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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.
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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.
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Q:
What are the different magnitude
scales, and why are there so many?
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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.
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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.
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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.
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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
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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
area 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.
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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.
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Note: original source of this image is unknown |
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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.
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Note: original source of this image is unknown |
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Q: What was the duration of the earthquake?
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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.
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Q: What does an earthquake look like?
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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:
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Local or Near-Field Earthquake |
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Regional Earthquake |
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Q: How do seismologists locate an earthquake?
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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?
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.
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Q: What is intensity? What is the Modified Mercalli Intensity Scale?
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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.
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Q: What is the difference between intensity scales and magnitude scales?
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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.
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Q: How much energy is released in an earthquake?
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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.
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Magnitude |
Approximate Equivalent TNT Energy |
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4.0 |
1010 tons |
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5.0 |
31800 tons |
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6.0 |
1,010,000 tons |
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7.0 |
31,800,000 tons |
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8.0 |
1,010,000,000 tons |
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9.0 |
31,800,000,000 tons |
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Q: What is acceleration, velocity, and displacement?
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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.
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Q: What is spectral acceleration?
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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.
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Q: What are those beachball figures?
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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.
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Q: What are UTC and GMT (in reference to the time of an EQ)?
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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.
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Q: What does it mean that the earthquake occurred at a depth of 0 km?
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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.
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