USGS/Cascades Volcano Observatory, Vancouver, Washington
DESCRIPTION:
Earthquakes and Seismicity
- Seismic Swarms
- Volcanic (Long-Period) vs. Tectonic (Short-Period)
- Deep - Shallow - Surface Events - Harmonic Tremor
- Harmonic Tremor
- Seismic Waves and Signals
- Magnitude and Intensity
- Magnitude - Richter Scale
- Intensity - Modified Mercalli Scale
From:
USGS Earthquake Hazards Program, National Earthquake Information Center Website, 2002
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A seismic swarm is
by definition a localized surge of earthquakes, with no one
shock being conspicuously larger than all other shocks of the
swarm. Seismic swarms typically last longer than more typical
earthquake sequences that consist of a main shock followed by
significantly smaller aftershocks. Seismic swarms occur in a variety of
geologic environments. They are not known to be indicative of any
change in the long-term seismic risk of the region in which they occur.
Volcanic (Long-Period) vs. Tectonic (Short-Period)
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From:
Heliker, et.al., 1986,
Volcano Monitoring at the U.S.Geological Survey's Hawaiian Volcano Observatory,
Earthquake Information Bulletin, v.18, n.1
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Seismic monitoring at HVO (Hawaiian Volcano Observatory) has helped to clarify
the distinction between the two main classes of earthquakes, volcanic and
tectonic. 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. Tectonic
earthquakes on Hawaii share characteristics with seismic events elsewhere
that are not associated with volcanic systems, such as the earthquakes generated
by the San Andreas fault zone in California.
From:
Tilling, et.al., 1987, Eruptions of Hawaiian Volcanoes:
Past, Present, and Future: USGS General Interest Publication
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During inflation the rocks surrounding the 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 summit. These earthquakes
(called short-period or tectonic) are recorded as high-frequency
features on a seismograph. During deflation the stress is completely relieved.
The short-period earthquakes stop, but their place is taken by 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. The long-period earthquakes are related to
harmonic tremor,
the continuous seismic record of underground magma movement.
Deep - Shallow - Surface Events - Harmonic Tremor
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From:
Brantley and Topinka, 1984,
Volcanic Studies at the David A. Johnston Cascade Volcano Observatory,
Earthquake Information Bulletin, March-April 1984, v.16, n.2
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The following major types of
seismograms
have been recognized at Mount St. Helens:
(1) deep earthquakes and those located away from the volcano,
which produce high-frequency signatures and sharp arrivals similar to
tectonic earthquakes,
(2) shallow earthquakes, located under the dome at depths of less than 3
kilomenters, which produce medium-to low-frequency seismic arriavals,
(3) 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, and
(4) harmonic tremor,
which is a long-lasting, very rhythmic signal whose
origin is not well understood but which is often associated with active
volcanoes.
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[Graphic,20K,GIF]
Four major types of seismograms, or "seismic signatures" --
Tectonic-like Earthquakes, Shallow Volcanic Earthquakes,
Surface Events, and Harmonic Tremor
From:
Foxworthy and Hill, 1982,
Volcanic Eruption of 1980 at Mount St. Helens: The First 100 Days,
USGS Professional Paper 1249
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Harmonic Tremor:
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.
From:
Tilling, Topinka, and Swanson, 1990, Eruptions of Mount St. Helens:
Past, Present, and Future: USGS General Interest Publication
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... volcanic tremor, a type of continuous, rhythmic ground shaking
different from the discrete sharp jolts characteristic of earthquakes. Such
continuous ground vibrations, commonly associated with eruptions at volcanoes in
Hawaii, Iceland, Japan, and elsewhere, are interpreted to reflect subsurface
movement of fluids, either gas or magma. ...
From:
Tilling, Heliker, and Wright, 1990, Eruptions of Hawaiian Volcanoes:
Past, Present, and Future: USGS General Interest Publication
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All Hawaiian eruptions are accompanied by harmonic tremor (also called
volcanic tremor). Quite distinct from the discrete seismic shocks
associated with rupture-caused earthquakes, harmonic tremor is a continuous
vibration of the ground caused by magma movement. Harmonic tremor generally is
detectable and recorded only by seismic instrumentation; however, if especially
vigorous, tremor can be felt by people as far as 5 miles from the eruption
site.
Seismic Waves and Signals
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From:
USGS National Earthquake Information Center (NEIC) Website, 1999
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Seismic waves are the vibrations from earthquakes
that travel through the Earth; they are recorded on instruments called
seismographs.
Seismographs record a zig-zag trace that shows the
varying amplitude of ground oscillations beneath the
instrument. Sensitive seismographs, which greatly
magnify these ground motions, can detect strong earthquakes from
sources anywhere in the world. The time, locations, and
magnitude
of an earthquake can be determined from the data
recorded by seismograph stations.
From:
USGS National Earthquake Information Center (NEIC) Website, 1999
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When an earthquake occurs, it releases energy in
the form of waves that radiate from the earthquake source in all
directions. The different types of energy waves
shake the ground in different ways and also travel through the earth at
different velocities. The fastest wave, and therefore
the first to arrive at a given location, is called the P wave. The P
wave, or compressional wave, alternately compresses
and expands material in the same direction it is traveling. The
S wave is slower than the P wave and arrives next,
shaking the ground up and down and back and forth perpendicular to
the direction it is traveling. Surface waves
follow the P and S waves.
From:
Noson, Qamar, and Thorsen, Washington State Earthquake Hazards,
1988, Washington Division of Geology and Earth Resources Information Circular 85
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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.)
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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.
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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.
-
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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.
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S (Secondary or Shear) Waves:
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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.
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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.
From:
Noson, Qamar, and Thorsen, Washington State Earthquake Hazards,
1988, Washington Division of Geology and Earth Resources Information Circular 85
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The size of an earthquake is indicated by a
number called its magnitude.
Magnitude is calculated from a measurement
of either the amplitude
or the duration of specific types of recorded seismic waves.
Magnitude is determined from measurements made from
seismograms
and not on reports of shaking or interpretations of building damage.
In general, the
different magnitude scales (for example,
local or Richter magnitude
and surface wave magnitude)
give similar numerical estimates
of the size of an earthquake, and all display a logarithmic relation to recorded ground
motion. That means each unit increase in magnitude represents an increase in the size of the
recorded signal by a factor of 10. Therefore, a magnitude 7 earthquake would have a maximum
signal amplitude 10 times greater than that of a magnitude 6 earthquake and 100 times greater
than that of a magnitude 5 earthquake. Seismologists sometimes refer to the size of an
earthquake as moderate (magnitude 5), large (magnitude 6), major (magnitude 7), or great
(magnitude 8). ...
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The intensity of an earthquake is a measure of the amount of ground shaking at a
particular site, and it is determined from reports of human reaction to shaking, damage done
to structures, and other effects. The
Modified Mercalli Intensity Scale
is now the scale most commonly used to rank earthquakes felt in the United States. If
magnitude is compared to the power output of a radio broadcasting station, then the intensity
of an earthquake is the signal strength at a particular radio receiver. In practice, an
earthquake is assigned one magnitude, but it may give rise to reports of intensities at many
different levels. The magnitude 6.5 April 29, 1965, Seattle-Tacoma earthquake produced
intensity VII to VIII damage near its epicenter, intensity V damage 150 kilometers from the
epicenter. ...
Magnitude - Richter Scale
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From:
Noson, Qamar, and Thorsen, Washington State Earthquake Hazards,
1988, Washington Division of Geology and Earth Resources Information Circular 85
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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.
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In general, the different magnitude scales
(for example, local or Richter magnitude
and surface wave magnitude)
give similar numerical estimates
of the size of an earthquake, and all display
a logarithmic relation to recorded ground
motion. That means each unit increase in
magnitude represents an increase in the size of the
recorded signal by a factor of 10. Therefore,
a magnitude 7 earthquake would have a maximum
signal amplitude 10 times greater than that of
a magnitude 6 earthquake and 100 times greater
than that of a magnitude 5 earthquake.
Seismologists sometimes refer to the size of an
earthquake as moderate (magnitude 5), large (magnitude 6),
major (magnitude 7), or great (magnitude 8).
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An earthquake magnitude scale, more properly called local magnitude
scale, based on measurements of the amplitude of
earthquake waves
recorded on a standard Wood-Anderson type seismograph
at a distance of less than 600 kilometers from the epicenter.
From:
USGS National Earthquake Information Center (NEIC) Website, 2000
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The idea of a logarithmic earthquake magnitude scale was first
developed by Charles Richter in the 1930's for measuring the size of
earthquakes occurring in southern California using relatively
high-frequency data from nearby seismograph stations. This
magnitude scale was referred to as ML, with the L standing for local.
This is what was to eventually become known as the Richter magnitude.
From:
USGS National Earthquake Information Center (NEIC) Website, 1998
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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.
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At first, the Richter Scale could be applied only to the records from
instruments of identical manufacture. Now, instruments
are carefully calibrated with respect to each other. Thus, magnitude
can be computed from the record of any calibrated
seismograph.
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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. Events with magnitudes of
about 4.5 or greater - there are several
thousand such shocks annually - are strong enough to be recorded by
sensitive seismographs all over the world. Great
earthquakes, such as the 1964 Good Friday earthquake in Alaska,
have magnitudes of 8.0 or higher. On the average, one
earthquake of such size occurs somewhere in the world each year.
Although the Richter Scale has no upper limit, the largest
known shocks have had magnitudes in the 8.8 to 8.9 range.
Recently, another scale called the moment magnitude scale has
been devised for more precise study of great earthquakes.
The Richter Scale is not used to express damage. An earthquake
in a densely populated area which results in many deaths and
considerable damage may have the same magnitude as a shock
in a remote area that does nothing more than frighten the wildlife.
Large-magnitude earthquakes that occur beneath the oceans
may not even be felt by humans.
Intensity - Modified Mercalli Scale
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From:
USGS National Earthquake Information Center (NEIC) Website, 1998
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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.
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The Modified Mercalli Intensity value
assigned to a specific site
after an earthquake has a more meaningful measure of
severity to the nonscientist than the magnitude because intensity
refers to the effects actually experienced at that place. After
the occurrence of widely-felt earthquakes, the Geological Survey
mails questionnaires to postmasters in the disturbed area
requesting the information so that intensity values can be assigned.
The results of this postal canvass and information furnished
by other sources are used to assign an intensity within the felt area.
The maximum observed intensity generally occurs near the
epicenter.
From:
Noson, Qamar, and Thorsen, Washington State Earthquake Hazards,
1988, Washington Division of Geology and Earth Resources Information Circular 85
-
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The intensity of an earthquake is a measure
of the amount of ground shaking at a
particular site, and it is determined from reports
of human reaction to shaking, damage done
to structures, and other effects. The
Modified Mercalli Intensity Scale
is now the scale most commonly used to rank earthquakes
felt in the United States.
If magnitude is compared to the power
output of a radio broadcasting station, then the intensity
of an earthquake is the signal strength at a
particular radio receiver. In practice, an
earthquake is assigned one magnitude,
but it may give rise to reports of intensities at many
different levels. The magnitude 6.5 April 29, 1965,
Seattle-Tacoma earthquake produced
intensity VII to VIII damage near its epicenter,
intensity V damage 150 kilometers from the
epicenter.
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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.
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Modified Mercalli Intensity Scale:
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I.
Not felt except by a very few under especially favorable circumstances.
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II.
Felt only by a few persons at rest, especially on upper floors of
buildings. Delicately suspended objects may swing.
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III.
Felt quite noticeably by persons indoors, especially on upper floors of
buildings. Many people do not recognize it as an earthquake. Standing motor
cars may rock slightly. Vibration similar to the passing of truck. Duration
estimated.
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IV.
Felt indoors by many, outdoors by few during the day. At night, some
awakened. Dishes, windows, doors disturbed; walls make cracking sound.
Sensation like heavy truck striking building. Standing motor cars rocked
noticeably.
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V.
Felt by nearly everyone; many awakened. Some dishes, windows broken.
unstable objects overturned. Pendulum clocks may stop.
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VI.
Felt by all; many frightened. Some heavy furniture moved; a few
instances of fallen plaster. Damage slight.
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VII.
Damage negligible in building of good design and construction; slight to
moderate in well-built ordinary structures; considerable damage in poorly built
or badly designed structures; some chimneys broken. Noticed by persons driving
motor cars.
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VIII.
Damage slight in specially designed structures; considerable in ordinary
substantial buildings with partial collapse. Damage great in poorly built
structures. Fall of chimneys, factory stacks, columns, monuments, walls. Heavy
furniture overturned.
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IX.
Damage considerable in specially designed structures; well-designed
frame structures thrown out of plumb. Damage great in substantial buildings,
with partial collapse. Buildings shifted off foundations.
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X.
Some well-built wooden structures destroyed; most masonry and frame
structures destroyed with foundations. Rails bent.
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XI.
Few, if any (masonry) structures remain standing. Bridges destroyed.
Rails bent greatly.
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XII.
Damage total. Lines of sight and level distorted. Objects thrown into
the air.
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09/22/04, Lyn Topinka