DICTIONARY OF GEOPHYSICS, ASTROPHYSICS, and ASTRONOMY

sedimentary
sedimentary Referring to rock that has been
formed by deposition and consolidation of preexisting
material. Divided into types: clastic
— consilidation (typically under water) of
fine “sand” particles; carbonate — including
oolitic limestones, which are consolidated diatom
shells; organic — coal, which formed from
compressed organic material; and evaporite —
rock salt, gypsum. Only sedimentary stones
contain fossils, since they form in ways that will
preserve the structure of organic material.
sedimentary basin In geophysics, a region
in which the Earth’s surface has subsided and
has been covered by sediments. One mechanism
for subsidence is thermal subsidence: the
lithosphere cools and thickens, and due to the
increased density of the cooler rocks the lithosphere
sinks. A typical thickness of a sedimentary
basin is a few kilometers but some basins
have a thickness of 10 km or more. Subsidence
is often impeded by the rigidity of the elastic
lithosphere and the resulting flexure of the lithosphere
results in near circular or linear basins
with a radius or width of about 200 km.
sediment budget See sand budget.
sediment load The amount of sediment being
transported by moving water. Consists of bed
load, suspended load, and wash load.
sediment sink A place or process that results
in sediment removal from a beach or littoral system.
A submarine canyon might serve as a sediment
sink, and sand mining provides another
example.
sediment source The opposite of a sand sink;
a source of sand for the littoral system or beach.
Cliffs that back a beach, or a river might serve
as sediment sources.
sediment transport The movement of sediment
due to the action of wind or water.
seeing The phenomenon of time dependent
refraction in the Earth’s atmosphere which
moves stellar images around, and often breaks
them into multiple subimages. For groundbased
telescope, seeing is the dominant factor
© 2001 by CRC Press LLC
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limiting resolution. Seeing arises principally in
the air within 50 m of the telescope. Seeing is
reported in seconds of arc, as the size of the apparent
disk of a point source (star). The best
Earth-based sites have seeing of the order of 2
arcseconds. Active optics in telescopes can reduce
the effects of seeing by factors of up to
10, thereby allowing ground-based observations
closer to the diffraction limit.
seiche A stationary water wave usually
caused by strong winds, changes in barometric
pressure, or seismic activity. It is found in lakes,
semi-enclosed bodies of water, and in areas of
the open ocean. The period, T ,ofaseiche in
an enclosed rectangular body of water is usually
represented by the formula 2L
√ gd , where L is the
length, d is the average depth of the body of water,
and g is the acceleration of gravity. Typical
periods range from a few minutes in small lakes
to a few hours in gulfs.
seismic coupling factor A parameter used
almost exclusively for subduction zone thrust
faults. It is defined as the ratio of the long-term
average rate of fault slip taking place during subduction
earthquakes to the rate of plate convergence
for a given fault area.
seismic gap First, a region that has a potential
to cause large earthquakes, but where large
earthquakes have not occurred for a long time
(seismic gap of a first kind). Second, a region
where seismicity of usual small earthquakes decreases
drastically for some time (seismic gap
of a second kind). The latter sometimes precedes
a forthcoming large earthquake. Seismic
gaps of the first and second kinds could be important
clues for long-term prediction of large
earthquakes.
seismic intensity Numerical values which
represent the intensity of earthquake motion at
a point, divided into several classes. Seismic
intensity is determined mainly based on degree
of shaking that the human body feels, also referring
to shaking of surrounding materials, degree
of damage, and phenomena associated with
the earthquake. Seismic intensity scales such as
MM, MSK, and JMA seismic intensity scales,
which are appropriate to each country, are used.