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Roger Weller, geology instructor

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 United States.

            Nicholas Cirillo developed the first mechanical device used to study earthquakes in 1731.  A series of earthquakes in Naples inspired Cirillo to design this mechanical device.  Cirillo’s device consisted of a pendulum that swung freely.  The tremors of the earthquake caused the pendulum to swing.  The lines caused by the pendulum indicated the amplitude of the ground motion.  In Calabria, Italy, a series of earthquakes in 1783 that killed over 50,000 civilians.  Because of these earthquakes, more mechanical devices were created (Kauffman and Judson 181).

              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 Italy designed a mercury seismometer.

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 Japan.  The society funded the invention of seismographs (“Inventors”).

            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 United States to record long period waves.  The difference between the Press-Ewing and the Horizontal pendulum seismograph is that the Press-Ewing seismograph has a pivot that hold the pendulum was replaced with elastic wire to avoid friction (“Inventors”). 

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 California.  The Richter scale is referred to ML, with “L” for local.  After many seismograph stations were established, it became clear the formula was only valid for a certain frequency and distance ranges (“Measuring Earthquakes”).  The original scale was modified to measure earthquakes at any distance, focal depth and compensate for geological variations from place to place (Lutgens and Edwards).  Adjustments were also included into the formula to compensate for the variation of the distance between different seismographs (“Severity”). 

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 (U.S. Geological Survey).  Although this process seems simple, it is actually very complex.  Seismologists are given seismograms then had to determine the magnitude at its source.  Earthquakes with a magnitude of 2.0 or less are called microearthquakes (Bellis).  The Richter scale has no upper limit.  Although the largest earthquake is recorded to be an 8.9, it is possible the earth can withstand the stress to produce an earthquake larger than 9.0 (McConnel).  According to the chart, large earthquakes will be felt by everyone and even cause serious damage.  However, people may not feel some large-magnitude earthquakes beneath the ocean at all (Bellis).

Richter magnitude

earthquake effects

less than 3.5

Generally not felt, but recorded.


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.


Can be destructive in areas up to about 100 kilometers across where people live.


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”). 



People do not feel any Earth movement.



A few people might notice movement if they are at rest and/or on the upper floors of tall buildings.



Many people indoors feel movement. Hanging objects swing back and forth. People outdoors might not realize that an earthquake is occurring.



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.



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.


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.



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.



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.



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.


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.



Most buildings collapse. Some bridges are destroyed. Large cracks appear in the ground. Underground pipelines are destroyed. Railroad tracks are badly bent.


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 Arizona occur mainly in the Northeast.  Since 1830, the Arizona Earthquake Information Center (AEIC) has recorded hundreds of earthquakes.  A majority of the earthquakes has occurred in Flagstaff, the Grand Canyon, Mogollon Plateau and Cataract Creek.  Many earthquakes occurred in southern Arizona as well.  Yuma is recorded to have had dozens of earthquakes in the last 175 years that occurs fairly regularly.  AEIC recorded the location of a numerous earthquakes simply as SW Arizona.  Because AEIC did not specify which area in SW Arizona, Sierra Vista may have had earthquakes in the past or recently.  In the last ten years at least, Sierra Vista has not experienced an earthquake that people have felt.  A slammed car door, vehicles colliding with each other, or building being built is the closest event Sierra Vista has experienced to an earth

                                                   Works Cited

Arizona Earthquakes 1830-2005.  10 Jan 2005.  Arizona Earthquake Information Center.  21 Apr. 2005.
Bellis, Mary.  Inventors: Seismograph. 2005. 24 Mar 2005.
 Charles Richter – The Richter Magnitude Scale.  2005. 21 Apr 2005.

Bolt, Bruce A.  Earthquakes: A Primer.  W.H. Freeman and Company: San Francisco 1978.

Bormann, Peter.  24 April 2005 
Chinese Science and Technology.  2003.  National Museum of Natural Science.  22 Apr 2005.

Intensity of the Earthquake.  2005.  THEmeter.  24 April 2005  
Kauffman, Marvin E. and Sheldon Judson.  Physical Geology.  8th ed Prentic Hall: New Jersey.  1990.

Lutgens, Fredrick K. and Tarbuck J Edwards.  Essentials of Geology.  8th ed.  Prentic Hall: New Jersey.  2003.

McConnel, David.  Earthquakes II.  14 Jan 1997.  Natural Science Geology.  258 Mar 2005. <>

Measuring Earthquakes.  2001.  SKLPC.  21 Apr 2005.
Nelson, Stephen A. Professor.  Earthquakes.  23 Oct 2003.  Tulane University.  24 April 2005
The Severity of an Earthquake.  29 Sep 2004.  U.S. Geological Survey.  25 Mar 2005. <>

Zhang Heng. Albertsons: College of Idaho.  24 April 2005. <>