scholarly journals Anomalous azimuthal variations with 360° periodicity of Rayleigh phase velocities observed in Scandinavia

2020 ◽  
Vol 224 (3) ◽  
pp. 1684-1704
Author(s):  
Alexandra Mauerberger ◽  
Valérie Maupin ◽  
Ólafur Gudmundsson ◽  
Frederik Tilmann
Keyword(s):  
Seismic Anisotropy ◽  
Broad Band ◽  
Shear Velocity ◽  
Velocity Variation ◽  
The South ◽  
Study Region ◽  
Lithospheric Thickness ◽  
Phase Velocities ◽  
The North ◽  
Southern Norway

SUMMARY We use the recently deployed ScanArray network of broad-band stations covering most of Norway and Sweden as well as parts of Finland to analyse the propagation of Rayleigh waves in Scandinavia. Applying an array beamforming technique to teleseismic records from ScanArray and permanent stations in the study region, in total 159 stations with a typical station distance of about 70 km, we obtain phase velocities for three subregions, which collectively cover most of Scandinavia (excluding southern Norway). The average phase dispersion curves are similar for all three subregions. They resemble the dispersion previously observed for the South Baltic craton and are about 1 per cent slower than the North Baltic shield phase velocities for periods between 40 and 80 s. However, a remarkable sin(1θ) phase velocity variation with azimuth is observed for periods >35 s with a 5 per cent deviation between the maximum and minimum velocities, more than the overall lateral variation in average velocity. Such a variation, which is incompatible with seismic anisotropy, occurs in northern Scandinavia and southern Norway/Sweden but not in the central study area. The maximum and minimum velocities were measured for backazimuths of 120° and 300°, respectively. These directions are perpendicular to a step in the lithosphere–asthenosphere boundary (LAB) inferred by previous studies in southern Norway/Sweden, suggesting a relation to large lithospheric heterogeneity. In order to test this hypothesis, we carried out 2-D full-waveform modeling of Rayleigh wave propagation in synthetic models which incorporate a steep gradient in the LAB in combination with a pronounced reduction in the shear velocity below the LAB. This setup reproduces the observations qualitatively, and results in higher phase velocities for propagation in the direction of shallowing LAB, and lower ones for propagation in the direction of deepening LAB, probably due to the interference of forward scattered and reflected surface wave energy with the fundamental mode. Therefore, the reduction in lithospheric thickness towards southern Norway in the south, and towards the Atlantic ocean in the north provide a plausible explanation for the observed azimuthal variations.

Scandinavian Lithosphere Structure derived from Surface Waves and Ambient Noise

2020 ◽  
Author(s):  
Alexandra Mauerberger ◽  
Valerie Maupin ◽  
Hamzeh Sadeghisorkhani ◽  
Olafur Gudmundsson ◽  
Frederik Tilmann
Keyword(s):  
Surface Wave ◽  
Phase Velocity ◽  
Surface Waves ◽  
Ambient Noise ◽  
Velocity Variation ◽  
Western Coast ◽  
Study Region ◽  
Phase Velocities ◽  
The North ◽  
Southern Norway

<p>The Scandinavian mountain chain runs approximately parallel to the western coast of Norway with topography up to 2500 m. Since this region lacks recent compressional tectonic forces, we can study the geodynamic evolution of crustal and upper mantle structures which were once participating in continental collision and are now deeply eroded. Together with the ScanArray network we use data from previous and permanent projects, in total more >220 stations, for a surface wave tomography of entire Scandinavia using both earthquake and ambient noise data.</p><p>Initially, we performed a beamforming of Rayleigh surface waves which yielded average phase velocities for the study region and several of its sub-regions. However, a remarkable sin(1Θ) phase velocity variation with azimuth is observed in northern Scandinavia and southern Norway/Sweden but not in the central study area. For periods >35 s a 5% deviation between the maximum and minimum velocities was measured for opposite backazimuths of 120° and 300°, respectively. Such a variation is incompatible with azimuthal anisotropy or weak heterogeneity and might be caused by an eastward dipping lithosphere-asthenosphere boundary (LAB), as is implied by the observations of low shallow velocities below southern Norway in previous studies.</p><p>In order to test this hypothesis, we carried out 2D full-waveform modeling of the Rayleigh wave propagation in a model with a steep gradient in the LAB in combination with a pronounced reduction in the shear velocity below the LAB. This setup resulted in faster phase velocities for propagation in the direction of shallowing LAB, and slower ones for propagation in the direction of deepening LAB, consistent with the observation. This effect is probably due to the interference of reflected surface wave energy.</p><p>From this observed azimuthal bias, we demonstrate that an isotropic distribution of earthquakes is vital for the tomography results, otherwise significant velocity artefacts occur.</p><p>Phase velocity maps were derived with the two plane wave method. We merge those ballistic surface wave observations at longer periods with tomographic maps constructed from inter-station phase velocities measured on ambient noise stacks. Finally, we use a 1D transdimensional Bayesian method to invert the merged phase dispersion curves at each grid point for the V<sub>SV</sub> structure. Below the entire mountain belt a crustal root is absent consistent with previous studies. The Lofoten peninsula shows very low crustal and lithospheric V<sub>SV</sub> with a shallowing Moho towards the continental margin. The LAB is deepening from west to east with a sharp step both in the South (120 km depth) and the North (150 km depth). A high-velocity spot above the LAB in the North can be related to a gravity anomaly. The central area shows rather smooth varying structures from west to east. Additionally, we find low-velocity areas below 150 km depth beneath the Paleoproterozoic Baltic Shield in northern Finland. The sharp gradients in the LAB imaged in southern and northern Scandinavia are consistent with our sin(1Θ) phase velocity variation with azimuth whereas the smoother velocity structure in the central study area explains the absence of 1Θ phase velocity variations there.</p>

On the Chronological Order of Deposition of the Highland Schists

1930 ◽  
Vol 67 (2) ◽  
pp. 68-73 ◽  
Author(s):  
Thorolf Vogt
Keyword(s):  
Broad Band ◽  
Difficult Problem ◽  
Chronological Order ◽  
The South ◽  
Scottish Highlands ◽  
North East ◽  
The North ◽  
South West ◽  
Mountain Chain

As is well known, the chronological order of deposition of the sediments of that broad band of schists which extends across Scotland from Banffshire and Aberdeenshire in the north-east to Argyllshire in the south-west is a most difficult problem to determine. The noteworthy development that has been made during recent years, leading to a more intimate understanding of the structure of the Southern Highlands, has made this problem much more vital. The question as to the top and bottom of these beds has been much discussed in recent years, and it can not be denied that views have been widely divergent on this matter. When a foreign geologist desires to call attention (through this paper) to certain conditions of fact, it may be pointed out that he is carrying on similar studies of another section—the Norwegian—of this same mountain chain, where one meets similar problems as in the Scottish Highlands.

scholarly journals Monitoring the Spatiotemporal Difference in Glacier Elevation on Bogda Mountain from 2000 to 2017

2021 ◽  
Vol 18 (12) ◽  
pp. 6374
Author(s):  
Weibing Du ◽  
Ningke Shi ◽  
Linjuan Xu ◽  
Shiqiong Zhang ◽  
Dandan Ma ◽  
...  
Keyword(s):  
Water Supply ◽  
Radar Data ◽  
Surface Elevation ◽  
North Slope ◽  
The South ◽  
Glacier Surface ◽  
Study Region ◽  
The North ◽  
South Slope ◽  
The Difference

The difference in glacier surface elevation is a sensitive indicator of climate change and is also important for disaster warning and water supply. In this paper, 25 glaciers on Bogda Mountain, in the eastern Tianshan Mountains, are selected as the study object as they are typical of glaciers in arid or semi-arid areas with importance for water supply. The Repeat Orbit Interferometry (ROI) method is used to survey the surface elevation of these glaciers using Sentinel-1A Radar data from 2017. Using data from the Shuttle Radar Topography Mission (SRTM) and a Digital Elevation Model (DEM), the difference in the glacier surface elevation between 2000 and 2017 is obtained. A scheme to evaluate the accuracy of estimated variations in glacier surface elevation is proposed in this article. By considering the surfaces of lakes in the study region as ideal horizontal planes, the average standard deviation (SD) value of the lake elevation is taken as the error caused by the radar sensor and observing conditions. The SD of the lake elevation is used as an index to evaluate the error in the estimated variation of the glacier surface elevation, and the obtained SD values indicate that the result obtained using the ROI method is reliable. Additionally, the glacier surface elevation variation pattern and a Logarithmic Fitting Model (LFM) are used to reduce the error in high-altitude glacial accumulation areas to improve the estimation of the difference in the glacier surface elevation obtained using ROI. The average SD of the elevation of the 12 lakes is ±2.87 m, which shows that the obtained glacier surface elevations are reliable. This article concludes that, between 2000 and 2017, the surface elevation of glaciers on Bogda Mountain decreased by an average of 11.6 ± 1.3 m, corresponding to an average decrease rate of 0.68 m/a, and glaciers volume decreased by an average of 0.504 km3. Meanwhile, the surface elevations of the lakes increased by an average of 8.16 m. The decrease of glacier surface elevation leads to the expansion of glacial lakes. From the north slope clockwise to the south slope, the glacier elevation variation showed a decreasing trend, and the elevation variation gradually increased from the south slope to the north slope. With the increase of glacier altitude, the variation of glacier surface elevation gradually changed from negative to positive. The findings of this article suggest that the rate of glacier retreat on Bogda Mountain increased from 2000 to 2017.

Detailed Investigation of Seismic Anisotropy in the Upper Mantle of the Northern Aegean Region

2020 ◽  
Author(s):  
Ceyhun Erman ◽  
Seda Yolsal-Çevikbilen ◽  
Tuna Eken ◽  
Tuncay Taymaz
Keyword(s):  
Upper Mantle ◽  
Seismic Anisotropy ◽  
Broad Band ◽  
Mantle Flow ◽  
Data Set ◽  
Study Region ◽  
Waveform Data ◽  
Seismic Stations ◽  
Aegean Region ◽  
Fast Polarization

<p>Seismic anisotropy studies can provide important constraints on geodynamic processes and deformation styles in the upper mantle of tectonically active regions. Seismic anisotropy parameters (e.g. delay time and fast polarization direction) can give hints at the past and recent deformations and can be most conventionally obtained through core-mantle refracted SKS phase splitting measurements. In order to explore the complexity of anisotropic structures in the upper mantle of a large part of the Aegean region, in this study, we estimate splitting parameters beneath 25 broad-band seismic stations located at NW Anatolia, North Aegean Sea and Greece mainland. To achieve this we employ both transverse energy minimization and eigenvalue methods. Waveform data of selected earthquakes (with M<sub>w</sub> ≥ 5.5; 2008-2018 and with epicentral distances between 85°–120°) were retrieved from Earthquake Data Center System of Turkey (AFAD; http://tdvm.afad.gov.tr/) and European Integrated Data Archive (EIDA; http://orfeus-eu.org/webdc3/). A quite large data set, the majority of which have not been studied before, were evaluated in order to estimate reliable non-null and null results. In general, station-averaged splitting parameters mainly exhibit the NE-SW directed fast polarization directions throughout the study area. These directions can be explained by the lattice-preferred orientation of olivine minerals in the upper mantle induced by the mantle flow related to the roll-back process of the Hellenic slab. We further observe that station-averaged splitting time delays are prone to decrease from north to south of the Aegean region probably changing geometry of mantle wedge with a strong effect on  the nature of mantle flow along this direction. The uniform distribution of splitting parameters as a function of back-azimuths of earthquakes refers to a single-layer horizontal anisotropy for the most part of the study area. However, back azimuthal variations of splitting parameters beneath most of northerly located seismic stations (e.g., GELI, SMTH etc.) imply the presence of a double-layer anisotropy. To evaluate this, we performed various synthetic tests especially beneath the northern part of study region. Yet, it still remains controversial issue due to the large azimuthal gap and thus requires further modelling which may involve the use of joint data sets.</p>

scholarly journals Association between anomalies of moisture flux and extreme runoff events in the south-eastern Alps

2011 ◽  
Vol 11 (3) ◽  
pp. 915-920 ◽  
Author(s):  
M. Müller ◽  
M. Kaspar
Keyword(s):  
Moisture Flux ◽  
Eastern Alps ◽  
The South ◽  
The West ◽  
Study Region ◽  
The North ◽  
Sava River ◽  
Moisture Fluxes ◽  
Annual Distribution ◽  
Runoff Events

Abstract. The aims of the paper are (i) to describe the annual distribution of extreme runoff events on the Mura, Drava and Sava Rivers, (ii) to demonstrate their association with moisture fluxes, and (iii) to explain their annual distribution by moisture flux climatology. Extreme runoff events were defined as rapid increases in daily mean discharge. Moisture flux anomalies were studied within six pixels of the ERA-40 database around the studied region. In general, extreme runoff events were concentrated in the summer and autumn and were usually associated with anomalies in moisture flux, mainly from the south. Nevertheless, while southern and western moisture fluxes were typical of Sava River events that occurred mainly in autumn, summer events prevailed on the Mura River and were frequently associated with moisture fluxes from the east or the north. It is remarkable that moisture fluxes from the west and south have their maxima in the autumn, whereas those from the east and north have their maxima in the summer. Therefore, the climatology of moisture flux seems to be one of the major reasons for the annual distribution of extreme runoff events in the study region. This result should be confirmed in other regions in the future.

scholarly journals The influence of night length: Activity of the northern bat Eptesicus nilssonii under conditions of continuous light in midnight sun compared to a southern population

BMC Zoology ◽  
2021 ◽  
Vol 6 (1) ◽  
Author(s):  
Karl Frafjord
Keyword(s):  
Continuous Light ◽  
Summer Season ◽  
Infrared Detection ◽  
The Arctic ◽  
Southern Population ◽  
The South ◽  
Active Season ◽  
The North ◽  
Light Levels ◽  
Southern Norway

Abstract Background Nearly all insectivorous bats (Chiroptera) are strictly nocturnal, flying and feeding only between sunset and sunrise despite lower insect availability than by day, most likely to avoid predation by diurnal birds. This may represent a great challenge to bats living north of the Arctic Circle, which are exposed to bright nights in the period of the midnight sun. The northern bat Eptesicus nilssonii was studied at different latitudes in Norway (69, 66 and 58°N) by three techniques; visual counts of exits from and returns to roosts, infrared detection with a datalogger and an ultrasound data recorder, to reveal how their activity varied across latitude, season, and night, as well as across light levels. How does a nocturnal bat adjust to perpetual light and what light levels are tolerated? Results In the north the bats’ active season lasted 2.5 months, 1.5 months shorter than in the south. The bats only flew in 3-4 weeks of midnight sun, and hardly ever left the roost until the sun went behind a hill in the evening. In addition, the timing of their nightly hunting was highly influenced by the darkness of the sky, and they very rarely flew in light levels above 200 foot-candles (FC). As the night became darker than twilight from early August, the bats restricted their activity to between sunset and sunrise. This was the normal situation in southern Norway, where the bats tracked sunset and sunrise throughout the entire season. Those bats appeared to prefer light levels below 100-50 FC and hence, also did fly in twilight conditions. Conclusions The willingness to fly in twilight by the southern population may be a prerequisite to the northern bat’s survival in the land of the midnight sun. These bats must accept short nights in the first part of their summer season and must be willing to fly in light levels 2-4 times higher than in the south. Most likely, this depends on a reduced predation risk and good abundance of insects at night.

Long-term Cyclic Variations of Prominences, Green and Red Coronae over Solar Cycles

2000 ◽  
Vol 179 ◽  
pp. 201-204
Author(s):  
Vojtech Rušin ◽  
Milan Minarovjech ◽  
Milan Rybanský
Keyword(s):  
Emission Lines ◽  
High Latitudes ◽  
Green Line ◽  
Solar Cycles ◽  
The South ◽  
Coronal Emission ◽  
Local Maxima ◽  
The North ◽  
Cyclic Variations

AbstractLong-term cyclic variations in the distribution of prominences and intensities of green (530.3 nm) and red (637.4 nm) coronal emission lines over solar cycles 18–23 are presented. Polar prominence branches will reach the poles at different epochs in cycle 23: the north branch at the beginning in 2002 and the south branch a year later (2003), respectively. The local maxima of intensities in the green line show both poleward- and equatorward-migrating branches. The poleward branches will reach the poles around cycle maxima like prominences, while the equatorward branches show a duration of 18 years and will end in cycle minima (2007). The red corona shows mostly equatorward branches. The possibility that these branches begin to develop at high latitudes in the preceding cycles cannot be excluded.

US and Russian policies and strategies in the Middle East: Iraq and Syria as a model

2019 ◽  
pp. 220
Author(s):  
Esraa Aladdin Noori ◽  
Nasser Zain AlAbidine Ahmed
Keyword(s):  
Middle East ◽  
East Asia ◽  
Balance Of Power ◽  
Western World ◽  
International Conflicts ◽  
The South ◽  
The West ◽  
The North ◽  
East Region ◽  
Middle East Region

The Russian-American relations have undergone many stages of conflict and competition over cooperation that have left their mark on the international balance of power in the Middle East. The Iraqi and Syrian crises are a detailed development in the Middle East region. The Middle East region has allowed some regional and international conflicts to intensify, with the expansion of the geopolitical circle, which, if applied strategically to the Middle East region, covers the area between Afghanistan and East Asia, From the north to the Maghreb to the west and to the Sudan and the Greater Sahara to the south, its strategic importance will seem clear. It is the main lifeline of the Western world.

Depositional Environment Drive as Overpressure Generation: Study Case in “Gap” Field, North West Java Basin

2020 ◽  
Author(s):  
A., C. Prasetyo
Keyword(s):  
Clay Mineral ◽  
Depositional Environment ◽  
Mineral Content ◽  
Hydrocarbon Generation ◽  
Well Test ◽  
Burial History ◽  
The South ◽  
North West ◽  
The North ◽  
Geological Processes

Overpressure existence represents a geological hazard; therefore, an accurate pore pressure prediction is critical for well planning and drilling procedures, etc. Overpressure is a geological phenomenon usually generated by two mechanisms, loading (disequilibrium compaction) and unloading mechanisms (diagenesis and hydrocarbon generation) and they are all geological processes. This research was conducted based on analytical and descriptive methods integrated with well data including wireline log, laboratory test and well test data. This research was conducted based on quantitative estimate of pore pressures using the Eaton Method. The stages are determining shale intervals with GR logs, calculating vertical stress/overburden stress values, determining normal compaction trends, making cross plots of sonic logs against density logs, calculating geothermal gradients, analyzing hydrocarbon maturity, and calculating sedimentation rates with burial history. The research conducted an analysis method on the distribution of clay mineral composition to determine depositional environment and its relationship to overpressure. The wells include GAP-01, GAP-02, GAP-03, and GAP-04 which has an overpressure zone range at depth 8501-10988 ft. The pressure value within the 4 wells has a range between 4358-7451 Psi. Overpressure mechanism in the GAP field is caused by non-loading mechanism (clay mineral diagenesis and hydrocarbon maturation). Overpressure distribution is controlled by its stratigraphy. Therefore, it is possible overpressure is spread quite broadly, especially in the low morphology of the “GAP” Field. This relates to the delta depositional environment with thick shale. Based on clay minerals distribution, the northern part (GAP 02 & 03) has more clay mineral content compared to the south and this can be interpreted increasingly towards sea (low energy regime) and facies turned into pro-delta. Overpressure might be found shallower in the north than the south due to higher clay mineral content present to the north.

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