90% Marginal Confidence Interval

Each parameter estimated (time, latitude, longitude and depth) includes an error value (eg. a +/- b). The marginal confidence interval is from a-b to a+b and has been derived so that the true value will fall into this interval 90 percent of the time REGARDLESS OF THE VALUES of other parameters estimated at the same time.

Aftershock

An earthquake which follows a larger earthquake or main shock and 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, with 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. S-waves 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 tp 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 pre-existing 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

An abrupt movement of soil and bedrock downhill in response to gravity. Landslides can be triggered by an earthquake or other natural causes. Undersea landslides can 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.

Lg 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

A major type of surface wave having a horizontal motion that is shear or transverse to the direction of propagation (travel). It is named after A.E.H. Love, the English mathematician who discovered it.

Low-velocity zone

Any layer in the Earth in which seismic wave velocities are lower than in the layers above and below.

Magnitude

A measure of the strength of an earthquake or strain energy released by it, as determined by seismographic observations. This is a logarithmic value originally defined by Charles Richter (1935). An increase of one unit of magnitude (for example, from 4.6 to 5.6) represents a 10-fold increase in wave amplitude on a seismogram or approximately a 30-fold increase in the energy released. In other words, a magnitude 6.7 earthquake releases over 900 times (30 times 30) the energy of a 4.7 earthquake - or it takes about 900 magnitude 4.7 earthquakes to equal the energy released in a single 6.7 earthquake! There is no beginning nor end to this scale. However, rock mechanics seems to preclude earthquakes smaller than about -1 or larger than about 9.5. A magnitude -1.0 event release about 900 times less energy than a magnitude 1.0 quake. Except in special circumstances, earthquakes below magnitude 2.5 are not generally 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 or less on the Richter scale.

Microseism

A more or less continuous motion in the Earth that is unrelated to an earthquake and that has a period of 1.0 to 9.0 seconds. It is caused by a variety of natural and artificial agents.

Modified Mercalli scale

Mercalli intensity scale modified for North American conditions. A scale, composed of 12 increasing levels of intensity that range from imperceptible shaking to catastrophic destruction, that is designated by Roman numerals. It does not have a mathematical basis; instead it is 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 and arrive at stations before the S waves, or secondary waves. The waves carry energy through the Earth as longitudinal waves, moving particles in the same line as the direction of the wave. P waves can travel through all layers of the Earth. P waves are generally felt by humans as a bang or thump.

Paleomagnetism

The natural magnetic traces that reveal the intensity and direction of Earth's magnetic field in the geologic past. Also, 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 which make up the Earth's crust. The plates are continuously moving.

Plate boundary

The place 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) is broken into a number of more or less 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. These are the slowest, but often the largest and most destructive, of the wave types caused by an earthquake. They are usually felt as a rolling or rocking motion and in the case of major earthquakes, can be seen as they approach. Named after Lord Rayleigh, the English physicist who predicted its existence.

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. It is a collection of mathematical formulas; it is 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, but 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 (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

A free or standing wave oscillation of the surface of water in an enclosed basin that is initiated by local atmospheric changes, tidal currents, or earthquakes. Similar to water sloshing in a bathtub.

Seismic belt

An elongated earthquake zone, for example, circum-Pacific, Mediterranean, Rocky Mountain. 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.

 

Spread

The layout of seismometer or geophone groups from which data from a single shot (the explosive charge) are recorded simultaneously.

Spreading Center

An elongated region where two plates are being pulled away from each other. New crust is formed as molten rock is forced upward into the gap. Examples of spreading centers include the Mid-Atlantic Ridge and the East African Rift.

Subduction

The process in which one lithospheric plate collides with and is forced down under another plate and drawn back into the Earth's mantle.

Subduction zone

An elongated region along which a plate descends relative to another plate, for example, the descent of the Nazca plate beneath the South American plate along the Peru-Chile Trench.

Surface of the Earth

The value given is the depth below the surface of the mean spheroid. The mean spheroid is a uniform approximation to the true shape of the Earth. No adjustment is made to the depth due to any differences between the true Earth and the mean spheroid. For example, the minimum depth that will be given is 0 kilometers, even though a quake directly under Mount Everest (elevation 8848 meters) could legitimately have a depth of -6 kilometers and still be 2 kilometers underground. On the other hand, a depth of 10 kilometers would actually be more than 1 kilometer above the ocean floor of Challenger Deep (elevation -11,033 meters) in the Marianas Trench of the Pacific Ocean.

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 (usually more than 20 degrees) from the recording station.

Tidal Wave

A term that seismologists hate. The correct word for the big waves people often call "tidal waves" is tsunami. True "tidal waves" - or waves caused by the tides - are the ordinary waves people see on the ocean.

Travel time

The time required for a wave train to travel from its source to a point of observation.

Tsunami

One or a series of huge sea waves caused by earthquakes or other large-scale disturbance of the ocean floor. (Referred to incorrectly by many as a tidal wave, but these waves have nothing to do with tides.) The word tsunami is Japanese, meaning "harbor wave."

Unconsolidated

Loosely arranged, not cemented together, so particles separate easily.

UTC

Coordinated Universal Time. The time scale based on the atomic second but corrected every now and again to keep it in approximate sync with the earth's rotation. The corrections show up as the leap seconds put into UTC - usually on New Year's Eve. In the most common usage, the terms GMT and UTC are identical.

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