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Roger
Weller, geology instructor
Earthquakes
Jessica Downer
Spring 2005
Physical Geology
Measuring Earthquakes
Seismologists
use two main devices to measure an earthquake: a seismograph and a
seismoscope. The seismograph is an instrument
that measures seismic waves caused by an earthquake. The seismograph has three main devices, the
Richter Magnitude Scale, the Modified Mercalli Intensity Scale, and the
Moment-Magnitude Scale. The seismoscope
is an instrument that measures the occurrence or the time of an occurrence of
an earthquake (“Inventors”). Unlike
other measuring devices, the seismoscope is a simple device without any
technological background. The
seismoscope is the oldest and most accurate instrument for measuring direction.
First
invented in 132 AD, the Dragon Jar was the first instrument for determining the
direction of an earthquake. 
(Photo by Peter Bormann). Chang Heng, a Chinese scientist, developed the Dragon
Jar. The Dragon Jar consists of a large
jar with eight dragons protruding around the top. Each dragonhead holds a ball in its mouth
while a frog sits with its mouth open directly underneath. Behind each dragonhead lies a trigger. Down the center of the jar is a thin stick
that is loosely secured. The tremors of
an earthquake cause the stick to fall on one of the eight triggers. When the trigger is set off, the dragonhead
linked to the trigger drops the ball into the frog’s mouth. The sound of the ball dropping into the
frog’s mouth indicated an earthquake had just occurred. By looking at which ball dropped, the
direction of the earthquake could be determined. Heng’s seismoscope was not only the first
seismoscope but also very accurate and precise.
In 138 AD, Heng’s seismoscope detected an earthquake 1,000 miles away
(“Chinese”). Wang Zhenduo recreated
Chang Heng’s seismoscope in 1951.
Instead of a thin stick loosely secured in the center of the jar,
Zhenduo replaced it with a copper pendulum shaft that connected to eight copper
arms (“Zhang Heng”). Like the
seismoscope, other devices used to gather information on earthquakes were also
first developed outside the
Nicholas Cirillo developed the first
mechanical device used to study earthquakes in 1731. A series of earthquakes in
D. Domenico Salsano, a clockmaker and mechanic, invented a similar
device to Nicholas Cirillo. Salsano’s
device was a long pendulum with a brush connected at the tip. The brush would then trace the motions of the
earthquake’s tremors with slow-drying ink onto an ivory slab. Salsano’s device had a bell attached that
rang when the tremors were large enough (Kauffman and Judson 181). James Forbes also created a similar
mechanical device. His device was called
the “Inverted-pendulum Seismometer.”
This device was a metal rid with a movable base (Kauffman and Judson
181). Later in 1855, Luigi Palmier of
The mercury seismometer had U-shaped tubes filled with
mercury and arranged along the compass points.
When an earthquake occurred, the movement of the mercury made an
electrical contact that stopped a clock and started the recording drum. The motion of a float on the surface of
mercury was recorded on the drum. Palmier’s
seismometer was the first device to accurately record the time and of an
earthquake while also recording the duration and the intensity of the
earthquake (“Inventors”). The
instruments used to gather information on earthquakes are seismographs.
In 1880 John Milne, an English
seismologist and geologist, is credited for the development of the first modern
seismograph in 1880. Milne called his
seismograph the “Horizontal seismograph” (“Inventors”). Milne’s seismograph consists of three parts:
the inertia member, the transducer, and a recorder. 
(Drawing by
Professor Stephen A. Nelson) The inertia
member is a weight suspended by a wire or a spring. It is similar to a pendulum; however, it can
only swing in one direction. The
transducer is a device that detected the motion between the mass and the
ground. This motion is then converted
into a form that can be recorded. The
transducer can be a mechanical lever or an electrodynamic system. In an electrodynamic system, a coil of wire
moved back and forth in a magnetic field.
This movement created an electric current that passed through a
galvanometer then recorded on a sheet of paper (Kauffman and Judson 182). Sir James Alfred Ewing, Thomas Grey and John
Milne founded the Seismological Society of
The Horizontal Pendulum seismograph
was improved after World War II. The new
device is called the Press-Ewing seismograph.
The Press-Ewing seismograph is widely used throughout the
The design of a seismograph is a weight freely
suspended from a support that is attached to bedrock. When the seismic waves reached the
seismograph, the inertia of the weight kept the device stable while the ground
and support shake. The movement of the
ground in relation to the movement of the weight it recorded onto a piece of
paper that is wrapped around a rotating drum (Lutgens and Edwards 306). To record motion in all directions, three
seismographs are required. One
seismograph is needed to measure vertical motion, and two to record horizontal
motion. The two seismographs recording
horizontal directions, record in 90-degree angles (Kauffman and Judson 182).
Seismographs record in a zigzag trace that shoes the
varying amplitude of ground oscillations beneath the instrument. Depending on the sensitivity of the
seismograph, earthquakes can be detected anywhere in the world. Using data collected by the seismographs, the
time, location, and the magnitude can be determined (Bellis). The magnitude of an earthquake can be
determined by a mathematical formula called the Richter Magnitude Scale.
Charles F. Richter developed the Richter Magnitude Scale
in 1934. Richter defined the scale as
“The logarithm to base 10 of the maximum seismic wave amplitude recorded on a
standard seismograph at a distance of 100 kilometers from the earthquake
epicenter.” The seismic wave used in the
calculation is not specified (Bolt 104).
Because it is not specified, S waves or P waves can be used. The Richter scale was originally designed by
Richter to differentiate between earthquakes with a low focus point in southern
Reading the Richter scale is a difficult task to
accomplish. Each whole number increase
in the scale releases 32 times more energy than the preceding number (
|
Richter magnitude |
earthquake effects |
|
less than 3.5 |
Generally not felt, but recorded. |
|
3.5-5.4 |
Often felt, but rarely causes damage. |
|
Under 6.0 |
At most slight damage to well-designed buildings. Can cause major damage to poorly constructed buildings over small regions. |
|
6.1-6.9 |
Can be destructive in areas up to about 100 kilometers across where people live. |
|
7.0-7.9 |
Major earthquake. Can cause serious damage over larger areas. |
|
8 or greater |
Great earthquake. Can cause serious damage in areas several hundred kilometers across. |
|
|
|
(“Intensity”) Like the Richter scale, the
Modified Mercalli Intensity Scale measures an earthquake at its source so where
the measurement is made is insufficient (“Measuring Earthquakes”).
American seismologists Harry Wood
and Frank Neumann developed the Modified Mercalli Intensity Scale in 1931. Wood and Neumann extended the Modified
Mercalli Intensity Scale to twelve Roman Numerals instead of the original
ten. Unlike the Richter scale, the
modified Mercalli scale ranks bases on observed effects. The effect an earthquake has on a particular
area of land is the intensity of the earthquake. These effects range from waking people up to
complete destruction (“Severity”).
|
I |
instrumental |
People do not feel any Earth movement. |
|
II |
lightest |
A few people might notice movement if they are at rest and/or on the upper floors of tall buildings. |
|
|
light |
Many people indoors feel movement. Hanging objects swing back and forth. People outdoors might not realize that an earthquake is occurring. |
|
IV |
mediocre |
Most people indoors feel movement. Hanging objects swing. Dishes, windows, and doors rattle. The earthquake feels like a heavy truck hitting the walls. A few people outdoors may feel movement. Parked cars rock. |
|
V |
strongly |
Almost everyone feels movement. Sleeping people are awakened. Doors swing open or close. Dishes are broken. Pictures on the wall move. Small objects move or are turned over. Trees might shake. Liquids might spill out of open containers. |
|
VI |
much fort |
Everyone feels movement. People have trouble walking. Objects fall from shelves. Pictures fall off walls. Furniture moves. Plaster in walls might crack. Trees and bushes shake. Damage is slight in poorly built buildings. No structural damage. |
|
|
strong |
People have difficulty standing. Drivers feel their cars shaking. Some furniture breaks. Loose bricks fall from buildings. Damage is slight to moderate in well-built buildings; considerable in poorly built buildings. |
|
VIII |
violent |
Drivers have trouble steering. Houses that are not bolted down might shift on their foundations. Tall structures such as towers and chimneys might twist and fall. Well-built buildings suffer slight damage. Poorly built structures suffer severe damage. Tree branches break. Hillsides might crack if the ground is wet. Water levels in wells might change. |
|
IX |
disastrous |
Well-built buildings suffer considerable damage. Houses that are not bolted down move off their foundations. Some underground pipes are broken. The ground cracks. Reservoirs suffer serious damage. |
|
X |
most disastrous |
Most buildings and their foundations are destroyed. Some bridges are destroyed. Dams are seriously damaged. Large landslides occur. Water is thrown on the banks of canals, rivers, lakes. The ground cracks in large areas. Railroad tracks are bent slightly. |
|
XI |
catastrophic |
Most buildings collapse. Some bridges are destroyed. Large cracks appear in the ground. Underground pipelines are destroyed. Railroad tracks are badly bent. |
|
XII |
great catastrophe |
Almost everything is destroyed. Objects are thrown into the air. The ground moves in waves or ripples. Large amounts of rock may move. |
(“Intensity”) After the occurrence of an earthquake, the
Geological Survey mails questionnaires to post offices in that particular
area. The questionnaires are distributed
to everyone in that area requesting information about the earthquake’s
damage. That information is used to
create an isoseismal math that shows the various levels of intensity in that
area (“Severity”). The Modified Mercalli
Scale however does have its dependencies: population density, building methods
and materials, and distance from the epicenter.
The intensity of an earthquake can be underestimated in an area that is
densely populated. The amount of damage
done by earthquakes depends on how well a building is built. The building methods and materials used to
build the structures affect how much damage is caused. This variance in damage could give the same
earthquake a different intensity value.
An area farther from the epicenter will receive less damage than closer
areas. Because each area has a different
intensity value, it is difficult to compare individual results (McConnel). Moment magnitude is similar to Modified Mercalli Scale
however is based on physical changed of the earth.
The moment magnitude is based on the amount of displacement that
occurred along a fault zone rather than the measurement of ground motion at a
given point (Lutgens and Edwards 314).
The Moment magnitude measures energy released by the earthquake more
accurately than the Richter scale. The
amount of energy released is dependent of a rock’s properties, area of the
fault surface, and amount of movement along the fault zone (McConnel). Seismologists calculate Moment magnitude from
seismograms after long-period waves are examined (Lutgens and Edwards 314). Moment magnitude is calculated with more accuracy
with large earthquakes (McConnel).
Moment magnitude gained support and acceptance among many seismologists
and engineers. Seismologists and
engineers accepted Moment Magnitude because:
1)
It is the only
magnitude scale that adequately measures the size of large earthquakes
2)
It is a measure
established from the size of the rupture surface and the amount of
displacement, which better determines the amount of energy released
3)
It can be verified
by two different methods, field studies that are based of measurements of fault
displacement and by seismograph methods that uses long-period waves (Lutgens
and Edwards 314).
Displacement
is the measurement of the actual change of location of the ground due to
shaking (“Measuring Earthquakes”).
Earthquakes in
Works Cited
Bellis, Mary. Inventors: Seismograph.
2005. About.com 24 Mar 2005.
http://inventors.about.com/library/inventors/blseismograph.htm
Charles Richter – The Richter
Magnitude Scale. 2005. About.com. 21 Apr 2005. http://inventors.about.com/od/qrstartinventors/a/Charles_Richter.htm
Bolt, Bruce A. Earthquakes:
A Primer. W.H. Freeman and Company:
Bormann, Peter.
24 April 2005 http://www.gfz-potsdam.de/pb2/pb21/Kurs/bilder/photo16.html
Chinese Science and Technology.
2003.
http://www.nmns.edu.tw/eng/pdf/chinese-science.pdf
Intensity of the Earthquake. 2005. THEmeter.
24 April 2005
http://www.themeter.net/sism_e.htm
Kauffman, Marvin E. and Sheldon Judson. Physical
Geology. 8th ed Prentic
Hall:
Lutgens, Fredrick K. and Tarbuck J Edwards. Essentials of Geology. 8th ed. Prentic Hall:
McConnel, David.
Earthquakes II. 14 Jan
1997. Natural Science Geology. 258 Mar 2005. <http://lists.vakron.edu/geology/natscigeo/Lectures/equake2/eq2.htm>
Measuring Earthquakes. 2001. SKLPC.
21 Apr 2005. http://www.sklponline.co.uk/earthquake/contents/splash/measuring_earthquakes.htm
Nelson, Stephen A. Professor. Earthquakes. 23 Oct 2003.
The Severity of an Earthquake. 29
Sep 2004.
Zhang Heng. Albertsons: