that follow the largest shock of an earthquake sequence.
They are smaller than the "mainshock" and can occur over
a period of weeks, months, or years. In general, the
larger the mainshock, the larger and more numerous the
aftershocks and the longer they will continue.
more or less continuous movement occurring on some
faults. Creep does not cause shaking.
shaking caused by a sudden movement on a fault or by
point on the Earth's surface above the point at depth in
the Earth's crust where an earthquake begins.
fracture or crack along which two blocks of rock slide
past one another. This movement may occur rapidly, in the
form of an earthquake, or slowly, in the form of
earthquake that precedes the largest quake ("mainshock")
of an earthquake sequence. Foreshocks may occur seconds
to weeks before the mainshock.
measure of ground shaking describing the local severity
of an earthquake in terms of its effects on the Earth's
surface and on humans and their structures. The Modified
Mercalli Intensity (MMI) scale, which uses Roman
numerals, is one way scientists measure
mass movement of soil, mud, and (or) rock down a
process that occurs when an earthquake shakes wet sandy
soil until it behaves like a liquid, allowing sand to
"boil up" to the surface, buildings to sink, or sloping
ground to move.
number that represents the size of an earthquake source,
as determined from seismographic observations. The
original earthquake magnitude scale was the Richter or
"local" scale (ML), defined by Charles Richter in 1935,
but it has limited range and applicability. Modern
magnitude scales are based on the area of fault rupture
times the amount of slip (seismic moment).The moment
magnitude (MW) is the preferred magnitude scale, as it
provides the most reliable estimate of the size of the
largest quakes. For smaller quakes, ML and MW values are
nearly the same. An increase of one unit of moment
magnitude (for example, from 4.6 to 5.6) corresponds
approximately to a 31.6-fold increase in energy released
[by definition, a two-unit increase in magnitude
&emdash;for example, from 4.7 to 6.7&emdash;represents an
increase in energy released of 1,000 times
(31.6_31.6)]. Quakes below magnitude 2.5 are not
generally felt by humans.
scientific theory that the Earth's outer shell is
composed of several large, thin, relatively strong
"plates" that move relative to one another. Movements on
the faults that define plate boundaries produce most
an existing structure to improve its resistance to the
effects of earthquakes.
area of the Earth through which fault movement occurred
during an earthquake. For large quakes, the section of
the fault that ruptured may be several hundred miles in
length. Ruptures may or may not extend to the ground
potential for damaging effects caused by earthquakes. The
level of hazard depends on the magnitude of likely
quakes, the distance from the fault that could cause
quakes, and the type of ground materials at a
chance of injury, damage, or loss resulting from seismic
hazards. There is no risk, even in a region of high
seismic hazard, if there are no people or property that
could be injured or damaged by a quake.
building story that has significantly less stiffness than
the story above. Some buildings with parking at ground
level (and thus fewer walls or columns) or an otherwise
open ground story have this condition. The term is
sometimes also applied to a story that has less strength
than the one above, a condition that is more precisely
termed a "weak story."
generally vertical fault along which the two sides move
horizontally past each other. The most famous example is
California's San Andreas Fault.
boundary along which one plate of the Earth's outer shell
descends (subducts) at an angle beneath another. A
subduction zone is usually marked by a deep trench on the
sea floor. An example is the Cascadia Subduction Zone
offshore of Washington, Oregon, and northern California.
Most tsunamis are generated by subduction-zone
sea wave of local or distant origin that results from
large sea-floor displacements associated with powerful
earthquakes, major submarine landslides, or exploding