Accelerograph:

• A seismograph whose output is proportional to ground acceleration (in comparison to the usual seismograph whose output is proportional to ground velocity). Accelerographs are typically used as instruments designed to record very strong ground motion useful in engineering design; seismographs commonly record off scale in these circumstances. Normally, strong motion instruments do not record unless triggered by strong ground motion. (Noson, et.al., 1988)

Aftershock:

• One of many earthquakes that often occur during the days to months after some larger earthquake (mainshock) has occurred. Aftershocks occur in the same general region as the mainshock and are believed to be the result of minor readjustments of stress at places in the fault zone. (Noson, et.al., 1988)
• 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. (USGS National Earthquake Information Center, 1999)

Amplitude:

• The amplitude of a seismic wave is the amount the ground moves as the wave passes by. (As an illustration, the amplitude of an ocean wave is one-half the distance between the peak and trough of the wave. The amplitude of a seismic wave can be measured from the signal recorded on a seismogram.) (Noson, et.al., 1988)
• The maximum height of a wave crest of depth of a trough. (USGS National Earthquake Information Center, 1999)

Broad Band Seismic Station:

Crust:

• The Earth's outermost layer. (Teacher's Packet)

Deep Earthquakes:

• ...deep earthquakes and those located away from the volcano, which produce high-frequency signatures and sharp arrivals similar to tectonic earthquakes (Brantley and Topinka, 1984)

Earthquake:

• Shaking of the Earth caused by a sudden movement of rock beneath its surface. (USGS National Earthquake Information Center, 1999)
• The release of stored elastic energy caused by sudden fracture and movement of rocks inside the Earth. Part of the energy released produces seismic waves, like P, S, and surface waves, that travel outward in all directions from the point of initial rupture. These waves shake the ground as they pass by. An earthquake is felt if the shaking is strong enough to cause ground accelerations exceeding approximately 1.0 centimeter/second squared. (Noson, et.al., 1988)

Earthquake swarm:

• A series of minor earthquakes, none of which may be identified as the main shock, occurring in a limited area and time. (USGS National Earthquake Information Center, 1999)

Epicenter:

• That point on the Earth's surface directly above the hypocenter of an earthquake. (USGS National Earthquake Information Center, 1999)
• The location on the surface of the Earth directly above the focus, or place where an earthquake originates. An earthquake caused by a fault that offsets features on the Earth's surface may have an epicenter that does not lie on the trace of that fault on the surface. This occurs if the fault plane is not vertical and the earthquake occurs below the Earth's surface. (Noson, et.al., 1988)

Fault:

• A break in the Earth along which movement occurs. Sudden movement along a fault produces earthquakes. Slow movement produces aseismic creep. (Noson, et.al., 1988)

Focus:

• That point within the Earth from which originates the first motion of an earthquake and its elastic waves. (USGS National Earthquake Information Center, 1999)

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. (USGS National Earthquake Information Center, 1999)

Harmonic tremor:

• Continuous rhythmic earthquakes in the Earth's upper lithosphere that can be detected by seimographs. Harmonic tremors often precede or accompany volcanic eruptions. (Teacher's Packet)
• A continuous release of seismic energy typically associated with the underground movement of magma. It contrasts distinctly with the sudden release and rapid decrease of seismic energy associated with the more common type of earthquake caused by slippage along a fault. (Foxworthy and Hill, 1982)
• ... harmonic tremor, which is a long-lasting, very rhythmic signal whose origin is not well understood but which is often associated with active volcanoes ... (Brantley and Topinka, 1984)

Hypocenter:

• The calculated location of the focus of an earthquake. (USGS National Earthquake Information Center, 1999)

Intensity:

• A measure of severity of shaking at a particular site. It is usually estimated from descriptions of damage to buildings and terrain. The intensity is often greatest near the earthquake epicenter. Today, the Modified Mercalli Scale is commonly used to rank the intensity from I to XII according to the kind and amount of damage produced. Before 1931 earthquake intensities were often reported using the Rossi-Forel scale. (Noson, et.al., 1988)
• 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 opint and the local geology at that point. (USGS National Earthquake Information Center, 1999)

Intensity scale:

• The effect of an earthquake on the Earth's surface is called the intensity. The intensity scale consists of a series of certain key responses such as people awakening, movement of furniture, damage to chimneys, and finally - total destruction. Although numerous intensity scales have been developed over the last several hundred years to evaluate the effects of earthquakes, the one currently used in the United States is the Modified Mercalli (MM) Intensity Scale. (USGS National Earthquake Information Center, 1999)

Lg wave:

• A surface wave which travels through the continental crust. (USGS National Earthquake Information Center, 1999)

Long-period earthquakes:

• ... low-frequency earthquakes (called long-period or volcanic), which reflect adjustments related to the exit of magma from the summit reservoir to feed the eruption ... (Tilling, et.al., 1987)

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. (USGS National Earthquake Information Center, 1999)

Magnitude:

• A quantity characteristic of the total energy released by an earthquake, as contrasted with intensity, which describes its effects at a particular place. A number of earthquake magnitude scales exist, including local (or Richter) magnitude, body wave magnitude, surface wave magnitude, moment magnitude, and coda magnitude. As a general rule, an increase of one magnitude unit corresponds to ten times greater ground motion, an increase of two magnitude units corresponds to 100 times greater ground motion, and so on in a logarithmic series. Commonly, earthquakes are recorded with magnitudes from 0 to 8, although occasionally large ones (M=9) and very small ones (M= -1 or -2) are also recorded. Nearby earthquakes with magnitudes as small as 2 to 3 are frequently felt. The actual ground motion for, say, a magnitude 5 earthquake is about 0.04 millimeters at a distance of 100 kilometers from the epicenter; it is 1.1 millimeters at a distance of 10 kilometers from the epicenter. (Noson, et.al., 1988)
• A numerical expression of the amount of energy released by an earthquake, determined by measuring earthquake waves on standardized recording instruments (seismographs). The number scale for magnitudes is logarithmic rather than arithmetic; therefore, deflections on a seismograph for a magnitude 5 earthquake, for example, are 10 times greater than those for a magnitude 4 earthquake, 100 times greater than for a magnitude 3 earthquake, and so on. (Foxworthy and Hill, 1982)

Mainshock:

• The largest in a series of earthquakes occurring closely in time and space. The mainshock may be preceded by foreshocks or followed by aftershocks. (Noson, et.al., 1988)

Mantle:

• A zone in the Earth's interior between the crust and the core that is 2,900 kilometers (1,740 miles) thick. (The lithosphere is composed of the topmost 65-70 kilometers (39-42 miles) of mantle and the crust). (Teacher's Packet)

Microearthquakes:

• Earthquakes with magnitude of about 2.0 or less are usually call microearthquakes; they are not commonly felt by people and are generally recorded only on local seismographs. (USGS National Earthquake Information Center, 1998)

Modified Mercalli Intensity Scale:

• The effect of an earthquake on the Earth's surface is called the intensity. The intensity scale consists of a series of certain key responses such as people awakening, movement of furniture, damage to chimneys, and finally - total destruction. Although numerous intensity scales have been developed over the last several hundred years to evaluate the effects of earthquakes, the one currently used in the United States is the Modified Mercalli (MM) Intensity Scale. It was developed in 1931 by the American seismologists Harry Wood and Frank Neumann. This scale, composed of 12 increasing levels of intensity that range from imperceptible shaking to catastrophic destruction, is designated by Roman numerals. It does not have a mathematical basis; instead it is an arbitrary ranking based on observed effects. (USGS National Earthquake Information Center, 1998)

P (Primary) waves:

• Also called compressional or longitudinal waves, P waves are the fastest seismic waves produced by an earthquake. They oscillate the ground back and forth along the direction of wave travel, in much the same way as sound waves, which are also compressional, move the air back and forth as the waves travel from the sound source to a sound receiver. (USGS National Earthquake Information Center, 1998)

Plates:

• Pieces of crust and brittle uppermost mantle, perhpas 100 kilometers thick and hundres or thousands of kilometers wide, that cover the Earth's surface. The plates move very slowly over, or possibly with, a viscous layer in the mantle at rates of a few centimeters per year. (Noson, et.al., 1988)

Plate tectonics:

• A widely accepted theory that relates most of the geologic features near the Earth's surface to the movement and interaction of relatively thin rock plates. The theory predicts that most earthquakes occur when plates move past each other. (Noson, et.al., 1988)

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. (USGS National Earthquake Information Center, 1999)

Richter Magnitude Scale

• The Richter magnitude scale was developed in 1935 by Charles F. Richter of the California Institute of Technology as a mathematical device to compare the size of earthquakes. The magnitude of an earthquake is determined from the logarithm of the amplitude of waves recorded by seismographs. Adjustments are included for the variation in the distance between the various seismographs and the epicenter of the earthquakes. On the Richter Scale, magnitude is expressed in whole numbers and decimal fractions. For example, a magnitude 5.3 might be computed for a moderate earthquake, and a strong earthquake might be rated as magnitude 6.3. Because of the logarithmic basis of the scale, each whole number increase in magnitude represents a tenfold increase in measured amplitude; as an estimate of energy, each whole number step in the magnitude scale corresponds to the release of about 31 times more energy than the amount associated with the preceding whole number value. (USGS National Earthquake Information Center, 1998)

S (Secondary or shear) waves:

• S waves oscillate the ground perpendicular to the direction of wave travel. They travel about 1.7 times slower than P waves. Because liquids will not sustain shear stresses, S waves will not travel through liquids like water, molten rock, or the Earth's outer core. (USGS National Earthquake Information Center, 1998)

Seiche:

• A standing wave in a closed body of water such as a lake or bay. It can be characterized as the sloshing of water in the enclosing basin. Seiches can be produced by seismic waves from earthquakes. The permanent tilting of lake basins caused by nearby fault motions has produced very entergetic seiches. (Noson, et.al., 1988)

Seismicity:

• Earthquake activity. (USGS National Earthquake Information Center, 1999)

Seismic waves:

• A vibrational disturbance in the Earth that travels at speeds of several kilometers per second. There are three main types of seismic waves in the earth: P (fastest), S (slower), and Surface waves (slowest). Seismic waves are produced by earthquakes. (Noson, et.al., 1988)
• Seismic waves are the vibrations from earthquakes that travel through the Earth; they are recorded on instruments called seismographs. (USGS National Earthquake Information Center, 1998)

Seismogram:

• A graph showing the motion of the ground versus time. (Noson, et.al., 1988)
• A written record of an earthquake, recorded by a seismograph. (USGS National Earthquake Information Center, 1999)

Seismograph:

• A sensitive instrument that can detect, amplify, and record ground vibrations too small to be perceived by human beings. (Noson, et.al., 1988)
• An instrument that records the motions of the Earth, especially earthquakes. (USGS National Earthquake Information Center, 1999)
• A scientific instrument that detects and records vibrations (seismic waves) produced by earthquakes. (Teacher's Packet)

Seismograph station:

• A site at which one or more seismographs are set up and routinely monitored. (USGS National Earthquake Information Center, 1999)

Seismologist:

• A scientist who studies earthquakes. (USGS National Earthquake Information Center, 1999)

Seismometry:

• The instrumental aspects of seismology. (USGS National Earthquake Information Center, 1999)

Shallow earthquakes:

• ... shallow earthquakes, located under the dome at depths of less than 3 kilomenters, which produce medium-to low-frequency seismic arriavals ... (Brantley and Topinka, 1984)

Short-period earthquakes:

• ... During inflation the rocks surrounding the (magma) reservoir become stressed, and this stress is partly relieved by increasing numbers of earthquakes, too small to be felt, but easily recorded by seismometers at Kilauea (Hawaii) summit. These earthquakes (called short-period or tectonic) are recorded as high-frequency features on a seismograph ... (Tilling, et.al., 1987)

Short Period Seismic Station:

Strong Motion Seismic Station:

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. (USGS National Earthquake Information Center, 1999)

Subduction zone:

• The place where two lithosphere plates come together, one riding over the other. Most volcanoes on land occur parallel to and inland from the boundary between the two plates. (Teacher's Packet)

Subduction zone boundary:

• The region between converging plates, one of which dives beneath the other. The Cascadia subduction zone boundary is an example. (Noson, et.al., 1988)

Surface events:

• ... surface events, such as gas and tephra events, rockfalls associated with dome growth, and snow and rock avalanches from the crater walls, which produce complicated signatures with no clear beginning or end ... (Brantley and Topinka, 1984)

Surface waves:

• Waves that move over the surface of the Earth. Rayleigh waves and Love waves are surface waves. (USGS National Earthquake Information Center, 1999)
• Seismic waves, slower than P or S waves, that propagate along the Earth's surface rather than through the deep interior. Two principal types of surface waves, Love and Rayleigh waves, are generated during an earthquakes. Rayleigh waves cause both vertical and horizontal ground motion, and Love waves cause horizontal motion only. They both produce ground shaking at the Earth's surface but very little motion deep in the Earth. Because the amplitude of surface waves diminishes less rapidly with distance than the amplitude of P or S waves, surface waves are often the most important component of ground shaking far from the earthquake source. (USGS National Earthquake Information Center, 1999)

Tectonic:

• Pertaining to the forces involved in the deformation of the Earth's crust, or the structures or features produced by such deformation.

Tectonic earthquakes:

• Although all earthquakes associated with active volcanoes are ultimately related to volcanic processes, Volcanic earthquakes are directly associated with magma movement, while tectonic earthquakes occur in zones separated from the principal areas of magma movement. (Heliker, et.al., 1986)

Tremor:

• See: Harmonic tremor.

Tsunami:

• A tsunami is a series of very long wavelength ocean waves caused by the sudden displacement of water by earthquakes, landslides, or submarine slumps. Ordinarily, tsunamis are produced only by earthquakes exceeding magnitude 7.5. In the open ocean, tsunami waves travel at speeds of 600-800 kilometers per hour, but their wave heights are usually only a few centimeters. As they approach shallow water near a coast, tsunami waves travel more slowly, but their wave heights may increase to many meters, and thus they can become very destructive. (Noson, et.al., 1988)
• Tsunamis are large, rapidly moving ocean waves triggered by a major disturbance of the ocean floor, which is usually caused by an earthquake but sometimes can be produced by a submarine landslide or a volcanic eruption. (Heliker, 1990)
• Damaging earthquakes and sea waves (tsunami) may also be closely realted to volcanoes and volcanic activity. Large earthquakes related to intrusion of magma into Hawaii's active rift zones of Mauna Loa and Kilauea have caused extensive damage on land and also triggered tsunami in 1868 and 1975 that devastated low-lying coastal areas. Large landslides from Alaskan volcanoes near the sea have also genereated tsunami that destroyed coastal villages. (Wright and Pierson, 1992)
• Major earthquakes occurring along subduction zones are especially hazardous, because they can trigger tsunami (from the Japanese word tsunami meaning "harbor wave") and pose a potential danger to coastal communities and islands that dot the Pacific. ... tsunamis are seismic sea waves caused by earthquakes, submarine landslides, and, infrequently, by eruptions of island volcanoes. During a mojor earthquake, the seafloor can move by several meters and an enormous amount of water is suddenly set into motion, sloshing back and forth for several hours. Thes result is a series of waves that race across the ocean at speeds of more than 800 kilometers per hour, comparable to those of commercial jetliners. The energy and momentum of these transoceanic waves can take them thousands of kilometers from their origin before slamming into far-distant islands or coastal areas. ... The 1883 eruption of Krakatau Volcano, located in the Sunda Straits between the islands of Sumatra and Java, Indonesia, provides an excellent example of an eruption-caused tsunami. A series of tsunamis washed away 165 coastal villages on Java and Sumatra, killing 36,000 people ... (Kious and Tilling, 1996)

Volcanic earthquakes:

• Although all earthquakes associated with active volcanoes are ultimately related to volcanic processes, Volcanic earthquakes are directly associated with magma movement, while tectonic earthquakes occur in zones separated from the principal areas of magma movement. (Heliker, et.al., 1986)

Volcanic tremor:

• See: Harmonic tremor

• Brantley and Topinka, 1984, Volcanic Studies at the David A. Johnston Cascade Volcano Observatory: Earthquake Information Bulletin, March-April 1984, v.16, n.2
• Foxworthy and Hill, 1982, Volcanic Eruption of 1980 at Mount St. Helens: The First 100 Days: USGS Professional Paper 1249
• Heliker, 1990, Volcanic and Seismic Hazards on the Island of Hawaii: USGS General Interest Publication
• Heliker, et.al., 1986, Volcano Monitoring at the U.S.Geological Survey's Hawaiian Volcano Observatory, Earthquake Information Bulletin, v.18, n.1
• Kious and Tilling, 1996, This Dynamic Earth: The Story of Plate Tectonics: USGS General Interest Publication
• Noson, Qamar, and Thorsen, 1988, Washington State Earthquake Hazards: Washington Division of Geology and Earth Resources Information Circular 85
• Tilling, et.al., 1987, Eruptions of Hawaiian Volcanoes: Past, Present, and Future: USGS General Interest Publication
• U.S. Geological Survey's National Earthquake Information Center Website, 1999
• U.S. Geological Survey's VOLCANO! Teacher's Packet, 1997
• Wright and Pierson, 1992, Living With Volcanoes: The U.S. Geological Survey's Volcano Hazards Program: USGS Circular 1073

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