Geoscience Reference
In-Depth Information
3.8
3.7
Fig. 11.1
Group velocity of 152-s Rayleigh waves (km/s).
Tectonic and young oceanic areas are slow (dashed), and
continental shields and older oceanic areas are fast. High
temperatures and partial melting are responsible for low
velocities. These waves are sensitive to the upper several
hundred kilometers of the mantle (after Nakanishi and
Anderson, 1984a). This was the first seismic indication of a
profound mantle feature under the Afar.
Zealand--Campbell Plateau and the marginal
seas in the western Pacific. The fastest regions
are the western Pacific, New Guinea--western
Australia--eastern Indian Ocean, west Africa,
northern Europe and the South Atlantic. A high-
velocity region is located in the north central
Pacific, centered near the Hawaiian swell, sug-
gesting that the swell is not a thermal anomaly.
Mid-Atlantic ridge tomography
shows
low-velocities especially near triple junctions
in the North and South Atlantic; these show
up particularly well when the reference model
is ORM, the Oceanic Reference Model. [
www.
mantleplumes.org/Seismology. html
]
High-velocity regions in continents generally
coincide with Precambrian shields and Phanero-
zoic platforms (northwestern Eurasia, western
and southern parts of Africa, eastern parts of
North and South America, and Antarctica). Low-
velocity regions in or adjacent to continents coin-
cide with tectonically active regions, such as the
Middle East centered on the Red Sea, eastern and
southern Eurasia, eastern Australia, and west-
ern North America and island arcs or back-arc
basins such as the southern Alaskan margin, the
Aleutian, Kurile, Japanese, Izu-Bonin, Mariana,
to 600 km. The fastest regions are the western
Pacific, western Africa and the South Atlantic.
Western North America, the Red Sea area, South-
east Asia and the North Atlantic are the slow-
est regions. The velocities in the South Atlantic
and
the
Philippine
Sea
plate
are
faster
than
shields.
The overall pattern of velocity variations
shows a general correlation with surface tec-
tonics. The lowest velocity regions are located
in regions of extension or active volcanism: the
southeastern Pacific, western North America,
northeast Africa centered on the Afar region,
the central Atlantic, the central Indian Ocean,
Kerguelen--Indian Ocean triple junction, western
North America centered on the Gulf of Califor-
nia, the northeast Atlantic, the Tasman Sea--New
