P-wave velocities

Oldham discovered that there are actually two kinds of seismic vibrations, one called P (or "primary," because it travels faster and arrives first) and the other called S (or "secondary," because of its later arrival at the same station). The compressional motion of the P waves can be transmitted through most substances, although the speed at which the wave moves decreases as the stiffness of the medium decreases. In contrast, the transverse motion of S waves cannot be transmitted through a liquid, because the loosely bonded molecules in a liquid slip past each other too easily. S waves are observed to disappear at the top of the core. Then, at a depth of approximately 5100 km, the P wave velocity abruptly increases and there is a hint of the reappearance of an S wave. From such observations, Danish geophysicist Inge Lehman hypothesized in 1936 that the core was stratified, with an outer liquid portion and an inner solid portion. The existence of molten metal at core pressures requires some light element to act as antifreeze. 

Lee and Collett (2001) measured the compressional (P-wave) and shear (S-wave) velocities of natural hydrates in sediments (33% average total porosity) at the Mallik 2L-38 well. The P-wave velocity of nongas-hydrate-bearing sediment with 33% porosity was found to be about 2.2 km/s. The compressional velocity of gas-hydrate-bearing sediments with 30% gas hydrate concentration (water-filled porosity of 23%) was found to be about 2.7 km/s, and 3.3 km/s at 60% concentration (water-filled porosity of 13%), that is, about a 20% or 50% increase to nongas-hydrate-bearing sediment. The shear velocity was found to increase from 0.81 to 1.23 km/s. 

Similar to Sites 994 and 995, six indicators of hydrate were present (1) large gas exsolution from cores, (2) high methane sediment concentration, (3) BSRs, (4) low interstitial-water chlorinity, (5) low core temperatures (although IR technology was introduced just after this hydrate leg), and (6) P-wave velocity logs and resistivity logs. As in Sites 994 and 995, there was a discrepancy between the indicated base of the inferred hydrate zone (452 mbsf) and the phase-equilibria stability zone (491 to 524 mbsf). 

Most seismological constraints on mantle composition are derived by comparison of values of seismic wave velocities inferred for particular regions within the Earth to the values measured in the laboratory for particular minerals or mineral assemblages, with such comparisons being made under comparable regimes of pressure (P) and temperature (T). The primary parameters of interest, then, are the compressional (or P-) wave velocities (Vp) and the shear (or S-) wave velocities (Ej). These wave velocities are simply related to the density (p) and to the two isotropic elastic moduli, the adiabatic bulk modulus (Ks)... 

Kg/p = Vp — (4/3)V. This new parameter (sometimes thought of as the P-wave velocity of an equivalent fluid, for which G = 0) can be determined directly from static compression data V = Kg/p = (1 + Tay)(bP/bp)p. The bulk sound velocity possesses another desirable feature, in that it can also be constrained indirectly through chemical equilibrium experiments. Chemical equilibria describe free energy minima the pressure dependence of free energy is described by the molar volume, and the pressure dependence of volume (or density) is described by Kp and hence V. Thus, experimental determinations of equilibrium phase boundaries can... 

Saltzer R. L. and Humphreys E. D. (1997) Upper mantle P wave velocity structure of the eastern Snake River Plain and its relationship to geodynamic models of the region. 

This fact is well established by seismic data (P-wave velocity, bulk modulus, and density), geodynamo observations (the need for it to be reasonably good electrical conductor), and cos-mochemical constraints. This then requires that the core, an iron- and nickel-rich reservoir, chemically balances the silicate Earth to make... 

Changes in P-wave velocity of a rock as a function of temperature and pressure are generally assumed to be on the order of — 4 X 10 4kms °C and 2 X 10 km s MPa (see Rudnick and Fountain (1995 and references therein). Because most laboratory measurements... 

Except for active rifts and some intra-oceanic island arcs, which exhibit the highest middle-cmst P-wave velocities (6.7 0.3 km s (Rudnick and Fountain, 1995) and... 

Behn M. D. and Kelemen P. B. (2003) The relationship between seismic P-wave velocity and the composition of anhydrous igneous and meta-igneous rocks. Geochem. Geophys. Geosys. (4)1041, doi 10.1029/2002GC000393. 

Shaw et al, 1994), the Baltic Shield (Joeleht and Kukkonen, 1998), and the Indian Shield (Roy and Rao, 2000). Nevertheless, these efforts remain limited to a few exceptional areas (Rudnick and Fountain, 1995). One outstanding problem is that the lowermost crust cannot be sampled directly, except perhaps in the Ivrea Zone, Italy, or in the Kohistan arc, Pakistan (Miller and Christensen, 1994). In seismic Shield models, the lowermost crust, identified by seismic P-wave velocities in the range 6.8-7.2kms makes up 16 km out of 45 km of the crustal column (Durrheim and Mooney, 1991 Christensen and Mooney, 1995). This large fraction of the crust remains elusive. 

Fig. 5. Cross-sections through the P-wave velocity perturbation models obtained by inversion of delay times correeted for elevation and crustal thickness, (a), (b), and (c) show plan views of velocity perturbations at depths of 150, 200 and 300 km, respectively, (d), (e) and (f) show vertical cross-sections along profiles A-A, B-B and C-C, respectively, as shown in the horizontal sections. Surface topography is plotted at 20 times actual scale. Uppermost 50 km (shaded area) in vertical sections denotes regions where station delay time residuals are ineorporated in model calculations. Colour scale shows the velocity perturbation in percent. Colours fade to black for ray hit counts less than 10 (see Fig. 7).

Results from the Western Superior wide-angle refraction experiment support an interpretation of azimuthal anisotropy. Musacchio et al. (1999) found evidence for a thin high-velocity anisotropic subcrustal layer, where the P-wave velocities in the east-west direction are faster than those in the north-south direction. The results from SKS splitting (see below) also indicate azimuthal anisotropy with a fast east-west direction. As a whole, the results are consistent with a preferred olivine a-axis aUgnment in a roughly east-west direction. 

Fig. 2) from this analysis has a 42 km thick crust and a 120 km thick high-velocity upper-mantle lid, giving a total thickness of 162 km for the seismic lithosphere. The P and S velocities beneath the Moho are 8.09 km s and 4.62kms , respectively and the compressional and shear velocity gradients in the lid are 0.0008 s and 0.0013 s , respectively. Below the lid, the S-wave velocity drops to at least 4.45 km s at 250 km depth, but no decrease in the P-wave velocity is required by the data. Below 160 km depth, the P-wave gradient increases to 0.0015 s and increases again to 0.0035 s between 250 km depth and the 410 km discontinuity. 

We use the absolute P-wave velocity and the ratio between P- and S-wave velocities (Pp/Ps) to infer the composition of the lower crust beneath the Marcy Anorthosite (New York State,... 

Although high-resolution Fp/Fs ratios from refraction data are restricted to the Adirondack Mountains transect, estimates from teleseismic data indicate Fp/Fs=1.84 for the entire Grenville Province (Jordan Frazer 1975 Owens 1987 Zandt Ammon 1995). These data also confirm that the lower crust is 20 km thick and has a P-wave velocity of about 7kms (Mar-tignole Calvert 1996). 

Shen AH, Reichmann H-J, Chen G, Angel RJ, Bassett WA, Spetzler H (1998) GHz ultrasonic interferometry in a diamond anvil cell P-wave velocities in periclase to 4.4 GPa and 207°C. In Manghanni MH, Yagi T (eds) Properties of Earth and Planetary Materials at High Pressure and Temperature. Am Geophys Union, Washington, DC, p 71-77... 

Deep seismic reflection experiments in the continental crust show that P-wave velocities increase with depth and that in many cases there is a lower crustal layer several kilometers thick which has a P-wave velocity greater... 

P-wave velocity (Vp) Seismic body wave velocity associated with particle motion (alternating compression and expansion) in the direction of wave propagation. Shield Tectonically stable, low lying Precambrian nucleus of continents, commonly containing the metamorphosed roots of ancient orogenic activity. 

To measure the P-wave velocity and attenuation illustrated by Biot-StolTs model an automated, PC-controlled logging system was developed which records and stores digital ultrasonic P-waveforms transmitted radially across marine sediment cores (Breitzke and Spieli 1993). These transmission measurements can be done at arbitrary small depth... 

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