Instability of the African large low-shear-wave-velocity province due to its low intrinsic density
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Anomaly (physics)
Shear wave splitting
Seismic Tomography
Shear velocity
One‐dimensional upper mantle velocity structure is estimated by modeling P, SV and SH body‐waveforms sampling the central United States. The resulting model features a thick mantle lid with a positive velocity gradient. A significant shear wave low‐velocity zone (6% velocity reduction) is required below the lid however the compressional velocities are not reduced. The paths studied lie along the fast axes of teleseismic shear wave splitting measurements and absolute plate motion. We observe splitting of upper mantle refracted shear‐waves, with the transverse component slow relative to the radial component. In order to fit the shear wave arrivals and satisfy teleseismic shear‐wave splitting observations we infer azimuthal anisotropy (maximum 1.6%) distributed throughout the mantle lid and low‐velocity zone, with increasing anisotropy with depth.
Shear wave splitting
Shear velocity
Velocity gradient
S-wave
Low-velocity zone
Shear waves
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To study the change law of seismic velocity and shear- wave splitting with cyclic loading, used Autolab 2000 petrophysical testing system which was introduced from USA, experiment measured seismic velocity of quartz sandstone sample from Yangzhuang, Jixian, under the confining pressure (5-150MPa) of different cycles. Got the change law of compressional wave velocity and shear wave velocity with the changing confining pressure, and then analyzed the shear-wave splitting. Preliminary results showed that seismic velocity of unload is higher than the seismic velocity of load. P wave velocity of the second cycle is lower than the corresponding P wave velocity of the first cycle when the confining pressure is lower than 50MPa and S wave velocity of the second cycle is higher than the corresponding S wave velocity of the first cycle. When the confining pressure is under 60MPa, shear-wave splitting of unload is lower than the corresponding shear-wave splitting of load and shear-wave splitting of the second cycle is lower than the corresponding value of the first cycle.
Shear wave splitting
Wave velocity
Shear velocity
Overburden pressure
Wave shoaling
Phase velocity
Group velocity
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Seismic Tomography
Rayleigh Wave
Shear velocity
Indian Shield
Low-velocity zone
Group velocity
Mantle plume
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Inner Mongolia has complex gravity anomalies.The Da Hinggan Mountain in the east of Inner Mongolia is a mirror-image anomaly area.There exist low gravity of rock body,high gravity of Sungliao Plain,mountain root and anti-root,and the AREM model is tenable.The Yinshan mountain chain in the middle of Inner Mongolia is a mountain anomaly area.There exists high gravity of rock body,but mountain root effect on the gentle Moho surface is nonexistent,and the Pratt model is tenable.Low-gravity anomaly of the Dadong Mountain occurs in the middle of the mountain anomaly area.Comparative studies suggest that the low-gravity anomaly of the Dadong Mountain is neither a mountain anomaly nor a mirror-image anomaly,but a rock body anomaly.The isostatic compensation occurs in the lower boundary of the intracrustal granitic layer.It can be said that,in addition to the mountain anomaly and mirror-image anomaly,there is also a granite anomaly.
Anomaly (physics)
Inner mongolia
Free-air gravity anomaly
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Abstract This paper presents an overview of the recent height anomaly over Indonesia region in respect of Global Geopotential Model 2008 (EGM2008) which known as the most advance Global Geopotential Model. Recent height anomaly over Indonesia computed from Gravity Recovery and Climate Experiment (GRACE) satellite mission long term static solution using Bruns’ formula with degree and order up to 180. The results indicate that the deviation between geoid undulation with pseudo height anomaly deviation vary up to 12 meters with the largest deviations occur in several zones. Further investigation still needed to deduce the causes of the high height anomaly deviation that occur in several places in Borneo, Celebes and Papua, whether it is caused by a large compression forces or any other.
Anomaly (physics)
Geopotential
Geopotential height
Free-air gravity anomaly
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The correlation method separates the observed gravity anomaly into two parts,the regional anomaly approximated by the polynomial equation of the observed stations′ coordinates,and the residual anomaly calculated by subtracting the observed anomaly from the regional anomaly,based on the goal of maximizing the correlation between the depth of the targeted density interface and the residual anomaly.The above maximization goal can be written as a least squares equation and be solved iteratively by the linear or nonlinear optimization algorithm to get the polynomial coefficients,which can be used to approximate the regional anomaly and subsequently calculate the residual anomaly in the whole area.The test on the synthetic gravity anomaly shows that the correlation method separates the gravity anomaly effectively with good results.
Anomaly (physics)
Free-air gravity anomaly
Maximization
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Complex anisotropy in the Australian lithosphere from shear-wave splitting in broad-band SKS records
Shear wave splitting of seismic core phases reveals complex azimuthal anisotropy in continental Australia. Using the broad band width of the seismograms, we demonstrate that differing SKS splitting phenomena are manifested at different frequencies.
Shear wave splitting
Seismogram
Seismic anisotropy
Broad band
Shear waves
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Shear wave splitting
Seismic anisotropy
Seismic Tomography
Shear waves
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Shear wave splitting manifested as leading shear wave polarization, that is, parallel alignment of leading shear wave particle motions from a variety of sources, has been observed at a number of seismograph stations in Japan. Detected on shear wave seismograms from crustal earthquakes over a wide range of source zones and source‐receiver azimuths, the shear wave splitting can be attributed to crustal anisotropy. This paper discusses the relation between leading shear wave polarization directions and tectonic features of Japan. To explain the observed shear wave splitting, we propose that at least three phenomena should be taken into account: stress‐induced microcracks primarily aligned in vertical or subvertical planes; cracks or fractures in the vicinity of active faults having their orientation parallel to the fault planes; and intrinsic rock anisotropy resulting from preferred orientation of minerals. Travel time differences between leading and slower split shear waves from crustal and upper mantle earthquakes analyzed for about one third of the stations suggest that the crustal anisotropy which causes the observed shear wave splitting may be limited to the upper 15–25 km. This implies that the density of nonhorizontally aligned cracks or fractures below 15–25 km and into the upper mantle is much smaller than that in the crust above 15–25 km.
Shear wave splitting
Seismic anisotropy
Seismogram
Shear waves
Seismometer
S-wave
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