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Measuring the Shake



Measuring the Shake
Reading Level
     edHelper's suggested reading level:   grades 8 to 9
     Flesch-Kincaid grade level:   9.88

Vocabulary
     challenging words:    imprecise, lag-time, tracing, wave-shaped, epicenter, north-south, seismologists, seismic, east-west, seismograph, currently, geology, intensity, calculate, radius, precise
     content words:    Modified Mercalli Scale


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Measuring the Shake
By Trista L. Pollard
  

1     We know the Earth moves under our feet occasionally. So how do scientists measure and study this movement? Enter the seismologist. This scientist is responsible for studying and measuring earthquakes that occur on Earth. They use instruments called seismographs to detect and record ground vibrations produced by quakes. Today's seismographs have three components or three sensing devices. The vertical ground motion is recorded by one device. Horizontal motion is recorded by the other two devices; one for east-west motion and one for north-south motion. When an earthquake occurs, the seismograph traces wave-shaped lines onto a paper. The motion can also be transmitted through electronic signals. If it is sent electronically, then the signals are recorded on magnetic tape or loaded into a computer. This gives scientists the opportunity to analyze the seismic activity from the quake. Seismograms are the tracing of the motion produced by earthquakes and recorded on seismographs.
 
2     As you may have guessed, seismic waves are recorded in different orders on the seismograph. P waves are usually recorded first because they are the fastest seismic waves. The second fastest waves, S waves, are the next to be recorded on the seismograph. The last waves recorded are Rayleigh and Love waves. These are the slowest moving waves produced by a quake.
 
3     Scientists use the seismograms to determine the location of the quake. To find out this information, they study the arrival times of P waves and S waves. The shorter the lag time between the arrival of the P waves and the arrival of the S waves, the closer the earthquake. Seismologists then study a lag-time graph to find out the distance an earthquake's epicenter is from the seismograph station. The differences in arrival of both types of waves are converted into distance. This is the distance from the earthquake's epicenter to each station that recorded the quake. Lag-time graphs are also used to determine the start time of earthquakes.

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