A velocity structure of the Earth's core

1971 ◽  
Vol 61 (2) ◽  
pp. 429-456 ◽  
Author(s):  
Goetz G. R. Buchbinder
Keyword(s):  
Transition Zone ◽  
Inner Core ◽  
Velocity Structure ◽  
Velocity Model ◽  
Outer Core ◽  
Travel Times ◽  
Amplitude Ratios ◽  
The Core ◽  
Short Period ◽  
Core Boundary

abstract Travel times and amplitudes of PKP, P2KP and higher multiple K phases are determined from a worldwide distribution of short-period seismograms. The sources are one explosion in Novaya-Zemlya and seven earthquakes, consisting of one intermediate focus event in the New Hebrides, and deep-focus events in Fiji, Java, Kermadec Islands, and Peru. The data are used to determine a new velocity model of the lowest mantle and the core. In the new velocity model 132, the velocity of the bottom of the mantle is 13.44 km/sec; the core mantle boundary is placed at 2892 ± 2 km. The velocity model of the core produces the PKP caustic B1 at 143° and the P2KP caustic B2 at −125°. A velocity discontinuity of 0.01 km/sec at a depth of 4550 km represents the top of the transition zone to account for the earliest forerunners of PKP. To account for the later forerunners a second discontinuity of 0.02 km/sec is placed at a depth of 4850 km. Since the forerunner data could not be resolved into branches, neither discontinuity is well defined. The top of the inner core boundary is placed at a depth of 5145 km with an uncertainty of at least 10 km and represents a discontinuity of 0.576 km/sec. Older core models have transition zone discontinuities an order of magnitude larger than those of model 132 with a discontinuity at the inner core boundary of about 1 km/sec. The smaller velocity discontinuities are a result of interpreting the amplitudes and travel times of PKP so that the turning points D and G are located at 120° and 140°, respectively, rather than at 110° and 125° as in previous interpretations. Amplitude ratios of PKP phases yield an inner core Q of about 400 and amplitude ratios of P3KP, P4KP and P5KP result in an outer core Q of about 4000.

Revised velocities in the Earth's core

1973 ◽  
Vol 63 (3) ◽  
pp. 1073-1105 ◽  
Author(s):  
Anthony Qamar
Keyword(s):  
Transition Zone ◽  
Inner Core ◽  
Velocity Model ◽  
Outer Core ◽  
Travel Times ◽  
Zone Boundary ◽  
Underground Nuclear Explosions ◽  
Nuclear Explosions ◽  
Core Boundary ◽  
Velocity Jump

abstract Travel times and amplitudes of PKP and PKKP from three earthquakes and four underground nuclear explosions are presented. Observations of reflected core waves at nearly normal angles of incidence provide new constraints on the average velocities in the inner and outer core. Interpretation of these data suggests that several small but significant changes to Bolt's (1962) core velocity model (T2) are necessary. A revised velocity model KOR5 is given together with the derived travel times that are consistent with the 1968 tables for P. Model KOR5 possesses a velocity in the transition zone which is 112 per cent lower than that in model T2. In addition, KOR5 has a velocity jump at the transition zone boundary (r = 1782 km) of 0.013 km/sec and a jump at the inner core boundary (r = 1213 km) of 0.6 km/sec. These values are, respectively, 1/20 and 2/3 of the corresponding model T2 values.

The origin of precursors to core waves

1970 ◽  
Vol 60 (3) ◽  
pp. 939-952 ◽  
Author(s):  
Eystein Husebye ◽  
Raúl Madariaga
Keyword(s):  
Inner Core ◽  
Outer Core ◽  
P Wave ◽  
Long Tail ◽  
The Core ◽  
Short Period ◽  
Diffracted Waves ◽  
Multiple Paths ◽  
Core Boundary ◽  
The Waves

Abstract The origin of the precursors of the core waves in the range 105°-142° is studied. Between 105° and 125° a long tail is observed after the P wave diffracted by the core. In the range 130° ≦ Δ ≦ 142° we usually observe short-period onsets a few seconds before PKIKP; these are the waves called P(GH). Reflection at a discontinuity in the outer core, near the inner-core boundary, is shown to produce the P(GH) branch. Reflections in the outer core are rejected as a mechanism for the tail of the P diffracted wave. A theoretical study of diffraction of P by the core shows that higher modes of diffracted waves cannot explain the observations of the tail of P diffracted. We conclude, by elimination, that it is due to reflections or multiple paths in the upper mantle.

scholarly journals Wave velocities in the earth's core

1958 ◽  
Vol 48 (4) ◽  
pp. 301-314
Author(s):  
B. Gutenberg
Keyword(s):  
Travel Time ◽  
Transition Zone ◽  
Southern California ◽  
Inner Core ◽  
Travel Times ◽  
Earth’S Core ◽  
Earth's Core ◽  
The Core ◽  
Wave Velocities

Abstract More than 700 seismograms of 39 shocks recorded mainly in southern California at epicentral distances between 105 and 140 degrees are used to investigate records of phases which have penetrated the earth's core. Properties of PKIKP, SKP, SKIKP, PKS, and PKIKS are discussed. Portions of travel-time curves of these phases are revised. Travel times of waves starting and ending at the surface of the core, and wave velocities in the core, are recalculated. Between about 1,500 and 1,200 km. from the earth's center in the transition zone from the liquid outer to the probably solid inner core, waves having lengths of the order of 10 km. travel faster than longer waves. This is probably caused by a rather rapid increase in viscosity toward the earth's center in this transition zone.

Diffraction from the PKP caustic B

1974 ◽  
Vol 64 (1) ◽  
pp. 33-43 ◽  
Author(s):  
Goetz G. R. Buchbinder
Keyword(s):  
Transition Zone ◽  
Inner Core ◽  
Focal Depth ◽  
Travel Times ◽  
Depth Data ◽  
Long Period ◽  
Short Period ◽  
Amplitude Decrease

abstract Long-period energy preceding the PKPDF branch on long-period seismograms was identified as resulting from diffraction from the PKP caustic B. This conclusion is based on the measurements of the travel times, and, therefore, the slope of the travel times, on the amplitude decrease, and the very rapid dispersion of the periods of one earthquake of intermediate focal depth. Data from three deepfocus earthquakes support this interpretation. Thus, the long-period precursors are not due to the same cause as short-period precursors and are not produced by discontinuities in the transition zone between the inner and outer cores nor by structure in the inner core.

Multiple inner core reflections from a Novaya Zemlya explosion

1972 ◽  
Vol 62 (4) ◽  
pp. 1063-1071 ◽  
Author(s):  
R. D. Adams
Keyword(s):  
Lower Bound ◽  
Inner Core ◽  
Nuclear Explosion ◽  
Outer Core ◽  
Low Velocity ◽  
Mantle Boundary ◽  
Core Mantle Boundary ◽  
Novaya Zemlya ◽  
The Core ◽  
Velocity Channel

Abstract The phases P2KP, P3KP, and P4KP are well recorded from the Novaya Zemlya nuclear explosion of October 14, 1970, with the branch AB at distances of up to 20° beyond the theoretical end point A. This extension is attributed to diffraction around the core-mantle boundary. A slowness dT/dΔ = 4.56±0.02 sec/deg is determined for the AB branch of P4KP, in excellent agreement with recent determinations of the slowness of diffracted P. This slowness implies a velocity of 13.29±0.06 km/sec at the base of the mantle, and confirms recent suggestions of a low-velocity channel above the core-mantle boundary. There is evidence that arrivals recorded before the AB branch of P2KP may lie on two branches, with different slownesses. The ratio of amplitudes of successive orders of multiple inner core reflections gives a lower bound of about 2200 for Q in the outer core.

PcP from the nuclear explosion BILBY September 13, 1963

1965 ◽  
Vol 55 (2) ◽  
pp. 441-461
Author(s):  
Goetz G. R. Buchbinder
Keyword(s):  
Standard Deviation ◽  
Seismic Velocity ◽  
Nuclear Explosion ◽  
Amplitude Ratios ◽  
The Core ◽  
Core Boundary

Abstract The core-reflected phase, PcP, from the BILBY event, received at stations between 19° and 88°, arrived early by an average of 1.80 seconds with respect to the Jeffries-Bullen tables. The standard deviation of these data was 0.77 seconds. The corresponding P phases were early by 1.34 seconds. The tables therefore need adjustments. If the core boundary is to be moved by more than 10 km from the value of 2898 km then the mantle seismic velocity immediately above the core must be changed also. The PcP/P amplitude ratios are nearly always much larger than those predicted theoretically.

Analytical Evaluation of Core Bowing Reactivity in the 4S Reactor

2009 ◽  
Author(s):  
Satoshi Nishimura ◽  
Hirokazu Ohta ◽  
Nobuyuki Ueda
Keyword(s):  
Fast Reactor ◽  
Radial Displacement ◽  
Inner Core ◽  
Structural Characteristics ◽  
Outer Core ◽  
Radial Expansion ◽  
Neutron Absorption ◽  
Analytical Evaluation ◽  
Life Condition ◽  
The Core

The 4S (super-safe, small and simple) reactor is a sodium-cooled small fast reactor. The core reactivity is controlled by moving the reflectors installed around the core, and the reactor has a fixed absorber at the core center to accomplish a long core lifetime. To evaluate core bowing behavior and the resulting reactivity feedback in the 4S reactor, an analytical evaluation was conducted under various core power to flow ratios (P/F). The core bowing reactivity under the BOC (beginning of core life) condition becomes increasingly negative with increasing P/F up to 2.0, then becomes less negative with increasing P/F from 2.0 to 3.0, and finally becomes positive at P/F = 3.0. The bowing reactivity under the EOC (end of core life) condition becomes increasingly negative with increasing P/F up to 1.5, then becomes less negative then positive with increasing P/F from 1.5 to 3.0; the core bowing reactivity is positive when P/F ≥ 2.0. These results are mainly caused by the following two mechanisms originating from the structural characteristics of the 4S reactor: - a decrease in neutron absorption by the fixed absorber due to the radial displacement of the inner core subassemblies (under the BOC condition); - a decrease in neutron streaming caused by the small gaps between the outer core subassemblies and the reflectors due to core radial expansion (under the EOC condition).

Influence of Mantle Structures on Measurements of Anisotropy in the Inner Core

2020 ◽  
Author(s):  
Sandra Beiers ◽  
Christine Thomas
Keyword(s):  
Seismic Anisotropy ◽  
Inner Core ◽  
Incidence Angle ◽  
Outer Core ◽  
Western Hemisphere ◽  
Travel Times ◽  
Array Seismology ◽  
Two Parameters ◽  
The Waves ◽  
Ray Paths

<p>The seismological exploration of the Earth’s inner core has revealed some structural complexities such as seismic anisotropy and hemispherical separation. Investigating the travel times of PKP waves from at least two different ray paths, a polar and an equatorial one, is one of the commonly used methods to probe the inner core’s anisotropy. Since the waves are traversing anomalous structures in the lowermost mantle before entering the core, these heterogeneities have to be taken into account when investigating anisotropy in the inner core.</p><p>In this study we use data from an equatorial path with events from Indonesia recorded in Morocco and a nearly polar one with earthquakes in New Zealand recorded in England. The two waves used in our study, PKPdf and PKPab, both propagate through mantle and outer core and PKPab additionally traverses the inner core. Within this work, we do not only analyse the travel times of the waves but rather investigate their deviations from the originally assumed path along with their incidence angle. This is done with the methods of array seismology, mainly its two parameters slowness and backazimuth.</p><p>The results of this study reveal opposite deviations of slowness and backazimuth of the polar in contrast to the equatorial path. While the polar waves travel shallower and closer to North, the equatorial waves propagate deeper and farther from North than predicted by ak135. Additionally we observe hemispherical differences between waves that sample the eastern and the ones that sample the western hemisphere for both ray paths, PKPdf and PKPab, which leads us to the assumption that the deviations are not caused by the inner core but are rather due to mantle structures.</p>

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