Analysis of Sea-Ice Areas Undetectable by the ASI Algorithm Based on Satellite Microwave Radiometry in the Arctic Ocean

Izvestiya, Atmospheric and Oceanic Physics - In the period of intense ice melting, algorithms retrieving sea-ice concentration from satellite microwave radiometry (SMR) data may fail to detect vast...     

Estimation of heat flow in Tuva from data on helium isotopes in thermal mineral springs

Concentrations of helium isotopes were measured in gas and water samples from 28 thermal mineral springs in Tuva and adjacent regions of Buryatia and Gorny Altai. It is shown that fluids from 16 springs are rich in mantle helium (4–35%). With regard to the air contamination of the samples, the corrected ratios of helium isotopes (R_cor = ³He/⁴He) in these springs vary from 5.3 × 10^–8 to 422 × 10^–8. Using these R_cor values, we estimated the heat flow; these estimates were then applied to calculate the deep-level temperatures and thickness of thermal lithosphere. According to these parameters, the Tuva region is divided into two parts. Eastern Tuva (from ~96° E to the boundary with Buryatia) is characterized by abnormal helium isotope ratios and heat flow indicating the intense heating of the Earth’s crust in eastern Tuva: At a depth of 50 km, a temperature reaches 1000–1200 °C, and the thickness of thermal lithosphere is reduced to 70–50 km. This testifies to a rift process west (probably, up to 96° E) of the Baikal Rift Zone. In western Tuva, the average heat flow is much lower, ~45–50 mW/m², which is commensurate with that in the Altai–Sayan folded area as a whole. The deep-level temperatures here are twice lower, and the lithosphere thickness increases to 150 km.     

Hydrological structure of the Sheksna River Deep of the Rybinsk Reservoir and water quality assessment by biological parameters

Peculiarities of formation and seasonal dynamics of hydrological structure of the Sheksna River Deep of the Rybinsk Reservoir are considered. The contribution of various genetic water types, including run-offs of the town of Cherepovets, to the formation of its water masses is established on the grounds of model calculations. It is shown, that the hydrological structure effects the distribution of hydrobionts in sewage disposal areas. The assessment of the contribution of Cherepovets run-offs to the change of total water salinity of the deep is given.     

Characteristics of ring seismicity in different depth ranges before large and great earthquakes in the Sumatra region

Characteristics of the seismicity in depth ranges 0–33 and 34–70 km before ten large and great (M _w = 7.0−9.0) earthquakes of 2000–2008 in the Sumatra region are studied, as are those in the seismic gap zones where no large earthquakes have occurred since at least 1935. Ring seismicity structures are revealed in both depth ranges. It is shown that the epicenters of the main seismic events lie, as a rule, close to regions of overlap or in close proximity to “shallow” and “deep” rings. Correlation dependences of ring sizes and threshold earthquakes magnitudes on energy of the main seismic event in the ring seismicity regions are obtained. Identification of ring structures in the seismic gap zones (in the regions of Central and South Sumatra) suggests active processes of large earthquake preparation proceed in the region. The probable magnitudes of imminent seismic events are estimated from the data on the seismicity ring sizes.     

Ring structures of seismicity in central Tien Shan and Dzungaria: Possible precursory processes of large earthquakes

It is shown that episodes of comparative seismic quiescence that lasted about 20–25 years in the areas of study alternated with intervals of sharply increased seismicity as series of large (M ≥ 6.9) earthquakes occurred during two to three decades. Since no M ≥ 6.6 earthquake has occurred in the area for as long as 21 years after the 1992 Susamyr event, middle-term prediction would require identification of zones of imminent large earthquakes. More reliable identification of such zones rests on data relating to inhomogeneities in the field of S-wave attenuation in the lithosphere, as well as on the characteristics of ring structures of seismicity. Such structures are formed as zones of seismic quiescence that are bounded by M ? Mth earthquake epicenters, where Mth is the threshold magnitude value. Correlative relationships were previously derived, lgL(Mw) and Mth(Mw), for events with different focal mechanisms (L is the length of the longer axis of a seismicity ring and Mw is the magnitude of the associated large earthquake). These relationships were used to estimate the Mw of large events that can occur in these ring structures. The greatest earthquake with Mw ? 7.5 is probably about to occur in southern Tien Shan, east of the 1949 Khait earthquake rupture. A smaller event (Mw ～ 7.0) can occur in the Kyrgyz Range area. Still smaller earthquakes probably have their precursory areas north and east of Lake Issyk-Kul, as well as in Dzungaria.     

Crustal electrical conductivity of the Indian continental subduction zone: New data from the profile in the Garhwal Himalaya

We present the results of studying the geoelectrical structure of the zone of continental subduction of the Indian lithospheric plate within the Gahrwal Himalaya. In the framework of the Russian–Indian project, the data of the broadband magnetotelluric soundings conducted by the Indian Institute of Technology Roorkee on the regional profile across the structures of the orogen were expanded, processed, and interpreted by the new program tools adapted for the measurements in the mountain conditions and for the presence of industrial noise. The constructed model of the deep electrical conductivity cross section for Garhwal revealed its two-dimensional (2D) features and more accurately delineated the location of the midcrustal conductor associated with the ramp structure of the detachment plane. The correlations with the regional distribution of the earthquake hypocenters and the seismotomographic images suggest a common, fluid-related nature of the seismic and geoelectrical anomalies in the crust of the Garhwal Tectonic Corridor and enabled the identification of the seismogenerating zones. Among the data of the expanded profile set of magnetotelluric and magnetovariational transfer functions, the response of a poorly explored deep conductive body is revealed. This object is located east of the profile and is probably associated with the activation of the ancient trans-Himalayan cratonic structures which prepares the segmentation of the Himalayan arc.     

Electromagnetic soundings of the sedimentary cover and consolidated crust in the transition zone from the Moscow syneclise to the Voronezh anteclise: Problems and prospects

The methods and results of electromagnetic soundings (EMS) performed in the transition zone from the Moscow syneclise to the Voronezh anteclise in the vicinity of the MSU geophysical base are considered. This base is located in the village of Aleksandrovka in the Yukhnov district of Kaluga area. The composite EMS curves characterizing rock complexes composing the sedimentary cover are constructed, and changes in these complexes within the specified transition zone are traced. The standard curves of magnetotelluric (MT) and magnetovariational (MV) soundings are constructed from the results of long-term measurements at the ALX observation point located at the Moscow State University’s (MSU) geophysical base. The maps of thickness and total longitudinal conductance of the sedimentary cover are constructed from the results of interpretation of MT data obtained in the region. A conductor in the consolidated Earth’s crust is identified within the Voronezh anteclise. Prospects for further investigations of the region are associated with the tracing of the crustal conductor within the Voronezh anteclise, as well as with the organization of an observatory at the MSU’s geophysical base in order to perform long-term measurements of the electromagnetic (EM) and other geophysical fields.     

Geoelectric section of the Central Tien Shan: Analysis of magnetotelluric and magnetovariational responses along the Naryn geotraverse

During the past two decades, at the Research station (Bishkek) more than a hundred magnetotelluric and magnetovariational soundings were carried out on the Naryn geotraverse that intersects the Tien Shan region from Lake Balkhash to the Tarim Basin along the 76° E meridian. Integration and complex interpretation of the data of these soundings with improved resolution and reliability of the geoelectric model of the Central Tien Shan section became an urgent challenge. Our paper presents a complex of methods for processing and invariant analysis of the electromagnetic data developed for the solution of this problem. Its application allowed us to validate the choice of the 2D interpretation model for the Naryn Line and to form the adequate ensemble of the data to be inverted. The developed approaches will also be useful in similar studies in the other mountain regions.