|
Measuring
Earthquakes
Q: Where can I buy a Richter
scale?
|
A: The Richter scale is not
a physical device, but a mathematical formula. The magnitude
of an earthquake is determined from the logarithm of the
amplitude of waves recorded on a seismogram at a certain
period.
See next question and answer.
For further information, see:
Magnitude & Intensity Links
|
Q: How are earthquakes recorded?
How are earthquakes measured? How is the magnitude of an
earthquake determined?
|
A: Earthquakes are recorded
by a seismographic network. Each seismic station in the
network measures the movement of the ground at the site.
The slip of block of rock over another in an EQ releases
energy that makes the ground vibrate. That vibration pushes
the adjoining piece of ground and cause it to vibrate and
thus the energy travel out from the EQ in a wave. There
are many different ways to measure different aspects of
an earthquake. Magnitude is the most common measure
of an earthquake's size. It is a measure of the size of
the earthquake source and is the same number no matter where
you are or what the shaking feels like. The Richter scale
measures the largest wiggle on the recording, but other
magnitude scales measure different parts of the earthquake.
Intensity is a measure of the shaking and damage
caused by the earthquake, and this value changes from location
to location.
See also the discussion in the Effects
section.
For further information, see:
Magnitude
& Intensity, NEIC
UC
Berkeley Seismo Lab FAQ on Recording Earthquakes
UC
Berkeley Seismo Lab FAQ on Measuring Earthquakes
UC
Berkeley Seismo Lab FAQ on Different Magnitudes
|
Q: What are the different
magnitude scales, and why are there so many?
|
A: Earthquake size, as measured
by the Richter Scale is a well known, but not well
understood, concept. 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.
As more seismograph stations were
installed around the world, it became apparent that the
method developed by Richter was strictly valid only for
certain frequency and distance ranges. In order to take
advantage of the growing number of globally distributed
seismograph stations, new magnitude scales that are an extension
of Richter's original idea were developed. These include
body-wave magnitude, mb, and surface-wave magnitude,
Ms. Each is valid for a particular frequency range
and type of seismic signal. In its range of validity each
is equivalent to the Richter magnitude. Because of the limitations
of all three magnitude scales, ML, mb, and Ms, a new, more
uniformly applicable extension of the magnitude scale, known
as moment magnitude, or Mw, was developed. In particular,
for very large earthquakes moment magnitude gives the most
reliable estimate of earthquake size. New techniques that
take advantage of modern telecommunications have recently
been implemented, allowing reporting agencies to obtain
rapid estimates of moment magnitude for significant earthquakes.
|
Q: Why are there often different
magnitudes reported for the same earthquake?
|
A:
When an earthquake occurs, the first information
that is processed and relayed is usually based on a small
subset of the seismic stations in the network, especially
in the case of a larger earthquake. This is done so that
some information can be obtained immediately without waiting
for all of it to be processed. As a result, the first magnitude
reported is usually based on a small number of recordings.
As additional data are processed and become available, the
magnitude and location are refined and updated. Sometimes
the assigned magnitude is "upgraded" or slightly
increased, and sometimes it is "downgraded" or
slightly decreased.
Sometimes the earthquake magnitude is reported by different
networks based on only their recordings. In that case, the
different assigned magnitudes are a result of the slight
differences in the instruments and their locations with
respect to the earthquake epicenter.
For further information, see:
Measuring
the Size of an Earthquake, NEIC
|
Q: What is "moment magnitude"?
|
A: Moment is a physical quantity
proportional to the slip on the fault times the area of
the fault surface that slips; it is related to the total
energy released in the EQ. The moment can be estimated from
seismograms (and also from geodetic measurements). The moment
is then converted into a number similar to other earthquake
magnitudes by a standard formula. The results is called
the moment magnitude. The moment magnitude provides an estimate
of earthquake size that is valid over the complete range
of magnitudes, a characteristic that was lacking in other
magnitude scales.
For further
information, see:
Magnitude
& Intensity, NEIC
|
Q: What are the earthquake
magnitude classes?
|
A:
Great; M > =8
Major; 7 < =M < 7.9
Strong; 6 < = M < 6.9
Moderate: 5 < =M < 5.9
Light: 4 < =M < 4.9
Minor: 3 < =M < 3.9
Micro: M < 3
|
Q: How do you give a Richter
magnitude to earthquakes that occurred prior to the scale?
|
A: For earthquakes that occurred
between about 1890 (when modern seismographs came into use)
and 1935 when Charles Richter developed the magnitude scale,
people went back to the old records and compared the seismograms
from those days with similar records for later earthquakes.
For earthquakes prior to about 1890, magnitudes have been
estimated by looking at the physical effects (such as amount
of faulting, landslides, sandblows or river channel changes)
plus the human effects (such as the area of damage or felt
reports or how strongly a quake was felt) and comparing
them to modern earthquakes. Many assumptions have to be
made when making these comparisons. For example, how do
you compare the shaking for people living in log cabins
or tents in the early 1800's with shaking for people living
in high-rise steel and concrete buildings (with waterbeds!)
in the 1990's? Because different researchers can get widely
varying magnitudes from using different assumptions on how
to make these comparisons, many of the old earthquakes have
big differences in the magnitudes assigned to them. For
example, magnitude estimates for the quakes that occurred
near New Madrid, Missouri in 1811 and 1812 vary from the
upper magnitude 6 range to as high as 8.8, all because of
the choices the researchers made about how to compare the
data.
For further
information, see:
The
Richter Magnitude Scale
|
Q: When was the first instrument
that actually recorded an earthquake?
|
A: The earliest seismoscope
was invented by the Chinese philosopher Chang Heng in A.D.
132. This was a large urn on the outside of which were eight
dragon heads facing the eight principal directions of the
compass. Below each dragon head was a toad with its mouth
opened toward the dragon. When an earthquake occurred, one
or more of the eight dragon-mouths would release a ball
into the open mouth of the toad sitting below. The direction
of the shaking determined which of the dragons released
its ball. The instrument is reported to have detected an
earthquake 400 miles away that was not felt at the location
of the seismoscope. The inside of the seismoscope is unknown:
most speculations assume that the motion of some kind of
pendulum would activate the dragons.
|
Note: original source of this
image is unknown |
For further information, see:
The
Early History of Seismometry (to 1900)
|
Q: What is a P wave? An
S wave?
|
A: 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.
|
Note: original source of this image is unknown |
|
Q: What was the duration
of the earthquake?
|
A: The duration
of shaking you feel from an earthquake depends in part on
the distance you are from the epicenter of the earthquake.
If you are close, the shaking will be more violent, "faster",
and may not last as long. If you are further away, the high-frequency
"fast" shaking will have been "absorbed" into the earth's
crust, you will feel are the longer-period, more rolling motions,
and they may be of longer duration. In short, the duration
is different in different places. |
Q: What does an earthquake
look like?
|
A: In order to study earthquakes,
scientists deploy seismometers to measure ground motion.
Seismograms are recordings of ground motion as a function
of time and are the basic data which seismologists use to
study the waves generated by earthquakes. These data are
used to study the earthquakes themselves and to learn more
about the structure of the Earth.
Seismologists generally describe
earthquakes as local, regional, or teleseismic. These terms
refer to distance from the earthquake to the recording instrument.
Local events occur within the immediate area less than 100km
away. Regional events occur within 10 - 1400km away. Teleseismic
events are those which occur at great distances, greater
than 1400km away. Local and regional earthquakes are dominated
by crustal waves, i.e., by waves which propagate through
the crust. At greater distances, the seismic wavefield is
dominated by waves which sample the body of the earth -
the upper mantle, the lower mantle, and the core.
Earthquake Examples:
Local or Near-Field Earthquake
Regional Earthquake
Teleseismic Earthquake
|
Q: How do seismologists locate
an earthquake?
|
A:When an earthquake occurs,
one of the first questions is "where was it?" The location
may tell us what fault it was on and where damage (if any)
most likely occurred.
Unfortunately, the earth is not transparent
and we can't just see or photograph the earthquake disturbance
like meteorologists can photograph clouds. When an earthquake
occurs, it generates an expanding wavefront from the earthquake
hypocenter at a speed of several kilometers per second.
We observe earthquakes with a network
of seismometers on the earth's surface. The ground motion
at each seismometer is amplified and recorded electronically
at a central recording site. As the wavefront expands from
the earthquake, it reaches more distant seismic stations.
When an earthquake occurs, we observe
the times at which the wavefront passes each station. We
must find the unknown earthquake source knowing these wave
arrival times. Here is a map of U.S. Geological Survey seismic
stations in the San Francisco Bay Area and 6 seismograms
from an earthquake:
We want to find the location, depth
and origin time of an earthquake whose waves arrive at the
times measured on each seismograms. We want a straightforward
and general procedure that we can also program in a computer.
The procedure is simple to state:
guess a location, depth and origin time; compare the predicted
arrival times of the wave from your guessed location with
the observed times at each station; then move the location
a little in the direction that reduces the difference between
the observed and calculated times. Then repeat this procedure,
each time getting closer to the actual earthquake location
and fitting the observed times a little better. Quit when
your adjustments have become small enough and when the fit
to the observed wave arrival times is close enough.
You can try to fit an earthquake
location on the map just to see how the procedure goes.
Note that the earthquake arrives first on station C, thus
C is a good first guess for the location. Many earthquakes
in California occur between 2 and 12 kilometers depth and
we will guess a 6 km. depth. The origin time should be a
few seconds before the time of the wave at the first station.
Let's guess an origin time of 10 seconds, measured on the
same clock that made the time scale at the bottom of the
figure and timed the seismograms. Then we can list the tentative
travel times by subtracting the origin time from the observed
arrival times:
station..................A B C D E F
observed time..........16.5 17.8 11.3 15.2 22.3 18.3
tentative travel time...6.5 7.8 1.3 5.2 12.3 8.3
Note the scale at the left of the figure.
It shows travel times for waves from an earthquake at a depth
of 6 kilometers. The scale starts at 1.3 seconds because the
wave reaches the surface 1.3 seconds after the earthquake
origin time. You can make a tracing of the scale and move
the earthquake on the map until the tentative travel times
match the travel times from the scale. Where do you think
the earthquake was? Are the times for each station systematically
early or late, requiring a shift in the origin time?
To open a window with the earthquake
location shown on the map, CLICK
HERE.
The earthquake was near station C.
The depth was about 6 km and the origin time was about 10
seconds. (We guessed very well!) A real magnitude 3.4 earthquake
occurred at this location on April 29, 1992. It was felt
by many people who were sitting or at rest.
Mathematically, the problem is solved
by setting up a system of linear equations, one for each
station. The equations express the difference between the
observed arrival times and those calculated from the previous
(or initial) hypocenter, in terms of small steps in the
3 hypocentral coordinates and the origin time. We must also
have a mathematical model of the crustal velocities (in
kilometers per second) under the seismic network to calculate
the travel times of waves from an earthquake at a given
depth to a station at a given distance. The system of linear
equations is solved by the method of least squares which
minimizes the sum of the squares of the differences between
the observed and calculated arrival times. The process begins
with an initial guessed hypocenter, performs several hypocentral
adjustments each found by a least squares solution to the
equations, and iterates to a hypocenter that best fits the
observed set of wave arrival times at the stations of the
seismic network. |
Q: What is intensity? What
is the Modified Mercalli Intensity Scale?
|
A:
The Mercalli Scale is based on observable
EQ damage. From a scientific standpoint, the Richter scale
is based on seismic records while the Mercalli is based
on observable data which can be subjective. Thus, the Richter
scale is considered scientifically more objective and therefore
more accurate. For example a level I-V on the Mercalli scale
would represent a small amount of observable damage. At
this level doors would rattle, dishes break and weak or
poor plaster would crack. As the level rises toward the
larger numbers, the amount of damage increases considerably.
The top number, 12, represents total damage.
For further information, see:
Modified
Mercalli Intensity Scale
Magnitude/Intensity
Comparison
|
Q: What is the difference
between intensity scales and magnitude scales?
|
A: Intensity scales, like
the Modified Mercalli Scale and the Rossi-Forel scale, measure
the amount of shaking at a particular location. So the intensity
of an earthquake will vary depending on where you are. Sometimes
earthquakes are referred to by the maximum intensity they
produce. Magnitude scales, like the Richter magnitude and
moment magnitude, measure the size of the earthquake at
its source. So they do not depend on where the measurement
is made. Often, several slightly different magnitudes are
reported for an earthquake. This happens because the relation
between the seismic measurements and the magnitude is complex
and different procedures will often give slightly different
magnitudes for the same earthquake.
For further information, see:
Magnitude/Intensity
Comparison
Modified
Mercalli Intensity Scale
|
Q: How much energy is released
in an earthquake?
|
A:
Earthquakes release a tremendous amount of energy,
which is why they can be so destructive. The table below
shows magnitudes with the approximate amount of TNT needed
to release the same amount of energy.
Magnitude |
Approximate Equivalent TNT Energy |
4.0 |
1010 tons |
5.0 |
31800 tons |
6.0 |
1,010,000 tons |
7.0 |
31,800,000 tons |
8.0 |
1,010,000,000 tons |
9.0 |
31,800,000,000 tons |
|
Q: What is acceleration,
velocity, and displacement?
|
A: Acceleration
is the rate of change in velocity of the ground shaking (how
much the velocity changes in a unit time), just as it is the
rate of change in the velocity of your car when you step on
the accelerator or put on the brakes. Velocity is the measurement
of the speed of the ground motion. Displacement is the measurement
of the actual changing location of the ground due to shaking.
All three of the values can be measured continuously during
an earthquake. |
Q: What is spectral acceleration?
|
A: PGA (peak acceleration)
is what is experienced by a particle on the ground. SA is
approximately what is experienced by a building, as modeled
by a particle on a massless vertical rod having the same
natural period of vibration as the building.
For further information see:
Natl.
Seismic Hazard Mapping Program FAQ
|
Q: What are those beachball
figures?
|
A:
In addition to determining the location
and magnitude of earthquakes, seismologists are now routinely
determining the "fault plane" solutions or "focal mechanisms"
of events. A fault plane solution illustrates the direction
of slip and the orientation of the fault during the earthquake.
These solutions, which are displayed in lower-hemisphere
projections frequently described as "beachballs", can be
determined from the first-motion of P-waves and from the
inversion of seismic waveforms. These figures help identify
the type of earthquake rupture: strike-slip, normal, or
thrust. Strike-slip earthquakes are typical of the San Andreas
fault zone, which forms part of the boundary between the
North American and Pacific plates. Normal earthquakes are
associated with extension, particularly with formation of
plates at mid-ocean ridges. Thrust or reverse earthquakes
are associated with compression, particularly with the subduction
of one plate under another as in Japan. (UC Berkeley)
For further information see:
Focal
Mechanisms
|
Q: What are UTC and GMT
(in reference to the time of an EQ)?
|
A: UTC stands for Coordinated Universal
Time, and GMT stands for Greenwich Mean Time. The
time that earthquakes occur around the world is reported
in UTC or GMT, which are essentially the same.
For further information see:
Time
Information - from NEIC
|
Q:
What does it mean that the earthquake occurred at a depth
of 0 km?
|
A: An earthquake cannot occur
at depth of 0 km. In order for an earthquake to occur, two
blocks of crust must slip past one another, and it is physically
impossible for this to happen at the surface of the earth.
So why do we report that the earthquake occured at a depth
of 0 km sometimes? Sometimes it is simply a very shallow
event with poor depth resolution, but more often it is not
actually an earthquake, but a quarry blast. These explosions
are recorded by the seismic network and located by the software.
When they are reviewed by a seismic analyst, they are labaled
as a quarry blast in the earthquake catalog. |
Back to Earthquake FAQ Main
page
|
|