Seismographs. Lesson 7. Seismographs recording activity on Kilauea

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1 Seismographs Lesson 7 E 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 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. Seismographs recording activity on Kilauea European inventors had constructed several seismographs by the end of the 19 th 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 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

2 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 28- second 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

3 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 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.

4 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 and distance away an earthquake is by the information recorded on a seismograph. 5. The continuous recording of activity on volcanoes is the mainstay of volcano monitoring. 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 signal, which indicates that magma is on the move beneath a volcano. 8. The first seismograph to record an earthquake that took place on the other side of the was in The simplest seismographs have a heavy with a pencil attached to record seismic waves on a paper. 10. The World-Wide Seismographic Station Network was originally set up to monitor underground testing of weapons. Word Bank seismographs weathercock European Earth pendulum size rhythmic seismic nuclear Venus

5 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 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 and forth, gently to simulate the Tin can seismograph 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 Create a chart showing how seismographs have changed over time. Create an earthquake detector like Zhang Heng did that is your own design.