Seismographs
Lesson 7
arthquakes generate seismic waves that travel all around the world and can be detected by
sensitive instruments called seismographs. The earliest instrument to detect earthquakes
was a seismoscope invented by Zhang Heng in A.D. 132. He called his instrument an
earthquake weathercock. He constructed a large brass urn with eight dragonheads on the outside
that held a ball in their mouth. Inside the urn was a mechanism that would release the ball during
an earthquake. The ball fell into the waiting mouth of a brass frog below the dragon’s mouth
making enough noise so the Imperial Watchman would know an earthquake had occurred and let
people know the direction of the earthquake. The Chinese used the instrument for 400 years.
E
Seismographs recording activity on Kilauea
European inventors had constructed several seismographs by the end of the 19th century. Most of
the instruments suspended a pendulum or magnetic mass within an electric coil. A moving
magnet, inside the coil, created an electric current during an earthquake. The electric current
converted into an electrical signal that moved a needle across paper and would trace out the
wiggles of the Earth’s shaking. The first time a seismograph recorded an earthquake that took
place on the opposite side of the Earth was in 1889. The earthquake generated in Tokyo, Japan
was recorded on a horizontal pendulum seismograph in Potsdam. Evon Rebleur Paschwitz
reported his findings and the discovery that earthquake waves traveled through the Earth after a
major earthquake.
The simplest seismographs have a heavy pendulum with a pencil attached to record seismic
waves on a paper. A manual seismograph that is a little more complicated is made with a drum
that turns constantly with a piece of paper attached to it. A bar or spring with a hinge at one end
holds a weight with a pen. The pen attaches to a pole or metal box that is bolted to the ground.
The pen does not move because of inertia when an earthquake shakes the table and the drum.
The drum vibrates during the earthquake. The unmoving pen records the distance the table
moves during the earthquake on a seismogram, paper attached to the drum. Today these manual
mechanisms are no longer used. Scientists have seismographs that work by measuring electronic
changes produced by the motion of the ground with respect to the mass. The electronic changes
transmitted to a recording station are recorded and analyzed.
One seismograph station will have three different pendulums to record earthquake waves in
north-south, east west, and vertical ground motions.
Seismograms record all types of earth movement including the vibrations created by a car
driving by on a road near the seismograph. Seismograms indicate the distance from the reporting
station but not the direction of the epicenter. To determine the exact location of the epicenter of
an earthquake a scientist must first determine the time lapse between the arrival of the P waves
and S waves at three stations. Scientists determining the distance they were from the focus of an
earthquake would first write down the exact time the S waves arrived at the first station, which
was 10:45 a.m. They would subtract the exact time the P waves arrived, which was 10:17 a.m.
from the S wave arrival time. The waves arrived 28 seconds apart. Next, they multiply the 28second time difference by 8 km/s to determine the number of kilometers from the first station.
After using the same formula for determining, the distance from two other stations you can find
the epicenter by drawing three circles representing the distance from each reporting station. The
place where the circles intersect is the epicenter of the earthquake.
Volcanologists monitoring Mount St. Helens have found four major types of seismic waves
recorded on seismograms. Deep earthquakes located away from the volcano produce the first
type of seismic waves. These waves have high-frequency signals and sharp arrival times similar
to tectonic earthquakes. The second type of seismic wave produced by volcanoes is shallow
earthquakes. The focus of the earthquake is located under the dome less than three kilometers
from the summit with medium-to-low frequency seismic arrival times. Surface events like the
growth of a dome on a volcano produce the third type of seismic wave. The surface events
include gas and tephra eruptions, rock falls and other events with no clear beginning or end. The
fourth type of seismic wave is a harmonic tremor, which is a long lasting and rhythmic signal
that indicates magma is moving beneath the volcano.
The continuous recording of seismic activity on volcanoes is the mainstay of volcano
monitoring. Scientists have found at the Hawaii Volcano Observatory (HVO) that virtually all
eruptions are preceded by an increase in seismic activity. Scientists can track magma moving
into shallow reservoirs beneath the summit of Kilauea by the rocks fracturing. New magma
enlarges the reservoir to contain the molten rock. The ground motions detected by a sensor
transmit them into electronic signals to a seismograph at the volcano observatory. Seismologists
tracking the subsurface movement of magma have gone to locations where earthquake waves
indicated the volcano would erupt. They have observed the actual breakthrough of magma from
subsurface conduits at the start of an eruption.
Seismographs are the principal tool of scientists studying earthquakes. Thousands of seismic
stations are in operation around the world. The World-Wide Seismographic Station Network
(WWSSN) with a network of 120 seismographs in 60 countries has set up seismographs deep in
mineshafts or vaults to make sensitive recordings free of noise created by automobiles and other
environmental factors. These instruments were originally set up to monitor underground testing
of nuclear weapons. Today the global network of seismographs share information among
reporting stations worldwide. Instruments transported to the Moon, Mars and Venus gather data
from outer space.
Lesson summary
♦
The earliest instrument to detect earthquakes was a seismoscope invented by Zhang Heng
in A.D. 132.
♦ The simplest seismographs have a heavy pendulum with a pencil attached to record
seismic waves on a paper.
♦ Seismologists can tell how far away and how strong an earthquake by information on a
seismograph.
♦ The continuous recording of seismic activity on volcanoes is the mainstay of volcano
monitoring.
Name __________________________
Date ___________________________
Quiz 7
Seismograph
Fill in the blanks using words from the Word Bank
1. Seismographs have been transported to the moon, Mars and
to gather data from outer space.
2.
seismograph inventors suspended a pendulum or
magnetic mass within an electric coil.
3. Zhang Heng in A.D. 132 invented the first seismograph that he called an earthquake
.
4. Seismologists can tell the
the information recorded on a seismograph.
5. The continuous recording of
mainstay of volcano monitoring.
and distance away an earthquake is by
activity on volcanoes is the
6. Earthquakes generate seismic waves that travel all around the world and can be detected by
.
sensitive instruments called
7. Harmonic tremors have a long lasting and
that magma is on the move beneath a volcano.
signal, which indicates
8. The first seismograph to record an earthquake that took place on the other side of the
was in 1889.
9. The simplest seismographs have a heavy
pencil attached to record seismic waves on a paper.
with a
10. The World-Wide Seismographic Station Network was originally set up to monitor
underground testing of
weapons.
Word Bank
seismographs
size
weathercock
rhythmic
European
seismic
Earth
nuclear
pendulum
Venus
Coffee Can Seismograph
Activity 7
Introduction
Seismologists use several different seismographs when they record earthquakes. They need one
to record information about waves moving north and south, east and west, and up and down. In
this activity, you are going to make a simple seismograph to record earthquake waves moving in
one direction.
Materials
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Large tin can (large coffee can)
Sand
Rope
Tape
Pencil
White paper
Directions
1. Punch two holes in a large tin can.
2. Make a loop with a piece of rope so
that it makes a handle for the tin
can.
3. Tie another piece of rope to the
handle on the can.
4. Tape a pencil to the side of the tin
can.
5. Fill the tin can with sand.
6. Place a sheet of paper on the table.
7. One person holds the tin can so the
pencil just touches the table.
8. One person moves the table back
Tin can seismograph
and forth, gently to simulate the
Earth's movement during an
earthquake.
9. The third person pulls the paper as the pencil traces the table movement.
10. After you have finished making a seismogram, you should measure the amplitude of the
largest wave.
Extensions
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Create a chart showing how seismographs have changed over time.
Create an earthquake detector like Zhang Heng did that is your own design.