DICTIONARY OF GEOPHYSICS, ASTROPHYSICS, and ASTRONOMY

earthquake intensity
tude represents a 32-fold increase in the energy
released by the earthquake. The study of earthquakes
is called seismology. Similar releases
of energy are seen on other bodies, resulting in
moonquakes, Marsquakes, etc.
earthquake intensity A measure of the local
perceived strength of an earthquake based on the
local damage that is done.
earthquake magnitude The Richter magnitude
is a measure of the intensity of an earthquake.
It is a logarithmic scale with an increase
of one in magnitude corresponding to
about a factor of 32 increase in energy (a factor
of 10 in amplitude recorded on a seismogram).
Earthquake magnitudes are instrumental
in that they are obtained from earthquake
seismographs. The table on page 141 relates
the earthquake magnitude to earthquake intensity
and the associated damage.
earthquake moment The earthquake moment
M is defined by the relation
M = mdA
where m is the shear modulus in the rock where
the earthquake occurs, d is the mean displacement
across the fault during an earthquake, and
A is the rupture area. The earthquake moment is
empirically related to the earthquake magnitude.
earthquake precursor Any phenomenon
that occurs prior to an earthquake that warns of
its future occurrence. Examples are foreshocks,
ground motion (uplift, tilt), electromagnetic signals,
emission of gases (for example, radon),
changes in the water table, and animal behavior.
Although earthquake precursors have been documented
in some cases, no reliable precursors to
earthquakes have been found.
earthquake prediction Predictions of earthquakes
can be divided into two classes:
(1) earthquake hazard assessment and (2) prediction
of a particular earthquake. Hazard assessment
provides an estimate of probability that
an earthquake of a specified magnitude will occur
in a specified region in a specified time interval.
Hazard assessments are based on a number
of observations including the number of smaller
© 2001 by CRC Press LLC
140
earthquakes that occur in a region. Reasonably
accurate hazard assessments are available. No
reliable short-term predictions of actual earthquakes
have been documented.
earthquake swarm A swarm of earthquakes
that takes place, concentrating spatially and temporally.
The relation among foreshocks, mainshock,
and aftershocks is obscure, and an earthquake
swarm does not include a remarkably
large event (mainshock). Earthquake swarms
tend to occur in markedly heterogeneous crustal
structures such as a volcanic region and crush
zone. Possible generation mechanisms are
(1) region of accumulated elastic strain is divided
into blocks, and earthquakes take place
in the respective subregions, or (2) earthquakes
occur at the same place repeatedly during short
period due to fast supply of energy. A remarkable
example of earthquake swarm is the Matsushiro
earthquake swarm in Japan (Maximum
magnitude M 5.4), for which more than 60,000
felt earthquakes were observed for several years
from 1965.
earth radius (RE) A widely used unit in
measuring distances in the Earth’s magnetosphere,
usually measured from the center of the
Earth. (In other planetary magnetospheres, the
planet’s radius is often used in a similar way.)
The mean earth radius is 6,371,315 m. The average
distance to the subsolar point of the magnetopause
is about 11 RE, to that of the bow
shock 14 RE, to the synchronous orbit 6.6 RE,
to the moon about 60 RE and to the L1 and L2
Lagrangian points 236 RE.
earth rotation parameters (ERP) Earth orientation
parameters and Universal Time.
earthshine Reflected light from the dayside
of the Earth that illuminates the part of the moon
that is not directly lit by sunlight.
East Australian current An ocean current
flowing southward along the east coast of Australia.
East Greenland current An ocean current
flowing southward along the east coast of Greenland.