Low velocity zones

Sharp A. D. L., Davis P. M., and Gray F. (1980) A low velocity zone beneath Mount Etna and magma storage. Nature 287, 587-591. 

Tomographic results show that high-velocity mantle roots extend locally to depths of at least 250 km beneath undisturbed Archaean craton, with no comparable root structures beneath post-Archaean terranes. Neither body- nor surface-wave analyses have yet produced evidence of a low-velocity zone beneath the cratonic keel (Ritsema van Heijst 2000 Freybourger et al. 2001 ... 

Although the results reported here represent only an initial examination of a vast seismic dataset, there is little evidence to date in body-wave tomography, surface-wave inversion, or receiver function analysis of deep discontinuity structure for a low-velocity zone beneath the cratonic root. High-resolution seismic studies of the upper mantle are continuing, however, and more sophisticated analysis may yet reveal a velocity reversal at the base of the tectosphere. 

Fig. 4. Inversion for shear-wave velocity model (continuous line) which best fits the Rayleigh-wave interstation phase velocities. The grey area shows range of acceptable solutions. Three other velocity models are shown for comparison dashed line, Brune Dorman (1963) dot-dashed line, Grand Helmberger (1984) dotted line, PREM (Dziewonski Anderson 1981). It should be noted that the TWiST model shows no indication of a low-velocity zone below a continental root.

Fig. 3. (a) Sensitivity test of the higher-mode waveforms to the depth to the base of the upper-mantle lid for the SLR seismogram of the 18 July 1986 earthquake (Fig. 1, event 2). The continuous line is the observed waveform, the dotted line is the synthetic for the southern Africa velocity model of Qiu et al. (1996), and the dashed line is the synthetic for the same velocity model but with the lid base increased to the depth indicated at the left of each seismogram, (b) Same as (a) but for the SLR seismogram of the 10 March 1989 earthquake (Fig. 1, event 5). (c) Same as (a) but for the SUR seismogram of the 24 July 1991 earthquake to the minimum S-wave velocity of the low-velocity zone (LVZ). 

James Pouch (2002) showed intermediate period fundamental mode Rayleigh wave phase and group velocity data measured across the Kalahari Craton and a shear-wave velocity model from inversion of these data. They find evidence for a weak low-velocity zone below 120-130 km depth. The phase velocity data of James Pouch (2002) are not significantly different from the Rayleigh wave phase velocity measured by Priestley (1999). However, such intermediate period fundamental mode dispersion data do not provide stringent constraints on mantle velocities below c. 200 km depth. 

Seismic, heat-flow and xenolith data provide constraints on different physical properties, and differences between estimates are expected. Seismic methods determine the depth to a low-velocity zone at the base of higher-velocity lithospheric lid (e.g. James Pouch 2002 Ken-... 

In solid-liquid systems the size and shape of the baffles are important design parameters. The standard baffling is illustrated in Fig 7.1. As the solid concentration increases and the viscosity becomes high, narrower baffles (approximately 1/24T) placed a distance from the wall, should be used. This design is normally employed to avoid permanent settling of particles in the low velocity zones. In some processes such fillets (settled particles) can nevertheless be advantageous for the power consumption. 

Partitioning into overlying partial melts. The ultra-low velocity zone at the coremantle boundary may reflect the presence of mantle melt (see Garnero 2000 Ohtani and Maeda 2001), and partitioning from the core back into the overlying mantle may occur if conditions are favorable. However, a flux cannot be easily calculated without constraints on partition coefficients and the volume and residence time of melts at the core-mantle boundary. 

The fault thickness depends on the ratio of solid friction resistance (to slippage) and the strength of the intact rock material. At the transition to the middle crust they begin to have the same order, the breaking of faults edges takes place. The stick-slip phenomenon. Brace (1972), is an indicator of competition of fault asperity crushing and slippage. This happens in the PT zone (III) where the faults are widened due to dilatancy. Then low velocity zones (LVZ) as porous/fractured layers appear. 

Zollo AEA, Gasparini P, Virieux J, Le Meur H, De Nafale G, Biella G, Vilardo G (1996) Seismic evidence for a low-velocity zone in the upper crust beneath Mount Vesuvius. Science 274(5287) 592-594... 

The equation demonstrates that a high pore pressure plays the same role as a low external stress, causing the compressional wave velocity to be reduced. This is used in the estimation of abnormal pore pressures from logs or seismic velocities (e.g. Japsen et al., 2006). Since the expected trend in a homogeneous formation would be a monotonous increase of velocity with depth (because the effective stress increases with depth), an overpressure zmie shows up as a low-velocity zone breaking the expected trend. 

Note that this formula works only locally. However, we can apply it step by step, in which case it is possible to show that in the absence of a low velocity zone < 0, we can find V z) in the whole layer [0, H]. This result was obtained by G. Herglotz [13] and E. Wiechert [22] about hundred years ago. 

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