Travel times and velocities in the outer core from PmKP

1972 ◽  
Vol 14 (2) ◽  
pp. 161-168 ◽  
Author(s):  
Goetz G.R. Buchbinder
Keyword(s):  
Outer Core ◽  
Travel Times

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.

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>

scholarly journals Calculated travel times of seismic core waves*

1955 ◽  
Vol 45 (3) ◽  
pp. 187-195
Author(s):  
R. D. Forester
Keyword(s):  
Velocity Distribution ◽  
Grazing Incidence ◽  
Outer Core ◽  
Continuous Functions ◽  
Fair Agreement ◽  
Travel Times ◽  
The Earth ◽  
The Times

Abstract Travel times for the seismic core waves, PKP, PKS, and SKS, were computed by integration along the travel paths. For this purpose the velocity distribution within the earth was broken into segments which were represented by continuous functions. Except for rays of grazing incidence to the outer core the times calculated for PKP and PKS are intermediate between the smoothed times given by Jeffreys and times based upon recent observed data. The times calculated for SKS are in fair agreement with the smoothed times given by Jeffreys.

The velocities in the outer core

1971 ◽  
Vol 61 (4) ◽  
pp. 1051-1059
Author(s):  
A. L. Hales ◽  
J. L. Roberts
Keyword(s):  
Velocity Distribution ◽  
Lower Limit ◽  
Outer Core ◽  
Time Of Arrival ◽  
Travel Times ◽  
Mantle Boundary ◽  
Core Mantle Boundary ◽  
The Core ◽  
Arc Distance ◽  
The Difference

abstract Earlier studies of the velocity distribution in the outer core have been based on the travel times of SKS.SKS arrivals can only be observed satisfactorily for arc distances at the surface greater than 85°. This lower limit of observation of SKS corresponds to an arc distance of 40.2° within the core. Thus the velocities in the outermost 250 km of the core are based upon an extrapolation. We have used observations of the difference in time of arrival of SKKS and SKS to obtain core travel times extending the range of observation down to a Δ within the core of about 14°. The velocity distribution thus found is significantly lower than those of Jeffreys (Bullen, 1963) and Randall (in press) near the core mantle boundary.

scholarly journals Lowermost mantle shear-velocity structure from hierarchical trans-dimensional Bayesian tomography

2021 ◽  
Author(s):  
Sima Mousavi ◽  
Hrvoje Tkalčić ◽  
Rhys Hawkins ◽  
Malcolm Sambridge
Keyword(s):  
Velocity Structure ◽  
Spatial Scales ◽  
Model Space ◽  
Shear Velocity ◽  
Outer Core ◽  
Travel Times ◽  
S Wave ◽  
Low Velocity ◽  
Spatial Coverage ◽  
Lowermost Mantle

The core-mantle boundary (CMB) is the most extreme boundary within the Earth where the liquid, iron-rich outer core interacts with the rocky, silicate mantle. The nature of the lowermost mantle atop the CMB, and its role in mantle dynamics, is not completely understood. Various regional studies have documented significant heterogeneities at different spatial scales. While there is a consensus on the long scale-length structure of the inferred S-wave speed tomograms, there are also notable differences stemming from different imaging methods and datasets. Here we aim to overcome over-smoothing and avoid over-fitting data for the case where the spatial coverage is sparse and the inverse problem ill-posed. Here we present an S-wave tomography model at global scale for the Lowermost Mantle (LM) using the Hierarchical Trans-dimensional Bayesian Inversion (HTDBI) framework, LM-HTDBI. Our HTDBI analysis of ScS-S travel times includes uncertainty, and the complexity of the model is deduced from the data itself through an implicit parameterization of the model space. Our comprehensive resolution estimates indicate that short-scale anomalies are significant and resolvable features of the lowermost mantle regardless of the chosen mantle-model reference to correct the travel times above the D’’ layer. The recovered morphology of the Large-Low-Shear-wave Velocity Provinces (LLSVPs) is complex, featuring small high-velocity patches among low-velocity domains. Instead of two large, unified, and smooth LLSVPs, the newly obtained images suggest that their margins are not uniformly flat.

The structure of the outer core from SKS amplitudes and travel times

1977 ◽  
Vol 67 (6) ◽  
pp. 1541-1554
Author(s):  
R. Kind ◽  
G. Müller
Keyword(s):  
Outer Core ◽  
Chemical Inhomogeneity ◽  
Travel Times ◽  
Amplitude Ratios ◽  
New Model ◽  
Long Period ◽  
Extensive Search ◽  
Deep Focus

abstract Worldwide long-period observations of five deep-focus earthquakes from the Tonga-Fiji region have been studied. Observed amplitude ratios SKS/SKKS and difference travel times of SKS and SKKS have been compared with theoretical values for conventional core models. The correction of observed discrepancies leads to a new model of the outer core with a zone of increased velocities around 3750-km depth. In spite of extensive search for other, significantly different models explaining the observations, none has been found. Our model implies pronounced chemical inhomogeneity of the outer core.

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.

scholarly journals Longitudinal waves through the Earth's Core*

1952 ◽  
Vol 42 (2) ◽  
pp. 119-134
Author(s):  
M. E. Denson
Keyword(s):  
Velocity Distribution ◽  
Travel Time ◽  
Focal Point ◽  
Epicentral Distance ◽  
Outer Core ◽  
Travel Times ◽  
Time Curve ◽  
Longitudinal Waves ◽  
The Core ◽  
The Waves

Abstract Amplitudes, periods, and travel times of the longitudinal P′ or PKP core waves have been investigated. Results indicate that the epicentral distance of the main focal point and the travel time of P′ phases vary with the periods of the waves. This variation would seem reasonably explained in terms of dispersion. The point of reversal in the travel-time curve of the waves through the outer core is believed to lie near 157°. Data suggest a discontinuity between 120° and 125° rather than 110°. Anomalies existing in energy, period, and travel-time relationships of the P′ phases indicate that current concepts of velocity distribution and of propagation paths within the core are in need of modification.

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