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Seismograph in Sunset Crater National Monument visitor center

Earthquakes - At Sunset Crater Volcano!

Measuring Earthquakes at Sunset Crater

If you've ever experienced an earthquake first hand, you know that earthquakes produce vibrations. Sometimes BIG vibrations. These vibrations can be detected by sensitive devices called seismometers. Data from the seismometer is sent to a seismograph where it is recorded.

Close up of an earthquake recorded at Sunset Crater

The image above shows the seismograph in Sunset Crater Volcano National Monument's visitor center. A pen at the top of the device records a zig-zag line on the moving, paper-covered cylinder whenever an earthquake is detected. Each day the paper record, called a seismogram , is removed and replaced with a new one. Scientists then analyze the earthquake data from the seismogram. Seismographs can help us determine the time, epicenter, focus, and the type of faulting that produced an earthquake as well as estimate how much energy was released.

Seismograms: up close and personal

The image on the right shows a close-up of one of eight earthquakes recorded on Sunset Crater's seismograph on October 18, 1998. Most of the time the seismograph pen draws a fairly smooth line, telling us that no earthquakes have occurred within range of our seismograph's sensitivity. Most of the lines in this image are smooth, with regular 'blips' that scientists use to help keep track of the time of day. It doesn't take an expert to see the earthquake record on this seismogram!

Birth of an earthquake

Where it all starts

The story starts beneath the surface. As you may know, the continents we live on are parts of moving plates. Most of the action takes place where plates meet. Plates may collide, pull apart, or scrape past each other.

All this stress and strain builds up in the rock until it simply can't take it any more. All at once, CRACK!, the rock fractures and the two rocky blocks move in opposite directions along a fault. The sudden movement generates an earthquake at a point called the focus. The energy from the earthquake spreads out as seismic waves in all directions.

Close up of an earthquake recorded at Sunset Crater

Watch out for the waves!

Did you know that earthquakes pack a double punch? If you look carefully at the seismogram, you'll notice that the earthquake waves do not arrive in a single vibrational blast. On this seismogram you can easily see two distinct zig-zags.

The first zig-zag, the one near the bottom of the seismogram, records the arrival of the first type of wave, called P waves. P waves, or primary waves, travel fast! They are usually the first type of earthquake-generated wave to reach the surface.

The second zig-zag records the arrival of the another type of seismic wave, called S waves or secondary waves. S waves do not travel as quickly as P waves so they arrive at the seismograph after the P wave. This is important! Why? Well, we know that both S and P waves blast out from the same place at exactly the same time. The farther the waves travel, the more the slower S waves lag behind the speedy P waves. Seismologists can use the lag time between the arrival of the P waves and S waves to figure out where earthquakes originate!

 


More on seismology coming soon! In the meantime, why not try your hand at locating an earthquake at the Virtual Earthquake site at California State University, Los Angeles?

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