scholarly journals 2.5 dimensional model of mantle heterogeneities under the Ukrainian shield according to the gradients of the velocities of seismic waves

2020 ◽  
Vol 29 (2) ◽  
pp. 431-441
Author(s):  
Liudmyla O. Shumlianska ◽  
Yurii I. Dubovenko ◽  
Petro H. Pigulevskyy
Keyword(s):  
Velocity Gradient ◽  
Upper Mantle ◽  
Seismic Waves ◽  
Deep Structure ◽  
Ukrainian Shield ◽  
Depth Range ◽  
Tectonic Structures ◽  
P Waves ◽  
Useful Signal ◽  
Background Density

We analyze the basic techniques for the investigation of the deep structure of the mantle and the shortcomings of the models of mantle structures derived from them. Thus, we reveal that there is no analysis of the velocity field by means of analytical transformants. Therefore, we developed and tested a new approach to define the mantle boundaries based on the calculations of the sequence of P-waves velocity derivatives. As a result, we obtain some new set of velocity gradient distributions for the principal tectonic structures of the Ukrainian Shield along the composite profile. The boundaries of the mantle discontinuities according to the velocity gradient we define in a special manner to eliminate the false anomalies and the fluctuations of the velocity curves that occur due to the conversion of the hodograph into the mean velocities. The smoothing of the velocity curve we perform with a previously defined wavelength step being equal to 50 km. We treat the calculated velocity gradient anomalies as the useful signal response above the appropriate sections, which have different velocity accelerations levels inside the upper mantle. We assume that the mantle anomalies have the same physical background (density/viscosity distributions, temperature gradients etc.) within each range with the equal acceleration value. However, the singular points determined by the inflections of the gradient curve could be the possible boundaries of additional inhomogeneities within the mantle. We calculate both the 1st and the 2nd derivatives for the velocity curves obtained. The excesses 2.5-D model of the 1-th and 2-th gradient curves (the acceleration of the gradvp itself) determine the position of the max / min anomalies of gradvp at the consolidated seismic profile within the Ukrainian Shield. Finally, we analyze in detail the distribution of velocity gradients of P-waves within the upper mantle in the depth range of 50–750 km. It results in the identification of a series of additional gradient velocity boundaries within three principal structural horizons of the upper mantle (under ~ 200–300 km, ~ 410–500 km, and ~ 600–650 km respectively).

scholarly journals CRUST AND UPPER MANTLE IN THE ZONE OF JUNCTION BETWEEN THE CENTRAL ASIAN AND PACIFIC FOLD BELTS

2021 ◽  
Vol 40 (5) ◽  
pp. 16-32
Author(s):  
A.M. Petrishchevsky ◽  
Keyword(s):  
Upper Mantle ◽  
Deep Structure ◽  
Sedimentary Basins ◽  
Density Contrast ◽  
Tectonic Structures ◽  
Crust And Upper Mantle ◽  
North East ◽  
The North ◽  
The Pacific ◽  
Amurian Plate

Spatial distributions of gravity sources and density contrast of geological media, which is reflected by the values of parameter μz , into the crust and upper mantle of Northeast China are analyzed. Features of rheological layering of the tectonosphere and deep spatial relationships of tectonic structures (cratonic blocks, marginal terranes, and sedimentary basins) are defined. In the density contrast distributions the formal signs of Paleozoic subduction of the North-China Craton and Mesozoic subduction of the Pacific Plate under the Amurian Plate were revealed. Crustal deformations are in sharp contrast with upper mantle deformations in structural planes resulting from different directions of tectonic stresses in the Paleozoic and Mesozoic. Thrusting of marginal terranes (Jamusi, Khanka) over the Amurian Plate lithosphere is revealed. Rheology and deep structure of North East China bear many similarities to other regions of the Pacific western margin in Asia and Australia.

Travel-time curves for a low-velocity channel in the upper mantle

1964 ◽  
Vol 54 (6A) ◽  
pp. 1981-1996 ◽  
Author(s):  
John Dowling ◽  
Otto Nuttli
Keyword(s):  
Velocity Gradient ◽  
Upper Mantle ◽  
Body Wave ◽  
Shadow Zone ◽  
Low Velocity ◽  
P Waves ◽  
Underground Nuclear Explosions ◽  
Nuclear Explosions ◽  
Time Distance ◽  
Velocity Channel

abstract Velocities within the earth can be determined from body wave time-distance (T-D) data by the Herglotz-Wiechert method provided the velocity does not decrease too rapidly with depth. Until the present time, the properties of T-D curves for rapid decreases of velocity with depth have been considered only qualitatively. This paper presents a technique for calculating a T-D curve for any velocity distribution, including continuous and discontinuous increases and decreases of velocity with depth. Some properties of T-D curves are quantitatively studied by systematically varying the characteristics of a single model and noting the corresponding variations in the calculated T-D curves. From this it is concluded that a significant low-velocity channel may not be evidenced by a shadow zone but rather by an overlapping of two distinct branches of the T-D curve. It is further concluded that the presence of a shadow zone implies a very gentle velocity gradient below the low-velocity channel. By fitting a calculated T-D curve to observed data one can determine velocity as a function of depth even when the velocity decreases rapidly with depth, when a low-velocity channel exists. Observed T-D data for two underground nuclear explosions (gnome and bilby) measured in four different azimuths were fitted with T-D curves calculated for assumed velocity distributions. It is concluded that these data can be satisfied by a low-velocity channel for P waves in the upper mantle. The character of this channel (depth, thickness and velocity) was determined in each azimuth. The depth to its top was shallow (70 ± km) in the western U.S. and deep (125 ± km) in the eastern U.S. The velocity gradient below the channel is sharp enough to produce no prominent shadow zones. There are significant lateral changes in upper mantle velocities in the western U. S.

Azimuth and slowness anomalies of seismic waves measured on the central California seismographic array. Part II. Interpretation

1966 ◽  
Vol 56 (3) ◽  
pp. 655-675 ◽  
Author(s):  
Michio Otsuka
Keyword(s):  
Structural Feature ◽  
Upper Mantle ◽  
Seismic Waves ◽  
Alternative Hypothesis ◽  
P Waves ◽  
Central California ◽  
Structural Trend ◽  
Surface Examination ◽  
Mohorovičić Discontinuity ◽  
Velocity Decrease

abstract An interpretation is given here of the azimuth and slowness anomalies for P waves measured on the seven northern stations in the telemetry seismographic array in central California which was described in Part I of this paper. The array provides a new method of investigating the structure of the upper mantle in an important oceanic-to-continental transition region. The structural feature which is primarily responsible for these anomalies is apparently located in the upper mantle within a depth of order 100 km of the surface. Examination shows that if these anomalies are attributed to a thickening crust alone, a dip of the Mohorovičić discontinuity as steep as 20° must be assumed. Further analysis suggests a solution having a moderately dipping Mohorovičić discontinuity below which a second, deeper interface has a velocity decrease downward and a steep dip toward the ocean. An alternative hypothesis to that of Press and Biehler is formulated which explains traveltime anomalies of certain California stations in terms of a regional structural trend.

scholarly journals Deep structure of South Kamchatka according to geophysical data

2018 ◽  
Vol 9 (4) ◽  
pp. 1147-1161
Author(s):  
Yu. F. Moroz ◽  
L. I. Gontovaya
Keyword(s):  
Electrical Conductivity ◽  
Upper Mantle ◽  
Deep Structure ◽  
Geophysical Data ◽  
P Wave ◽  
P Waves ◽  
Crust And Upper Mantle ◽  
Common Features ◽  
Deep Faults ◽  
North And South

Differences have been discovered in the deep structures of North and South Kamchatka, which raises a question about a boundary between these regions. This problem has been studied on the basis of the seismologic, inverse seismic tomographic (P-waves) and geoelectrical data obtained in recent years, as well as the information on magnetometry, gravimetry, tectonics and magmatism of the study area. Comprehensive analysis of the geological and geophysical data shows differences in the structure of the crust and upper mantle in North and South Kamchatka and common features of the deep structures of South Kamchatka and the Kuril Islands. A presumed boundary between North and South Kamchatka is related to the zone of transverse deep faults crossing the peninsula. It is evidenced by P-wave velocity anomalies at different levels of the lithosphere. This fault zone is associated with a change in the geometry and strike of the high-velocity focal layer, and reflected in the modern tectonic plan as the Petropavlovsk-Malkinsky zone of transverse dislocations. In this zone, the Avacha-Koryak group of modern volcanoes is also NW-oriented. We propose a geoelectric model showing the depths along the profile constructed across the zone. In its deep part, the model includes sub-vertical anomalies of increased electrical conductivity. The anomalies are related to deep faults. Increased electrical conductivity may be due to the presence of magmatic melts feeding volcanoes. The results obtained in our study give evidence of the common features of the deep structures of South Kamchatka and the Kuril island arc and demonstrate the differences between the deep structures of North and South Kamchatka, being separated by the zone of faults, which may penetrate into the upper mantle. It is suggested that the identified features in the South Kamchatka structure are due to deep processes taking place not only at the side of the Pacific Ocean, but also at the southern margins of the Sea of Okhotsk. These findings are of interest for geodynamics, volcanology, tectonics and other Earth sciences.

A periodic seismic isolation foundation with an extremely broad low-frequency attenuation zone: Theoretical analysis and experimental verification

2021 ◽  
pp. 136943322110646
Author(s):  
Peng Zhou ◽  
Shui Wan ◽  
Xiao Wang ◽  
Yingbo Zhu ◽  
Muyun Huang
Keyword(s):  
Seismic Isolation ◽  
Seismic Waves ◽  
Periodic Structures ◽  
Finite Size ◽  
Low Frequency ◽  
Bridge Structure ◽  
Engineering Structures ◽  
P Waves ◽  
New Type ◽  
Dominant Frequencies

The attenuation zones (AZs) of periodic structures can be used for seismic isolation design. To cover the dominant frequencies of more seismic waves, this paper proposes a new type of periodic isolation foundation (PIF) with an extremely wide low-frequency AZ of 3.31 Hz–17.01 Hz composed of optimized unit A with a wide AZ and optimized unit B with a low-frequency AZ. The two kinds of optimized units are obtained by topology optimization on the smallest periodic unit with the coupled finite element-genetic algorithm (GA) methodology. The transmission spectra of shear waves and P-waves through the proposed PIF of finite size are calculated, and the results show that the AZ of the PIF is approximately the superposition of the AZs of the two kinds of optimized units. Additionally, shake tests on a scale PIF specimen are performed to verify the attenuation performance for elastic waves within the designed AZs. Furthermore, numerical simulations show that the acceleration responses of the bridge structure with the proposed PIF are attenuated significantly compared to those with a concrete foundation under the action of different seismic waves. Therefore, the newly proposed PIF is a promising option for the reduction of seismic effects in engineering structures.

Reflections from discontinuities beneath antarctica

1971 ◽  
Vol 61 (5) ◽  
pp. 1441-1451
Author(s):  
R. D. Adams
Keyword(s):  
North American ◽  
Upper Mantle ◽  
Deep Structure ◽  
Low Velocity ◽  
Crust And Upper Mantle ◽  
Nuclear Explosions ◽  
Novaya Zemlya ◽  
The Earth ◽  
Velocity Channel ◽  
Upper Boundary

abstract Early reflections of the phase P′P′ recorded at North American seismograph stations from nuclear explosions in Novaya Zemlya are used to examine the crust and upper mantle beneath a region of eastern Antarctica. Many reflections are observed from depths less than 120 km, indicating considerable inhomogeneity at these depths in the Earth. No regular horizons were found throughout the area, but some correlation was observed among reflections at closely-spaced stations, and, at many stations, reflections were observed from depths of between 60 and 80 km, corresponding to a likely upper boundary of the low-velocity channel. Deeper reflections were found at depths of near 420 and 650 km. The latter boundary was particularly well-observed and appears to be sharply defined at a depth that is constant to within a few kilometers. The boundary at 420 km is not so well defined by reflections of P′P′, but reflects well longer-period PP waves, arriving at wider angles of incidence. This boundary appears to be at least as pronounced, but not so sharp as that near 650 km. The deep structure beneath Antarctica presents no obvious difference from that beneath other continental areas.

Diffraction of elastic waves by a fluid-filled crack in a fluid-saturated poroelastic half-space

2021 ◽  
Author(s):  
Zhongxian Liu ◽  
Jiaqiao Liu ◽  
Sibo Meng ◽  
Xiaojian Sun
Keyword(s):  
Half Space ◽  
Seismic Waves ◽  
Incident Angle ◽  
Excitation Frequency ◽  
Vertical Displacement ◽  
Crack Model ◽  
P Waves ◽  
Amplification Effect ◽  
Resonance Frequencies ◽  
Angle Crack

Summary An indirect boundary element method (IBEM) is developed to model the two-dimensional (2D) diffraction of seismic waves by a fluid-filled crack in a fluid-saturated poroelastic half-space, using Green's functions computed considering the distributed loads, flow, and fluid characteristics. The influence of the fluid-filled crack on the diffraction characteristics is investigated by analyzing key parameters, such as the excitation frequency, incident angle, crack width and depth, and medium porosity. The results for the fluid-filled crack model are compared to those for the fluid-free crack model under the same conditions. The numerical results demonstrate that the fluid-filled crack has a significant amplification effect on the surface displacements, and that the effect of the depth of the fluid-filled crack is more complex compared to the influence of other parameters. The resonance diffraction generates an amplification effect in the case of normally incident P waves. Furthermore, the horizontal and vertical displacement amplitudes reach 4.2 and 14.1, respectively. In the corresponding case of the fluid-free crack, the vertical displacement amplitude is only equal to 4.1, indicating the amplification effect of the fluid in the crack. Conversely, for normally incident SV waves at certain resonance frequencies, the displacement amplitudes above a fluid-filled crack may be lower than the displacement amplitudes observed in the corresponding case of a fluid-free crack.

Evidence of tectonic release from underground nuclear explosions in long-period P waves

1983 ◽  
Vol 73 (2) ◽  
pp. 593-613
Author(s):  
Terry C. Wallace ◽  
Donald V. Helmberger ◽  
Gladys R. Engen
Keyword(s):  
Upper Mantle ◽  
High Stress ◽  
Test Site ◽  
Strike Slip ◽  
Body Waves ◽  
Release Time ◽  
P Waves ◽  
Underground Nuclear Explosions ◽  
Nuclear Explosions ◽  
Long Period

abstract In this paper, we study the long-period body waves at regional and upper mantle distances from large underground nuclear explosions at Pahute Mesa, Nevada Test Site. A comparison of the seismic records from neighboring explosions shows that the more recent events have much simpler waveforms than those of the earlier events. In fact, many of the early events produced waveforms which are very similar to those produced by shallow, moderate-size, strike-slip earthquakes; the phase sP is particularly obvious. The waveforms of these explosions can be modeled by assuming that the explosion is accompanied by tectonic release represented by a double couple. A clear example of this phenomenon is provided by a comparison of GREELEY (1966) and KASSERI (1975). These events are of similar yields and were detonated within 2 km of each other. The GREELEY records can be matched by simply adding synthetic waveforms appropriate for a shallow strike-slip earthquake to the KASSERI observations. The tectonic release for GREELEY has a moment of 5 ՠ1024 dyne-cm and is striking approximately 340°. The identification of the sP phase at upper mantle distances indicates that the source depth is 4 km or less. The tectonic release time function has a short duration (less than 1 sec). A comparison of these results with well-studied strike-slip earthquakes on the west coast and eastern Nevada indicate that, if tectonic release is triggered fault motion, then the tectonic release is relatively high stress drop, on the order of several hundred bars. It is possible to reduce these stress drops by a factor of 2 if the tectonic release is a driven fault; i.e., rupturing with the P velocity. The region in which the stress is released for a megaton event has a radius of about 4 km. Pahute Mesa events which are detonated within this radius of a previous explosion have a substantially reduced tectonic release.

Simplified Criteria to Select Ground Response Analysis Methods for Seismic Building Design: Equivalent Linear versus Nonlinear Approaches

2021 ◽  
Author(s):  
Mauro Aimar ◽  
Sebastiano Foti
Keyword(s):  
Seismic Waves ◽  
Site Response ◽  
Building Design ◽  
Soil Conditions ◽  
Response Analysis ◽  
Depth Range ◽  
Ground Response Analysis ◽  
Equivalent Linear ◽  
Acceleration Time Histories ◽  
Soil Deposits

ABSTRACT The possible amplification of seismic waves in soil deposits is crucial for the seismic design of buildings and geotechnical systems. The most common approaches for the numerical simulation of seismic site response are the equivalent linear (EQL) and the nonlinear (NL). Even though their advantages and limitations have been investigated in several studies, the relative field of applicability is still under debate. This study tested both methods over a wide population of soil models, which were subjected to a set of acceleration time histories recorded from strong earthquakes. A thorough comparison of the results of the EQL and the NL approaches was carried out, to identify the conditions in which the relative differences are significant. This assessment allowed for the definition of simplified criteria to predict when the two schemes are or are not compatible for large expected shaking levels. The proposed criteria are based on simple and intuitive parameters describing the soil deposit and the ground-motion parameters, which can be predicted straightforwardly. Therefore, this study provides a scheme for the choice between the EQL and the NL approaches that can be used even at the preliminary design stages. It appears that the EQL approach provides reliable amplification estimates in soil deposits with thickness up to 30 m, except for very deformable soils, but this depth range may be extended at long vibration periods. This result reveals a good level of reliability of the EQL approach for various soil conditions encountered in common applications, even for high-intensity shaking.

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