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An S wave, or shear wave, is a seismic body wave that shakes the ground
back and forth perpendicular to the direction the wave is
moving.
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A sand boil is sand
and water that come out onto the ground surface during an
earthquake as a result of
liquefaction at shallow depth.
(Photo by John Tinsley, U.S. Geological Survey)
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Sea-floor spreading
is what happens at the mid-oceanic ridge where a divergent
boundary is causing two plates to move away from one another
resulting in spreading of the sea floor. As the plates move
apart, new material wells up and cools onto the edge of the
plates.
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Secular refers to
long-term changes that take place slowly and imperceptibly.
Commonly used to describe changes in elevation, tilt, and stress
or strain rates that are related to long-term
tectonic deformation. For example, a mountain that is
growing is getting taller so slowly that we cannot see it
happen, but if we were to measure the elevation one year and
then the next, we could see that it has grown taller.
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Segmentation is the
breaking up of a fault along its length into several smaller
faults. This can happen as a result of other faults crossing it,
topography changes, or bends in the
strike of the faults. Segmentation can limit the length of
faulting in a single earthquake to some fraction of the total
fault length, thus also limiting the size of the earthquake.
The segments of the San Andreas Fault, and the probability for
strong shaking in the next 30 years on each segment. (Image from
Working Group on California Earthquake Probabilities, 1988)
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A seiche is the
sloshing of a closed body of water from earthquake shaking.
Swimming pools often have seiches during earthquakes.
(Image courtesy of Professor Brennan, Geneseo
State Univ. of New York)
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A seismic gap is a
section of a fault that has produced earthquakes in the past but
is now quiet. For some seismic gaps, no earthquakes have been
observed historically, but it is believed that the fault segment
is capable of producing earthquakes on some other basis, such as
plate-motion information or strain measurements.
(From U.S. Geological Survey Circular 1045)
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Earthquake
hazard
Earthquake hazard
is anything associated with an earthquake that may affect the
normal activities of people. This includes
surface faulting,
ground shaking,
landslides,
liquefaction,
tectonic deformation,
tsunamis, and
seiches.
Earthquake
risk
Earthquake risk is
the probable building damage, and number of people that are
expected to be hurt or killed if a likely earthquake on a
particular fault occurs. Earthquake risk and earthquake hazard
are occasionally incorrectly used interchangeably.
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Seismicity refers
to the geographic and historical distribution of earthquakes.
Seismicity of the U.S 1975-1995.
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The seismic moment is
a measure of the size of an earthquake based on the area of fault
rupture, the average amount of slip,
and the force that was required to overcome the friction sticking
the rocks together that were offset by faulting. Seismic moment
can also be calculated from the amplitude
spectra of seismic waves.
Moment = µ A D
µ = shear modulus = 32 GPa in crust, 75 GPa in
mantle
A = LW = area
D = average displacement during rupture
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Seismic
refraction or
Seismic
reflection line |
A seismic refraction
or seismic reflection line is a set of seismographs usually lined
up along the earth's surface to record seismic waves generated by
an explosion for the purpose of recording reflections and
refractions of these waves from velocity discontinuities within
the earth. The data collected can be used to infer the internal
structure of the earth.
(Images courtesy of Geosphere, Inc.)
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A seismic wave is an
elastic wave generated by an impulse such as an earthquake or an
explosion. Seismic waves may travel either along or near the
earth's surface (Rayleigh
and
Love waves) or through the earth's interior (P
and
S waves).
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A seismic zone is an
area of seismicity probably sharing a common cause. Example: "The
New Madrid Seismic Zone."
The New Madrid
seismic zone in Central U.S.
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Seismogenic means
capable of generating earthquakes. The base of the seismogenic
zone is the top of the more ductile
asthenosphere.
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A seismogram is a
record written by a seismograph in response to ground motions
produced by an earthquake, explosion, or other ground-motion
sources.
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Seismograph
or
Seismometer |
A seismograph, or
seismometer, is an instrument used to detect and record
earthquakes. Generally, it consists of a mass attached to a fixed
base. During an earthquake, the base moves and the mass does not.
The motion of the base with respect to the mass is commonly
transformed into an electrical voltage. The electrical voltage is
recorded on paper, magnetic tape, or another recording medium.
This record is proportional to the motion of the seismometer mass
relative to the earth, but it can be mathematically converted to a
record of the absolute motion of the ground. Seismograph
generally refers to the seismometer and its recording device as a
single unit.
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Seismology is the
study of earthquakes and the structure of the earth, by both
naturally and artificially generated
seismic waves.
Charles Richter, inventor of the Richter Scale.
(Image courtesy of Caltech)
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Seismometer
or
Seismograph |
A seismograph, or
seismometer, is an instrument used to detect and record
earthquakes. Generally, it consists of a mass attached to a fixed
base. During an earthquake, the base moves and the mass does not.
The motion of the base with respect to the mass is commonly
transformed into an electrical voltage. The electrical voltage is
recorded on paper, magnetic tape, or another recording medium.
This record is proportional to the motion of the seismometer mass
relative to the earth, but it can be mathematically converted to a
record of the absolute motion of the ground. Seismograph
generally refers to the seismometer and its recording device as a
single unit.
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The shadow zone is
the area of the earth from angular distances of 104 to 140 degrees
that, for a given earthquake, does not receive any direct P waves.
The shadow zone results from S waves being stopped entirely by the
liquid core and P waves being bent (refracted) by the liquid core.
Did You
Know???
Through measuring how P and S waves travel through the earth and
out the other side, a seismic wave shadow zone was discovered in
about 1910. From the lack of S waves and a great slowing of the P
wave velocity (by about 40%) it was deduced that the outer core is
made of liquid. The shadow zone also defined the diameter of the
core.
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Shear stress is the
stress component parallel to a given surface, such as a fault
plane, that results from forces applied parallel to the surface or
from remote forces transmitted through the surrounding rock.
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An S wave, or
shear wave, is a seismic body wave that shakes the ground back
and forth perpendicular to the direction the wave is moving.
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A slab is the oceanic
crustal plate that underthrusts the continental plate in a
subduction zone and is consumed by the earth's
mantle.
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Slickensides are
polished striated rock surfaces caused by one rock mass moving
across another on a fault.
(Photo courtesy of David Laurent)
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Slip is the relative
displacement of formerly adjacent points on opposite sides of
a fault, measured on the fault surface.
1992 M7.2 Landers, California earthquake
slip model. (Image from Wald and Heaton, BSSA, 1994)
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The strain rate is
how fast the
lithosphere is being deformed from plate tectonic movement.
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Stress is the force
per unit area acting on a plane within a body. Six values are
required to characterize completely the stress at a point: three normal components and three shear components.
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The stress drop is
the difference between the
stress across a fault before and after an earthquake.
Stress builds up on the fault over time,
and then is released when an earthquake occurs.
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The strike is the
trend or bearing, relative to north, of the line defined by the
intersection of a planar geologic surface (for example, a fault or
a bed) and a horizontal surface such as the ground.
(Image courtesy of Charles Ammon, Penn
State)
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Strike-slip faults are vertical (or nearly vertical) fractures
where the blocks have mostly moved horizontally. If the block
opposite an observer looking across the fault moves to the right,
the slip style is termed right lateral; if the block moves to the
left, the motion is termed left lateral.
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Strong motion is
ground motion of sufficient
amplitude and duration to be potentially damaging to a
building or other structure.
This turn-of-the-century wooden
residence in Los Gatos sustained major damage when it
moved off its foundation during the October 17, 1989, M7.1
Loma Prieta, California earthquake. (Photo by E.V.
Leyendecker, U.S. Geological Survey)
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Subduction is the
process of the oceanic
lithosphere colliding with and descending beneath the
continental lithosphere.
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The subduction zone
is the place where two
lithospheric plates come together, one riding over the other.
Most volcanoes on land occur parallel to and inland from the
boundary between the two plates.
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Surface faulting is
displacement that reaches the earth's surface during slip along a
fault. Commonly occurs with shallow earthquakes, those with an
epicenter less than 20 km. Surface faulting also may accompany
aseismic
creep or natural or man-induced subsidence.
Hector Mine surface rupture after 1999
earthquake in southern California. (Photo by Katherine
Kendrick, U.S. Geological Survey)
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Seismic wave
A seismic wave is an
elastic wave generated by an impulse such as an earthquake or an
explosion. Seismic waves may travel either along or near the
earth's surface (Rayleigh
and
Love waves) or through the earth's interior (P
and
S waves).
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Magnitude
The magnitude is a
number that characterizes the relative size of an earthquake.
Magnitude is based on measurement of the maximum motion recorded
by a
seismograph. Several scales have been defined, but the most
commonly used are (1) local magnitude (ML), commonly referred to
as "Richter magnitude," (2) surface-wave magnitude (Ms), (3)
body-wave magnitude (Mb), and (4) moment magnitude (Mw). Scales
1-3 have limited range and applicability and do not satisfactorily
measure the size of the largest earthquakes. The moment magnitude
(Mw) scale, based on the concept of
seismic moment, is uniformly applicable to all sizes of
earthquakes but is more difficult to compute than the other types.
All magnitude scales should yield approximately the same value for
any given earthquake.
Compare the fault area of the magnitude
7.3 (top) with that of the magnitude 5.6 (smallest one near
the bottom).
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