Shear wave velocities in the lower mantle

1969 ◽  
Vol 59 (5) ◽  
pp. 1983-1999
Author(s):  
J. W. Fairborn
Keyword(s):  
Monte Carlo ◽  
Velocity Gradient ◽  
Lower Mantle ◽  
Travel Times ◽  
Core Mantle Boundary ◽  
Velocity Models ◽  
The Core ◽  
Wave Velocities ◽  
Shear Wave Velocities ◽  
Steep Slopes

abstract The Large Aperture Seismic Array in eastern Montana was used to measure the travel times and dT/dΔ of earthquake-generated shear waves for the purpose of determining lower-mantle shear velocities. The data were limited to epicentral distances between 27 and 95 degrees. To convert uncertainties in the dT/dΔ measurements to uncertainties in the computed velocities, the data were inverted by a Monte Carlo procedure. The computer randomly generated velocity models, and those which satisfied prescribed travel time and dT/dΔ-versus-Δ limits were considered as possible models for the real earth. The dT/dΔ curve possessed anomalously steep slopes between 27-30 and 65-75 degrees, and the corresponding velocity models had increased gradients between the approximate depths of 700-800 and 1550-2100 kilometers. The slope of the dT/dΔ curve also decreased at about 88 degrees which may indicate a decrease in velocity gradient several hundred kilometers above the core-mantle boundary. For epicentral distances greater than 27 degrees the observed travel times to LASA were greater than the expected J-B times. Although vague, a systematic increase in the observed residuals appeared to exist between 40 and 60 degrees which agreed with the J-B residuals of the computed models.

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.

Observations of PKKPab Diffraction Waves Well Beyond Cutoff Distance

2021 ◽  
Author(s):  
Yulin Chen ◽  
Sidao Ni ◽  
Baolong Zhang
Keyword(s):  
Epicentral Distance ◽  
P Wave ◽  
Source Depth ◽  
Large Area ◽  
Core Mantle Boundary ◽  
Velocity Models ◽  
Wave Velocities ◽  
The Earth ◽  
Cutoff Distance ◽  
Lowermost Mantle

Abstract The core mantle boundary (CMB) features the most dramatic contrast in the physical properties within the Earth and plays a fundamental role in the understanding of the dynamic evolution of the Earth’s interior. Seismic core phases such as PKKP sample large area of the lowermost mantle and the uppermost core, thus providing valuable information of the velocity structures on both sides of the CMB. Diffraction Waves Well Beyond Cutoff Distance (PKKPab) is one branch of the triplicated PKKP that can be observed beyond its ray theoretical cutoff distance as a result of diffraction along the CMB. The travel time and slowness of the diffracted PKKPab (denoted as PKKPabdiff) can be used to constrain the P-wave velocities at the lowermost mantle, thus have been investigated in numerous studies. Previous results (Rost and Garnero, 2006) suggest that most of the observations of the PKKPabdiff waves are in the epicentral distance range of 95°–105° (minor arc convention) (PKKPabdiff diffraction length less than 10°). However, high-frequency (∼1 Hz) synthetic seismograms show that the PKKPabdiff waveforms could be observable at distance down to 65°, which indicates that the PKKPabdiff signals could be detected at distances less than 95° in observations. To explore the distance ranges in which PKKPabdiff is observable, we collected global three-component broadband waveforms from 246 events with source depth deeper than 100 km and magnitude above M 6 from 2007 to 2017 available at the Incorporated Research Institutions for Seismology Data Management Center. We analyzed the slowness, polarization, and amplitude of the candidate PKKPabdiff signals, and found 95 events with clear PKKPabdiffsignals, with nearly 60% of the events show PKKPabdiff diffraction lengths greater than 10°, and the longest diffraction distance is beyond 20°. These newly identified PKKPabdiff waves would substantially augment the dataset of core phases for improvements of the CMB velocity models.

scholarly journals Smooth Crustal Velocity Models Cause a Depletion of High-Frequency Ground Motions on Soil in 2D Dynamic Rupture Simulations

2021 ◽  
Author(s):  
Yihe Huang
Keyword(s):  
Shear Wave ◽  
High Frequency ◽  
Ground Motions ◽  
Velocity Model ◽  
Dynamic Rupture ◽  
Ground Acceleration ◽  
Velocity Models ◽  
Wave Velocities ◽  
Shear Wave Velocities ◽  
Crustal Velocity

ABSTRACT A depletion of high-frequency ground motions on soil sites has been observed in recent large earthquakes and is often attributed to a nonlinear soil response. Here, I show that the reduced amplitudes of high-frequency horizontal-to-vertical spectral ratios (HVSRs) on soil can also be caused by a smooth crustal velocity model with low shear-wave velocities underneath soil sites. I calculate near-fault ground motions using both 2D dynamic rupture simulations and point-source models for both rock and soil sites. The 1D velocity models used in the simulations are derived from empirical relationships between seismic wave velocities and depths in northern California. The simulations for soil sites feature lower shear-wave velocities and thus larger Poisson’s ratios at shallow depths than those for rock sites. The lower shear-wave velocities cause slower shallow rupture and smaller shallow slip, but both soil and rock simulations have similar rupture speeds and slip for the rest of the fault. However, the simulated near-fault ground motions on soil and rock sites have distinct features. Compared to ground motions on rock, horizontal ground acceleration on soil is only amplified at low frequencies, whereas vertical ground acceleration is deamplified for the whole frequency range. Thus, the HVSRs on soil exhibit a depletion of high-frequency energy. The comparison between smooth and layered velocity models demonstrates that the smoothness of the velocity model plays a critical role in the contrasting behaviors of HVSRs on soil and rock for different rupture styles and velocity profiles. The results reveal the significant role of shallow crustal velocity structure in the generation of high-frequency ground motions on soil sites.

Estimation of PcP travel times and the depth to the core

1968 ◽  
Vol 58 (4) ◽  
pp. 1293-1303 ◽  
Author(s):  
James Taggart ◽  
E. R. Engdahl
Keyword(s):  
Velocity Distribution ◽  
Core Radius ◽  
Travel Times ◽  
Nuclear Explosions ◽  
Velocity Models ◽  
The Core

Abstract Based on a new P velocity distribution and observed PcP travel times from nuclear explosions, the core is estimated to have a mean radius = 3477 ± 2.0 km (depth = 2894 ± 2.0 km). Five velocity models were tested for the lower-most 90 km of the mantle. The PcP data suggest that the P velocity increases slightly with depth in this region. Tables of PcP travel times have been computed for the preferred model and a core radius of 3477 km.

scholarly journals Hydrous SiO2 in subducted oceanic crust and water transport to the core-mantle boundary

2020 ◽  
Author(s):  
Yanhao Lin ◽  
Qingyang Hu ◽  
Jing Yang ◽  
Yue Meng ◽  
Yukai Zhuang ◽  
...  
Keyword(s):  
Partial Melting ◽  
Oceanic Crust ◽  
Lower Mantle ◽  
Oceanic Lithosphere ◽  
Low Velocity ◽  
Water Storage Capacity ◽  
Mantle Boundary ◽  
Core Mantle Boundary ◽  
The Core ◽  
Subducted Oceanic Crust

Abstract Subduction of oceanic lithosphere transports surface water into the mantle where it can have remarkable effects, but how much can be cycled down into the deep mantle, and potentially to the core, remains ambiguous. Recent studies show that dense SiO2 in the form of stishovite, a major phase in subducted oceanic crust at depths greater than ~300 km, has the potential to host and carry water into the lower mantle. We investigate the hydration of stishovite and its higher-pressure polymorphs, CaCl2-type SiO2 and seifertite, in experiments at pressures of 44–152 GPa and temperatures of ~1380–3300 K. We quantify the water storage capacity of these dense SiO2 phases at high pressure and find that water stabilizes CaCl2-type SiO2 to pressures beyond the base of the mantle. We parametrize the P-T dependence of water capacity and model H2O storage in SiO2 along a lower mantle geotherm. Dehydration of slab mantle in cooler slabs in the transition zone can release fluids that hydrate stishovite in oceanic crust. Hydrous SiO2 phases are stable along a geotherm and progressively dehydrate with depth, potentially causing partial melting or silica enrichment in the lower mantle. Oceanic crust can transport ~0.2 wt% water to the core-mantle boundary region where, upon heating, it can initiate partial melting and react with the core to produce iron hydrides, providing plausible explanations for ultra-low velocity regions at the base of the mantle.

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