Low-velocity zone
Encyclopedia
The low-velocity zone occurs close to the boundary between the lithosphere
and the asthenosphere
in the upper mantle
. It is characterized by unusually low seismic shear wave
velocity compared to the surrounding depth intervals. This range of depths also corresponds to anomalously high electrical conductivity.It is present between about 80 and 300 km depth. This appears to be universally present for S waves, but may be absent in certain regions for P waves. A second low-velocity zone (not generally referred to as the LVZ, but sometimes as the ULVZ) has been detected in a thin ≈50 km layer at the core-mantle boundary
. These LVZs may have important implications for plate tectonics and the origin of the Earth's crust.
The LVZ has been interpreted to indicate the presence of a significant degree of partial melting
, and alternatively as a natural consequence of a thermal boundary layer and the effects of pressure and temperature on the elastic wave velocity of mantle components in the solid state. In any event, a very limited amount of melt (about 1%) is needed to produce these effects. Water in this layer can lower the melting point, and may play an important part in its composition.
s in 1959 by Beno Gutenberg
. He noted that between 1° to 15° from the epicenter the longitudinal arrivals showed an exponential decrease in amplitude after which they showed a sudden large increase. The presence of a low-velocity layer that defocussed the seismic energy, followed by a high velocity gradient that concentrated it, provided an explanation for these observations.
compared to P-waves
. As is evident from the figure, the reduction and depth over which reduction occurs varies with the choice of tectonic province, that is, regions differ in their seismic characteristics. Following the drop, the base of the zone is marked by an increase in velocity, but it has not been possible to decide whether this transition is sharp or gradual. This lower boundary, found beneath the continental lithosphere and oceanic lithosphere away from mid-ocean ridges, is sometimes referred to as the Lehmann discontinuity
and occurs at about 220±30 km depth. The interval also shows a reduction in Q, the seismic quality factor (representing a relatively high degree of seismic attenuation), and a relatively high electrical conductivity.
The LVZ is present at the base of the lithosphere except in areas of thick continental shield where no velocity anomaly is apparent.
in the LVZ, it is generally interpreted as a zone in which there is a small degree of partial melting. For this to occur at the depths where the LVZ is observed, small amounts of water and/or carbon dioxide must be present to depress the melting point of the silicate minerals. Only 0.05–0.1 % water would be sufficient to cause the 1% of melting necessary to produce the observed changes in physical properties. The lack of LVZ beneath continental shields is explained by the much lower geothermal gradient, preventing any degree of partial melting.
Lithosphere
The lithosphere is the rigid outermost shell of a rocky planet. On Earth, it comprises the crust and the portion of the upper mantle that behaves elastically on time scales of thousands of years or greater.- Earth's lithosphere :...
and the asthenosphere
Asthenosphere
The asthenosphere is the highly viscous, mechanically weak and ductilely-deforming region of the upper mantle of the Earth...
in the upper mantle
Mantle (geology)
The mantle is a part of a terrestrial planet or other rocky body large enough to have differentiation by density. The interior of the Earth, similar to the other terrestrial planets, is chemically divided into layers. The mantle is a highly viscous layer between the crust and the outer core....
. It is characterized by unusually low seismic shear wave
S-wave
A type of seismic wave, the S-wave, secondary wave, or shear wave is one of the two main types of elastic body waves, so named because they move through the body of an object, unlike surface waves....
velocity compared to the surrounding depth intervals. This range of depths also corresponds to anomalously high electrical conductivity.It is present between about 80 and 300 km depth. This appears to be universally present for S waves, but may be absent in certain regions for P waves. A second low-velocity zone (not generally referred to as the LVZ, but sometimes as the ULVZ) has been detected in a thin ≈50 km layer at the core-mantle boundary
Core-mantle boundary
The core–mantle boundary lies between the Earth's silicate mantle and its liquid iron-nickel outer core. This boundary is located at approximately 2900 km of depth beneath the Earth's surface. The boundary is observed via the discontinuity in seismic wave velocities at that depth...
. These LVZs may have important implications for plate tectonics and the origin of the Earth's crust.
The LVZ has been interpreted to indicate the presence of a significant degree of partial melting
Partial melting
Partial melting occurs when only a portion of a solid is melted. For mixed substances, such as a rock containing several different minerals or a mineral that displays solid solution, this melt can be different from the bulk composition of the solid....
, and alternatively as a natural consequence of a thermal boundary layer and the effects of pressure and temperature on the elastic wave velocity of mantle components in the solid state. In any event, a very limited amount of melt (about 1%) is needed to produce these effects. Water in this layer can lower the melting point, and may play an important part in its composition.
Identification
The existence of the low-velocity zone was first proposed from the observation of slower than expected seismic wave arrivals from earthquakeEarthquake
An earthquake is the result of a sudden release of energy in the Earth's crust that creates seismic waves. The seismicity, seismism or seismic activity of an area refers to the frequency, type and size of earthquakes experienced over a period of time...
s in 1959 by Beno Gutenberg
Beno Gutenberg
Beno Gutenberg was a German-American seismologist who made several important contributions to the science...
. He noted that between 1° to 15° from the epicenter the longitudinal arrivals showed an exponential decrease in amplitude after which they showed a sudden large increase. The presence of a low-velocity layer that defocussed the seismic energy, followed by a high velocity gradient that concentrated it, provided an explanation for these observations.
Characteristics
The LVZ shows a reduction in velocity of about 3–6% with the effect being more pronounced with S-wavesS-wave
A type of seismic wave, the S-wave, secondary wave, or shear wave is one of the two main types of elastic body waves, so named because they move through the body of an object, unlike surface waves....
compared to P-waves
P-wave
P-waves are a type of elastic wave, also called seismic waves, that can travel through gases , solids and liquids, including the Earth. P-waves are produced by earthquakes and recorded by seismographs...
. As is evident from the figure, the reduction and depth over which reduction occurs varies with the choice of tectonic province, that is, regions differ in their seismic characteristics. Following the drop, the base of the zone is marked by an increase in velocity, but it has not been possible to decide whether this transition is sharp or gradual. This lower boundary, found beneath the continental lithosphere and oceanic lithosphere away from mid-ocean ridges, is sometimes referred to as the Lehmann discontinuity
Lehmann discontinuity
The Lehmann discontinuity refers to an abrupt increase of P-wave and S-wave velocities in the vicinity of 220±30 km depth, discovered by seismologist Inge Lehmann. It appears beneath continents, but not usually beneath oceans, and does not readily appear in globally averaged studies...
and occurs at about 220±30 km depth. The interval also shows a reduction in Q, the seismic quality factor (representing a relatively high degree of seismic attenuation), and a relatively high electrical conductivity.
The LVZ is present at the base of the lithosphere except in areas of thick continental shield where no velocity anomaly is apparent.
Interpretation
The interpretation of these observations is complicated by the effects of seismic anisotropy, which may greatly reduce the actual scale of the velocity anomaly. However, because of the reductions in Q and electrical resistivityResistivity
Electrical resistivity is a measure of how strongly a material opposes the flow of electric current. A low resistivity indicates a material that readily allows the movement of electric charge. The SI unit of electrical resistivity is the ohm metre...
in the LVZ, it is generally interpreted as a zone in which there is a small degree of partial melting. For this to occur at the depths where the LVZ is observed, small amounts of water and/or carbon dioxide must be present to depress the melting point of the silicate minerals. Only 0.05–0.1 % water would be sufficient to cause the 1% of melting necessary to produce the observed changes in physical properties. The lack of LVZ beneath continental shields is explained by the much lower geothermal gradient, preventing any degree of partial melting.