E A R T H Q U A K E G L O S S A R Y
Fault—
a break or fracture in Earth’s crust along
which movement has taken place.
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Focus (pl. foci)—
the point within the Earth that is the
origin of an earthquake, where stored energy is first
released as wave energy.
Force—
the cause or agent that puts an object at rest
into motion or affects the motion of a moving object.
On Earth, gravity is a vertical force; earthquake
shaking includes both horizontal and vertical forces.
Foreshock—
an
earthquake that precedes a larger
earthquake, or
main shock
, usually originating along
the same fault as the main shock.
Friction—
mechanical resistance to the motion of
objects or bodies that touch.
Frequency—
the
rate at which a motion repeats, or
oscillates. The frequency of a motion is directly
related to the energy of oscillation. In this context,
frequency is the number of oscillations in an
earthquake wave that occur each second. In earthquake
engineering, frequency is the rate at which the top of a
building sways.
Generalization—
a statement made after observing
occurrences that seem to repeat and to be related.
Glazing—
glass surface.
Gravity
—the force of attraction between any two
objects with mass. Gravity is especially noticeable
when an object of great mass, such as Earth, attracts an
object of lesser mass.
Ground water—
subsurface or underground water.
Hazard
—an object or situation that holds the
possibility of injury or damage.
Hertz (Hz)—
the unit of measurement for frequency,
as recorded in cycles per second. When these rates are
very large, the prefixes
kilo
or
mega
are used. A
kilohertz
(kHz) is a frequency of 1,000 cycles per
second and a
megahertz
(MHz) is a frequency of
1,000,000 cycles per second.
Horizontal load—
the sum of horizontal forces (shear
forces) acting on the elements of a structure.
Index fossil—
a fossil that, because its approximate
date is known, allows scientists to determine the age of
the rock in which it is imbedded.
Infill
—a construction method that starts with a
structural steel or reinforced concrete frame and fills
the empty spaces between structural elements with
brick or hollow concrete block.
Intensity—
a subjective measure of the amount of
ground shaking an earthquake produces at a particular
site, based on human observations of the effect on
human structures and geologic features. The Modified
Mercalli Intensity scale uses Roman numerals from I
to XII.
Isoseismal line—
a line on a map that encloses areas of
equal earthquake intensity.
Joint—
a break or fracture in the Earth’s crust along
which movement has not taken place.
Joists—
the parallel planks or beams that hold up the
planks of a floor or the laths of a ceiling.
Lag time—
the difference between the arrival time of
P waves (T
p
) and S waves (T
s
).
Landfill—
a site where soil has been deposited by
artificial means—often, where garbage or rubbish has
been disposed of, then covered with dirt and
compacted.
Landslide—
an abrupt movement of soil and bedrock
downhill in response to gravity. Landslides can be
triggered by an earthquake or other natural causes.
Latitude—
the location of a point north or south of the
equator, expressed in degrees and minutes. Latitude is
shown on a map or globe as east-west lines parallel to
the equator.
Lifeline—
a service that is vital to the life of a
community. Major lifelines include transportation
systems, communication systems, water supply lines,
electric power lines, and petroleum or natural gas
pipelines.
Liquefaction—
the process in which a solid (soil)
takes on the characteristics of a liquid as a result of an
increase in pore pressure and a reduction in stress.
Load
—the sum of vertical force (gravity) and
horizontal forces (shear forces) acting on the mass of a
structure. The overall load is further broken down into
the loads of the various parts of the building. Different
parts of a building are designed and constructed to
carry different loads.
