Download Module 1: Earthquake Glossary

Earthquake Glossary
Aftershock
An earthquake which follows a larger earthquake or main
shock.
An aftershock originates in or near the rupture zone of the
larger earthquake. Generally, major earthquakes are followed
by a larger number of aftershocks, decreasing in frequency
with time.
Amplitude
The maximum height of a wave crest or depth of a trough.
Array
An ordered arrangement of seismometers or geophones, the
data from which feeds into a central receiver.
Arrival
The appearance of seismic energy on a seismic record.
Arrival time
The time at which a particular wave phase arrives at a
detector.
Aseismic
Not associated with an earthquake - as in Aseismic Slip. Also
used to indicate an area with no record of earthquakes - an
Aseismic zone.
Body wave
A seismic wave that can travel through the interior of the
earth. P-waves and S-waves are body waves.
Central Angle
An angle with the vertex at the center of the Earth. A Central
Angle has one ray passing through the hypocenter (and also
the epicenter) and the other ray passing through the recording
station.
Consolidated
Tightly packed. Composed of particles that are not easily
separated.
Core
The innermost layers of the Earth.
The inner core is solid and has a radius of about 1300
kilometers. (The radius of the Earth is about 6371
kilometers.)
The outer core is fluid and is about 2300 kilometers thick. Swaves cannot travel through the outer core.
Continental Drift
The theory, first advanced by Alfred Wegener, that Earth's
continents were originally one land mass. Pieces of the land
mass split off and migrated to form the continents.
Crust
The thin outer layer of the Earth's surface, averaging about 10
kilometers thick under the oceans and up to about 50
kilometers thick on the continents. This is the only layer of
the Earth that humans have actually seen.
Earthquake
Shaking of the Earth caused by a sudden movement of rock
beneath its surface.
Earthquake Swarm
A series of minor earthquakes, none of which may be
identified as the main shock, occurring in a limited area and
time.
Elastic wave
A wave that is propagated by some kind of elastic
deformation, that is, a change in shape that disappears when
the forces are removed. A seismic wave is a type of elastic
wave.
Epicenter
That point on the Earth's surface directly above the
hypocenter of an earthquake.
Fault
A weak point in the Earth's crust and upper mantle where the
rock layers have ruptured and slipped. Faults are caused by
earthquakes, and earthquakes are likely to reoccur on preexisting faults.
First arrival
The first recorded signal attributed to seismic wave travel
from a source.
Focus
That point within the Earth from which originates the first
motion of an earthquake and its elastic waves.
Focal Zone
See Rupture Zone.
Foreshock
A small tremor that commonly precedes a larger earthquake
or main shock by seconds to weeks and that originates in or
near the rupture zone of the larger earthquake.
Great Earthquake
An earthquake having a magnitude of 8 or greater on the
Richter scale.
Hazard
A risk. An object or situation that has the possibility of injury
or damage.
Hypocenter
The calculated location of the focus of an earthquake.
Intensity
A measure of the effects of an earthquake at a particular place
on humans, structures and (or) the land itself.
The intensity at a point depends not only upon the strength of
the earthquake (magnitude) but also upon the distance from
the earthquake to the point and the local geology at that point.
Isoseismal Line
A line connecting points on the Earth's surface at which
earthquake intensity is the same. It is usually a closed curve
around the epicenter.
Landslide
Abrupt movement of soil/bedrock downhill in response to
gravity. Landslides can be triggered by an earthquake or other
natural causes. Undersea landslides cause tsunamis.
Latitude
The location of a point north or south of the equator. Latitude
is shown on a map or globe as east-west lines parallel to the
equator.
Leaking Mode
A surface seismic wave which is imperfectly trapped so that
its energy leaks or escapes across a layer boundary causing
some attenuation, or loss of energy.
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. In other words, solid ground turns to jelly.

You will hear this term used repeatedly.
Large Wave
A surface wave which travels through the continental crust.
Longitude
The location of a point east or west of the prime meridian.
Longitude is shown on a map or globe as north-south lines
left and right of the prime meridian, which passes through
Greenwich, England.
Love Wave
Major type of surface wave. Has a horizontal motion that is
shear or transverse to the direction of propagation (travel).
Named after AEH Love, the English mathematician who
discovered it.
Low-Velocity Zone
Any layer in the Earth where seismic wave velocities are
lower than in the layers above and below.
Magnitude
An earthquake’s strength or the strain energy released by it, as
determined by seismographic observations. A logarithmic
value originally defined by Charles Richter (1935).
A one unit increase of magnitude (e.g., from 4.6 to 5.6)
represents a 10-fold increase in wave amplitude on a
seismogram or a 30-fold increase in the energy released.
For example, a 6.7 earthquake releases over 900 times (30 x
30) the energy of a 4.7 earthquake. In other words, 900 4.7
earthquakes equal the energy of one 6.7 earthquake!
Magnitude (continued)
There is no beginning or end to this scale.
However, rock mechanics seems to preclude earthquakes
smaller than about (-1) or larger than about 9.5.
A (-1) event release about 900 times less energy than a 1.0
quake. Except in special circumstances, earthquakes below
magnitude 2.5 are not generally not felt by humans.
Major Earthquake
An earthquake having a magnitude of 7 to 7.99 on the Richter
scale.
Mantle
The layer of rock that lies between the crust and the outer
core of the Earth. It is approximately 2900 kilometers thick
and is the largest of the Earth's major layers.
Microearthquake
An earthquake having a magnitude of 2.0 or less on the
Richter scale.
Microseism
A continuous motion in the Earth that is unrelated to an
earthquake and has a period of 1-9 seconds. It is caused by a
variety of natural and artificial agents.
Modified Mercalli Scale
Mercalli intensity scale modified for North American
conditions.
Composed of twelve increasing levels of intensity that range
from imperceptible shaking to catastrophic destruction.
Designated by Roman numerals.
Has no mathematical basis. An arbitrary ranking based on
observed effects.
Mohorovicic Discontinuity (the Moho)
The boundary surface or sharp seismic-velocity discontinuity
(pronounced Mo-ho-ro-vi-chich) that separates the Earth's
crust from the underlying mantle.
Named for Andrija Mohorovicic, the Croatian seismologist
who first suggested its existence.
P Wave
Primary, longitudinal, irrotational, push, pressure, dilatational,
compressional, or push-pull wave.
P waves are the fastest body waves. They can travel through
all layers of the Earth and are felt by humans as a bang or
thump.
P waves carry energy through the Earth as longitudinal
waves, moving particles in the same line as the direction of
the wave. They arrive before the S waves, or secondary
waves.
Paleomagnetism
The natural magnetic traces that reveal the intensity and
direction of Earth's magnetic field in the geologic past.
Another word for the study of these magnetic traces.
Paleoseismology
The study of ancient (prehistoric) earthquakes.
Period
The time between two successive wave crests.
Phase
The onset of a displacement or oscillation on a seismogram
indicating the arrival of a different type of seismic wave.
Plate
One of the huge sections that make up the Earth's crust. Plates
are constantly moving.
Plate Boundary
Where two or more plates in the Earth's crust meet.
Plate Tectonics
The theory that the Earth's crust and upper mantle (the
lithosphere) are broken into a number of rigid, but constantly
moving, segments or plates.
Rayleigh Wave
A type of surface wave having a retrograde, elliptical motion
at the Earth's surface, similar to the waves caused when a
stone is dropped into a pond.
Rayleigh waves are the slowest but often the largest and most
destructive wave types caused by an earthquake. They are
usually felt as a rolling or rocking motion. In the case of
major earthquakes, they can be seen as they approach.
Recurrence Interval
The approximate length of time between earthquakes in a
specific seismically active area.
Reflect
To bounce back from a surface.
Refract
To bend or change direction.
Richter Scale
The system used to measure the strength of an earthquake.
Developed by Charles Richter in 1935 as a means of
categorizing local earthquakes. A collection of mathematical
formulas - not a physical device.
Rupture Zone
The area of the Earth through which faulting occurred during
an earthquake.
For very small earthquakes, this zone could be the size of a
pinhead. In the case of a great earthquake, the rupture zone
may extend several hundred kilometers in length and tens of
kilometers in width.
S Wave
Shear, secondary, rotational, tangential, equivoluminal,
distortional, transverse, or shake wave.
These waves carry energy through the Earth in very complex
patterns of transverse waves, or crosswise waves. These
waves move more slowly than P waves, but in an earthquake
they are usually bigger.
S waves cannot travel through the outer core because these
waves cannot exist in fluids, such as air, water or molten rock.
Seiche
Free or standing wave oscillation on the surface of water in an
enclosed basin. Initiated by local atmospheric changes, tidal
currents, or earthquakes. Similar to water sloshing in a
bathtub.
Seismic Belt
Elongated earthquake zone. For example, Circum-Pacific,
Mediterranean, Rocky Mts. About 60% of the world's
earthquakes occur in the Circum-Pacific seismic belt.
Seismic Constant
In building codes dealing with earthquake hazards, an
arbitrarily-set acceleration value (in units of gravity) that a
building must withstand.
Seismicity
Earthquake activity.
Seismic
Of or having to do with earthquakes.
Seismic Sea Wave
A tsunami generated by an undersea earthquake.
Seismic Zone
A region in which earthquakes are known to occur.
Seismogram
A written record of an earthquake, recorded by a
seismograph.
Seismograph
An instrument that records the motions of the Earth,
especially earthquakes.
Seismograph Station
A site at which one or more seismographs are set up and
routinely monitored.
Seismologist
A scientist who studies earthquakes.
Seismometry
The instrumental aspects of seismology.
Signal-to-Noise Ratio
The comparison between the amplitude of the seismic signal
and the amplitude of noise caused by seismic unrest and (or)
the seismic instruments.
Spreading Center
An elongated region where two plates are being pulled away
from each other.
New crust is formed as molten rock, then forced upward into
the gap. Examples of spreading centers include the MidAtlantic Ridge and the East African Rift.
Subduction
When one lithospheric plate collides with and is forced down
under another plate, then drawn back into the Earth's mantle.
Subduction Zone
An elongated region where a plate descends relative to
another plate, for example, the descent of the Nazca plate
beneath the So. Amer. plate along the Peru-Chile Trench.
Surface Waves
Waves that move over the surface of the Earth.
Rayleigh waves and Love waves are surface waves.
Teleseism
An earthquake that is distant from the recording station
(usually more than 20 degrees).
Travel Time
The time required for a wave train to travel from its source to
a point of observation.
Unconsolidated
Loosely arranged, not cemented together, so particles separate
easily.
UTC
Coordinated Universal Time.
The time scale based on the atomic second but corrected usually on New Year's Eve - to keep it in approximate sync
with the earth's rotation.
The corrections show up as leap seconds put into UTC. In the
most common usage, the terms GMT and UTC are identical.
For more information on time, see Time Information Links.
Glossary Provided Courtesy of
United States Geological Survey
National Earthquake Information
Center – Public Information