Any earthquake which occurs after a larger earthquake (a
mainshock) within one rupture-length of the
original fault rupture and before the seismicity rate in that area
has returned to the background (pre-mainshock) level is generally
considered an aftershock. For some earthquakes, a specific "aftershock zone"
may be defined, in lieu of the one-rupture-length rule given above.
Aseismic slip, very similar to creep,
that occurs along a fault ruptured by a large earthquake in the months
following that event.
Loose material -- clay, silt, sand, gravel, and larger rocks -- washed
down from hills and mountains and deposited in low areas.
Zoning act passed in 1972 -- in response to the 1971
San Fernando earthquake to prevent building across the traces of
More information about the Alquist-Priolo Act, and Earthquake Fault
Zone Maps is available from the
California Geological Survey.
Basin and Range
An area of the southwestern United States characterized by roughly parallel
mountain ranges and valleys, formed by a series of tilted fault blocks,
and brought about by tectonic extension of the region. As is true of
any region experiencing crustal extension, normal faulting
is common here. (This name can apply generally to
any zone of similar landforms and tectonics.)
blind thrust fault
A shallow-dipping reverse fault which terminates before it reaches the surface.
When it breaks, therefore, it may produce uplift, but never
any clear surface rupture. Many still-unknown blind thrust faults
may exist in southern California. Two examples of known
blind thrust faults: the Elysian Park Thrust, which runs underneath
downtown Los Angeles and the Northridge Thrust Fault, which ruptured in
the 1994 Northridge quake.
Describes a pair of intersecting (or nearly intersecting) faults,
the slip motions of which are opposite (e.g., right-lateral and
left-lateral), so as to accomodate the rotation of the block they bound.
Conjugate faults will sometimes slip roughly "simultaneously" (within
hours or days of each other), causing pairs of earthquakes.
One example of this is the 1987 Elmore
Ranch/Superstition Hills earthquakes, in which the rupture of the
left-lateral Elmore Ranch fault was followed
only a day later by the rupture of the right-lateral
Superstition Hills fault. The
1992 Landers and Big
Bear earthquakes were also a conjugate pair.
Relatively slow, quiet movement along a fault. It is sometimes
called "seismic creep" to distinguish it from the slumping of rock
or soil on slopes (which is also known as creep), and sometimes
called "aseismic creep", since it does not trigger events greater than
A detachment fault; a fault where crustal deformation causes
separation along a boundary of rock types, typically between
so-called crystalline "basement" rock and overlying
The angle between a geologic surface -- for example, a fault plane --
and the horizontal. The direction of dip can be thought of as the
direction a ball, if placed upon the tilted surface, would roll.
Thus, a ball placed on a north-dipping fault plane would roll northward.
The dip of a surface is always perpendicular to the
strike of that surface.
Fault movement (slip) that is parallel to the dip of the fault.
This can describe both normal slip and reverse slip.
Eastern California Shear Zone (ECSZ)
A region of increased seismic activity which stretches from the
San Andreas fault near Indio, north-northeast
across the Mojave and northward into Owens Valley.
It may accommodate as much as 10 to 20 percent of the relative motion
between the North American and Pacific plates.
The point on the Earth's surface directly above the (subterranean) point of
origin (hypocenter) of an earthquake;
only two measurements, latitude and longitude,
are needed to locate it.
A fracture or zone of fractures along which there has been displacement of the
sides relative to one another, parallel to the fracture.
A commonly-used term that is synonymous with the surface
trace of a fault. It should never be used to describe the
fault itself; faults are planar, not linear, features.
An individual fault of a set of closely-spaced parallel or subparallel faults
of a fault system.
The direction and sense of slip on a fault plane at the point of
origin (hypocenter) of an earthquake,
as inferred from the first seismic waves which arrive at various
locations; often, they are draw on maps with a "beachball"-like symbol:
The black areas denote compression; the white areas denote dilation.
The fault plane which moved is parallel to one of the two planes
dividing the sphere in half.
Of the two sides of a non-vertical fault, the side below the fault plane.
It is called the footwall because where inactive faults have been "filled in"
with mineral deposits and then mined, this is the side on which miners walk.
(See hanging wall.)
Any earthquake which is followed, within a short time span, by a larger
earthquake in the exact same location can be labelled a "foreshock".
(In the case of an earthquake swarm, this
terminology is not generally applied.)
An elongate part of the Earth's crust bounded by faults on its long sides
and relatively down-dropped compared to its surroundings.
Of the two sides of a fault, the side above the fault plane.
It is called the hanging wall because where inactive faults have been
"filled in" with mineral deposits and then mined, this is the side on
which miners can hang their lanterns.
A fault with no dip. Still theoretical, this sort of fault should only
exist within a region of strong compression or extension where the
tectonic forces required for such movement could be present.
The most recent geologic era; from about 10,000 years
ago to the present. The Holocene is the latest epoch of
the Quaternary period.
Also known as the focus, the hypocenter is the point of origin
of an earthquake. It can be expressed with no fewer than three
measurements: latitude, longitude, and depth.
A measure of the effects at a particular place produced by shaking
during an earthquake. (Not to be confused with
A map showing the distribution of intensity across a region for
a particular earthquake using isoseismal
lines to connect points of equal intensity.
isoseisms (or isoseismal lines)
A line connecting points of identical intensity for a given earthquake.
A fault in which slips in such a way that the two sides move with
a predominantly lateral motion (with respect to each other). There
are two kinds of lateral slip: right-lateral and left-lateral.
They can be distinguished by standing on one side of the fault,
facing the fault (and, of course, the other side), and noting which
way the objects across the fault have moved with respect to you.
If they have moved to your right, the fault is right-lateral.
If the motion is to the left, then the fault is left-lateral.
A general term for a measure of the strength or energy of an earthquake
as determined from seismographic information.
ML (local magnitude)
A measure of the strain energy released by an earthquake within 100 kilometers
of its epicenter. Strictly defined by Charles Richter as the
base-10 logarithm of the amplitude, in microns, of the largest
trace deflection that would be observed on a standard torsion seismograph
at a distance of 100 km from the epicenter.
Ms (surface-wave magnitude)
A magntiude determined at teleseismic distances using the logarithm
of the amplitude of 20-second period surface waves generated
by an earthquake.
Mw (moment magnitude)
The seismic moment of an earthquake, converted to
a magnitude scale that roughly parallels the original
Richter magnitude scale. However, since it is
not based on the same measurements as
Richter (local or surface-wave) magnitudes, the different magnitudes
do not always agree, particularly for very large quakes.
Because it relates directly to the energy released by an earthquake,
it has become the standard in modern seismology.
The largest earthquake in any series of earthquakes; to be definitively
called a "mainshock", it should generally be at least half a magnitude
unit larger than the next largest quake in the series. Otherwise, the
series of quakes may more accurately fit the definition of a
A term used to describe earthquakes under Richter magnitude 2, and
occasionally, slightly larger quakes, especially those not felt
by people nearby.
Modified Mercalli Intensity Scale
An earthquake intensity scale adopted in 1931 that divides the
effects of an earthquake into twelve categories, from I (not
felt by people) to XII (damage total). A full
listing of the scale is available.
The tectonic region located between the Garlock fault and the San Andreas
fault, and extending eastward roughly to the California-Arizona (and
A fault characterized by predominantly vertical displacement in
which the hanging wall is moved
downward with respect to the footwall
of the fault. Generally, this kind of fault is a sign of
oblique (fault or slip)
Describing motion that is a combination of movement both perpendicular
and parallel to the strike of a fault -- a combination of strike-slip
and dip-slip (whether normal or reverse).
A term used primarily in the southwestern United States to describe a dry,
vegetation-free, flat area at the lowest part of an undrained desert basin,
underlain by stratified clay, silt, or sand, and commonly by soluble salts.
They are occasionally covered by shallow lakes in the wettest
parts of the year.
A name given to the geologic time period
between about 1.6 million years and 10,000 years before the present.
The Pleistocene is the earlier (older) epoch of the Quaternary period;
it is followed by the Holocene.
The fastest of seismic waves, and thus the first to arrive at a location
following an earthquake (the P stands for "primary"). This is a
compressional body wave; particle movement is parallel to the direction
of propagation of the wave. Its speed is 5.5 to 7.2 km/sec in the
crust and 7.8 to 8.5 km/sec in the upper mantle.
The period of geologic time starting 1.6 million years ago and continuing
to the present day. It is divided into two epochs: the
Pleistocene and the
Holocene, with the division between
these two falling at about 10,000
years before the present.
Late Quaternary refers
to the time between 700,000 years ago and the present day.
It does not necessarily exclude the Holocene epoch.
Pre-Quaternary refers to any time before 1.6 million years ago.
A fault in which the displacement is predominantly vertical, and
the hanging wall is moved
upward with respect to the footwall.
Some amount of reverse slip is often seen in predominantly lateral faults.
If a reverse fault has a dip angle of less than 45 degrees, it is called a
Introduced in 1935 by Charles F. Richter, the Richter scale is a numerical
scale for quantifying earthquake magnitude -- typically it refers to
local magnitude, but for larger quakes, it often
refers to surface-wave magnitude. (Currently,
large quakes are generally assigned a moment
magnitude, which is scaled to be similar, but is based on
seismic moment, and a better measure of the
energy of an earthquake.) Since the Richter scale is logarithmic,
very small earthquakes (microearthquakes)
can have a negative magnitudes. While the scale has no theoretical
upper limit, the practical upper limit, given the strength of materials
in the crust, is just below 9 for local or surface-wave magnitudes (and
just below 10 for moment magnitudes).
A roughly linear, cliff-like slope or face that breaks the continuity of
a surface into distinct levels. Scarps are often produced by faulting,
especially that which involves a significant amount of
A segment of an active fault zone that has not experienced a
major earthquake during a time interval when most other segments of
the zone have. Seismologists commonly consider seismic gaps to
have a high future-earthquake potential.
A measure of the strength of an earthquake, equal to the product of the
force and the moment arm of the double-couple system of forces that
produces ground displacements equivalent to that produced by the actual
earthquake slip. It is also equal to the product of the rigidity
modulus of the Earth material, the fault surface area, and the
average slip along the fault. Therefore, both seismological and
geological observations can produce the same result.
seismic zone (or seismic belt)
A region of the Earth's crust, generally linear, associated with active
seismicity. It may not necessarily be connected with a particular fault or
surface fault trace.
sense of slip
The sense with which one side of a fault slips relative to the other
side, in a reference frame defined by a horizontal (level) plane
oriented with the pull of Earth's gravity pointed "down" (perpendicular
to the plane).
The relative speed with which the two sides of a fault move past each
other. Typically, slip rates are measured in millimeters per year.
This figure applies to the motion of a fault over an extended period
of time, since most faults slip only during earthquakes; in between
earthquakes, the two sides are "locked." Thus, a slip rate of 6 mm/yr
does not mean that two structures built directly across from each other
on either side of a fault will move past each other at 0.5 millimeters per
month, 6 millimeters per year, or 60 millimeters per decade. They
may, for example, remain relatively "fixed" for many years until
they are suddenly offset several meters in a large earthquake.
The direction, or trend, of the line marking the intersection of a fault
plane (or another planar geologic feature) with the horizontal. Strike
is always at a right angle to dip.
A fault along which the slip motion is parallel to the
strike of the fault.
The breakage of ground along the surface trace of a fault caused by the
intersection of the fault surface area ruptured in an earthquake with
the Earth's surface.
The intersection of a fault plane with the surface of the Earth. It
is sometimes, but not always, expressed at the surface by geomorphic
evidence (ridges, valleys, saddles, etc.).
A series of minor earthquakes, none of which may be identified as the
mainshock, occurring in a limited area and time.
A seismic body wave which propagates by a shearing motion; particle
vibrate in a direction perpendicular to that of the propagation of the wave.
Slower than the P-wave, the S-wave always arrives
after the P-wave (the "S" stands for "secondary"). Its speed is roughly
3 to 4 km/sec in the crust and 4.4 to 4.6 km/sec in the upper mantle.
Because of its shearing motion, it cannot propagate through liquids.
The S-wave is responsible for our determination that the Earth's outer
core is liquid, since S-waves cannot travel through the outer core.
An adjective to refer to earthquakes, and any properties thereof,
that occur more than 100 km from a particular site. In this age of
modern seismic data-recording networks, this translates to more than
100 km from the nearest operational station in that network.
A specific kind of reverse fault in which the dip of the fault is less than
45 degrees over much if not all of its length. It is characterized not
so much by vertical displacement, but by horizontal compression.
Thrust faults are an obvious sign of compressional tectonics.
View an animation of
thrust fault motion (77 K).
The mountains formed by compression associated with the Big Bend
of the San Andreas fault zone -- primarily the San Gabriel
and San Bernardino Mountains. They are called transverse
because they stretch east-west, unlike the north-south trending
Sierra Nevada, Peninsular Ranges, and the mountains of the
Basin and Range province. Thus, they are
transverse to most other California mountains and to the overall
tectonic motion at this plate boundary.
A somewhat poorly understood process; involves the slipping of a fault
located in the same region as, but not directly associated with,
a fault which ruptures in a major earthquake. Most likely, the
shaking of the ground during the earthquake causes minor slippage
along temporarily "loosened" faults.