DICTIONARY OF GEOPHYSICS, ASTROPHYSICS, and ASTRONOMY

multiple ring basins
all be possible). This can lead to severe amplitude
fading, and hence signal distortion, because
of the small time delays (up to 1 msec and more
for several modes) in radio signals traveling by
different modes. The effects of this form of multipath
may be minimized by choosing a working
frequency that is supported on fewer modes, and
by selecting antennas that favor one mode over
others. A single ionospheric mode (e.g., 1-F
mode) may encompass four separate paths: the
ordinary and extraordinary paths and a high and
low ray for each of these polarizations. This
situation is common near the maximum useable
frequency for the circuit and time delays of the
order of 0.5 msec are possible. See fading.
multiple ring basins Very large impact
basins often display a pattern of three or more
complete or partial rings of mountainous materials.
These multiple ring basins provide information
on the thickness of the crust at the
time the basin formed. Multiple ring basins
are created by impact into a rigid surface layer
(lithosphere) which overlies a more fluid layer
(asthenosphere). If the crater resulting from
the impact is smaller than the thickness of the
lithosphere, a normal impact crater is produced.
However, if the impact crater is greater than
the thickness of the lithosphere, the asthenosphere
can flow inward beneath the portion of
the lithosphere which is pushed down by the
impact. This flow exerts a drag force on the descending
segment of the lithosphere, which can
cause fracturing in the lithosphere surrounding
the crater. The number of rings in a multi-ring
basin is related to the thickness of the crust at
the time of basin formation — fewer rings are
produced in a thicker crust, while many rings
indicate a relatively thin crust.
multiple shock An earthquake composed of
several spatially and temporally discontinuous
© 2001 by CRC Press LLC
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subevents. Most large earthquakes are multiple
shocks. From detailed analysis of seismograms,
we can find that an earthquake source fault does
not spread continuously and smoothly, but expands
intermittently and heterogeneously. This
kind of rupture pattern is considered to reflect
the inhomogeneous structure of a source region.
mushy zone As a melt containing more than
one component (e.g., a rock magma with diverse
chemistry, or the fluid of the Earth’s outer
core, which is thought to be composed of iron,
nickel, and some lighter elements) is cooled, the
composition of the first crystals to solidify are
generally different from the bulk composition of
the melt as a whole. As cooling continues, the
chemical makeup of the material being solidified
evolves, both because temperature of the melt
is changing and because the chemical makeup
of the remaining melt is itself changing (becoming
more enriched in the material excluded from
the first crystals). These factors also come into
play at the boundary between the Earth’s inner
core and outer core: Between the depths where
a parcel of outer core fluid would start to crystallize
and finish crystallizing, the boundary region
may be a mixture of liquid and solid material,
commonly referred to as a mushy zone. As neither
the composition of the core nor the thermodynamic
conditions at the inner core boundary
are particularly well constrained, the existence
and depth of the mushy zone are not known, but
it may be a factor both for the seismic properties
of the top of the inner core and the dynamic
boundary conditions for the geodynamo at the
base of the outer core. The crystallization of
aqueous analogs reveals some of the nature of
mushy zones: liquid circulates through the zone,
creating organized structures such as channels
called “chimneys”.