DICTIONARY OF GEOPHYSICS, ASTROPHYSICS, and ASTRONOMY

sferic
galactic nuclei, distinct from quasars since the
host galaxy is visible and since their luminosity
is lower. It is customary, albeit arbitrary, as there
is no discontinuity between nearby active galactic
nuclei and quasars, to distinguish between
Seyferts and quasars, defining Seyfert nuclei as
having luminosity lower than 10 11 solar luminosity,
the luminosity of a large galaxy.
sferic Theabbreviatedtermforanoccurrence
of atmospheric radio noise. See atmospheric
noise.
shadow bands Bright and dark bands that are
observed running along the ground just before
the onset of totality in a solar eclipse. They are
thought to arise from the refraction of the light
from a thin crescent of the sun behind the moon,
varying due to turbulence only tens or hundreds
of meters above the ground. They can have
wavelengths of centimeters to close to 1 m, and
move at rates of meters per second (determined
by the rate of motion of the atmospheric inhomogeneities).
Their contrast is typically low,
about 1%.
shadowzone Seismicrayspassingthecoreof
the Earth are refracted due to the rapid decrease
in seismic wave speed in the outer core, such
that direct P waves cannot be detected in a zone
of about 105 ◦ to 140 ◦ epicentral distance. This
zone is called the shadow zone.
shallow water A relative measure of water
depth used in the study of water waves. Shallow
water is (arbitrarily) defined as a ratio of water
depth to wavelength, h/L, of 1/25 (sometimes
1/10).
shallow water wave A wave in water more
shallow than 1/25 of a wavelength; in this case,
the speed of propogation is c = √ gh, where g
is the acceleration of gravity and h is the water
depth.
shatter cone In geology, a distinctively striated
conical rock structure ranging from a few
centimeters to a few meters in length, along
which fracturing has occurred, believed to have
been formed by the passage of a shock wave
following meteorite impact. The mechanism is
© 2001 by CRC Press LLC
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postulated to be a focusing of the shock wave behindobstacles,
concentratingitseffectinconical
regions.
shear The mathematical quantity (a symmetric
rank-2, 3-dimensional tensor) that determines
the rate of change of shape of a given
portion of a continuous medium surrounding
a given particle after factoring out the overall
change in volume (see also acceleration, expansion,
kinematical invariants, rotation, tidal
forces). Consider a fluid in which acceleration,
expansion, and rotation all vanish while shear
is nonzero. An observer could examine a parallelepiped
whose one vertex is at his position and
the other vertices are at neighboring particles
of the medium. With time, this parallelepiped
would change into another one whose volume
is the same, but its shape is different (sheared)
from the initial one. In real astronomical bodies
(and in all fluid mediums in everyday life)
shearing motion is typical.
shear modulus Also called rigidity. The proportionality
constant G between shear stressτij
and shear strainεij for a linearly elastic medium:
τij = 2Gεij . G is expressed in terms of Young’s
modulus E and Poisson’s ratio v as
G =
E
2(1 + v) .
shear probe See airfoil probe.
shear stress A stress (force per unit area)
exerted along a surface, as opposed to a stress
normal to a surface. In viscous fluids the shear
stress τ is τ = µ(du/dz), where du/dz is
a velocity gradient, u is the velocity in the xdirection,
and µ is the viscosity of the fluid.
shear velocity A measure of shear stress used
in the study of sediment transport. Defined as
u∗ = √ τ/ρ, where τ is the shear stress on the
bed, and ρ is density.
shear wave A seismic body wave in which
the displacement is perpendicular to the direction
of propagation. Shear waves can propagate
through solids but not through liquids or gases.