DICTIONARY OF GEOPHYSICS, ASTROPHYSICS, and ASTRONOMY

seismometer
the body’s interior can be determined. Surface
waves are subdivided into several types, but all
are concentrated along the surface rather than
propagating through the body. Surface waves
can cause much damage to structures on the
surface of the planet but give little information
about the body’s interior structure.
seismometer An instrument that measures
ground motions.
seismotectonics A research field to investigate
the tectonics by means of earthquakes.
Seismotectonics pursues the mutual relation between
regional characteristics of earthquake
generation and observed various geoscientific
structure and phenomena. To be more concrete,
the configuration of plate boundaries, relative
plate motion, interplate coupling, intraplate deformations,
and stresses are investigated based
on data such as seismicity, fault parameters like
fault plane solutions and seismic moment, stress
drop, and rupture process.
selective heating Solar process which, in impulsive
flares, leads to an enrichment of 3 He and
heavier ions such as Fe compared to coronal or
solar wind abundances. See impulsive flare.
self-generated turbulence See diffusive
shock acceleration.
semiclassical gravity The approximation in
which gravitational degrees of freedom are assumed
to be sufficiently well defined by classical
general relativity or another classical theory of
gravity, but sources (e.g., matter) are taken in
terms of a quantum description of matter.
semi-diurnal tides Tidal components that
have a cycle of approximately one-half day. The
largest amplitude tidal constituent, M2, is semidiurnal,
and therefore the tidal motion in most of
the world is dominated by two high waters and
two low waters each tidal day. The tidal current
is said to be semi-diurnal when there are two
flood and two ebb periods each day. All tidal
constituents with a subscript 2 are semi-diurnal
tides.
© 2001 by CRC Press LLC
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semimajor axis Half the major axis of an
ellipse.
separator A magnetic field line that connects
any two null points of a magnetic configuration.
The separator defines the intersection of two
separatrix surfaces.
separatrix In space plasmas, the magnetic
surface that divides topologically distinct magnetic
flux regions, for instance separating bundles
of magnetic field lines with different connections,
e.g., open and closed field lines.
sequence stratigraphy A subdiscipline of
stratigraphy (the study of stratified rocks, either
volcanic or sedimentary). The phrase sequence
stratigraphy is not uniquely defined but
is generally understood to involve the division
of rocks in a sedimentary system into distinct
packages, which are then related to each other
in time through observation of their spatial and
lithological relationships and the application of
some stratigraphic rules. In practice, this has
been associated with the development of algorithms
for interpreting seismic profiles of sedimentary
basins (“seismic stratigraphy”), as it became
clear that packets of rocks are laid down
in similar fashions in different basins. These
packets are related to different stages of local
transgression and regression of the sea, which
occurs across a broad range of frequencies. By
inferring that local sea level is related to global
sea level, and correlating sequences between different
basins, one may derive a curve of global
sea level with different “orders” of cycles from
timescales of around 100 Ma (first order cycles,
associated with supercontinent assembly
and break-up) through tens of thousands of years
(fourth order or higher, perhaps due to changes
in global glaciation); however, the global correlation
of higher order cycles is disputed and
local subsidence may often be important.
Sérsic–Pastoriza (1965) galaxies Galaxies
that exhibit an anomalous luminosity profile
near their center, and often structures due
to unresolved, compact emission regions (“hot
spots”), which are sites of intense star formation.
Several Sérsic–Pastoriza galaxies have
a nucleus showing a Seyfert-type or LINER