in the meridional tract 75–76° E from Lake Song-Köl in Kyrgyzstan to the town of Kashgar in China are considered. The seismic section demonstrating complex heterogeneous structure of the Earth’s crust and reflecting its near-horizontal delamination with vertical and inclined zones of compositional and structural differentiation was constructed from processing of initial data of reflection CMP seismic profiling, earthquake converted-wave method (ECWM), and seismic tomography. The most important is the large zone of underthrusting of the Tarim Massif beneath the Tien Shan.