Be an Earthquake Detective
Everyone seems interested in current earthquakes. They are eager to know what the magnitude is, what damage has occurred, and the number of aftershocks. Everyone wants to know what the seismograph says.
- What is a Seismograph?
- A seismograph is an instrument that detects and records, as a function of time, motions of the Earth's surface, especially those caused by earthquakes. Seismographs vary in the materials they are constructed from. Three illustrations of different seismographs are shown on this page. The seismograph gives us a permanent timed recording of ground motion called a seismogram. A seismogram shows the amplitude of body and surface waves, which indicates the amount of strain energy released by an earthquake. This measure is called the earthquake's magnitude.
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- But, you can actually find evidence of past earthquakes by looking at historical records and clues in your environment. This evidence can be found without building an apparatus or recording device. It can be found in chimney and cemetery damage, the observations of the people who experienced the earthquake, and in the piles of materials that may have fallen off shelves. These environmental clues can be thought of as "real world" earthquake indicators or seismoscopes.
- What is a seismoscope?
- A seismoscope is an instrument that indicates the occurrence of an earthquake. The first one is believed to have been invented by the Chinese philosopher, Chang Heng, in 132 A.D. It was a decorated metal jar ringed at the top with 8 dragon heads, each holding a ball in its mouth. Below each dragon was a frog with an open mouth. Inside the jar, a pendulum hung so that if the ground shook, it would swing and knock out a ball that would fall into a frog's mouth.
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How can environmental clues serve as seismoscopes?
Disruption of or damage to cemetery stones, chimneys, and the like can tell us that an earthquake has occurred and give an idea of the direction of the shaking. Depending upon damage patterns in an area, they can also tell us how close the epicenter is. We can learn about this damage in original newspaper accounts or by interviewing people who experienced the earthquake. Old newspapers can be found either at the library or historical society. If you are close to where the earthquake occurred, you can then see if the earthquake evidence is still visible. Some damaged markers and chimneys may not have been replaced for safety reasons.
Why look at environmental clues?
It is important to look at environmental clues when investigating a historical earthquake. Sometimes, they're the only clues we have, especially if the earthquake happened before there were seismographs. The evidence that is found can then help in defining the seismic hazard of a region and set the stage for earthquake preparedness activities.
- Chimneys as Seismoscopes
- One way of measuring an earthquake is by looking at its effects at a particular place. This is called Earthquake Intensity. We measure Intensity with a 12-value scale called the Modified Mercalli Scale. This scale uses Roman numerals to rank relative levels of ground motion, human impact, and structural damage. Usually chimney damage can be seen when there is an Intensity VII or VIII earthquake. But, there are different kinds of damage. Sometimes just a few bricks drop, other times the entire chimney falls. One kind of chimney damage, called "up-bumped," makes a mushroom shape of the chimney. This can tell us the structure was nearer the epicenter than a building where only part of the bricks have fallen.
Cemetery Stones as Seismoscopes
In some earthquakes, damage can be seen in cemeteries. Sometimes tombstones will rotate or shift. Some taller monuments will fall over in the primary direction of ground shaking. You can determine shift and rotation directions by:
- referring to a road map to determine the primary direction of disturbed tombstones,
- using a compass to determine the direction of original position and the direction after shaking,
- using a protractor to measure the degree a stone rotated,
- using a ruler to measure the amount of shift.
This can give an earthquake detective some rough direction information in relation to the earthquake's epicenter.
People as Seismoscopes
People who experienced an earthquake can also act as seismoscopes because they have perceptions about the amount, direction of ground shaking, and the length of the earthquake. However, it is important to remember that direction can be relative to the individual's geographic location and their distance from the epicenter. When someone says they felt shaking from the north, it is important to know whether they know that from a compass or from something else. It is also important to know how long the shaking was observed by different people. You can find out this information by either interviewing people or reading interviews in old newspapers.
- Shelf Damage as a Seismoscope
- Look at pictures of shelves and cupboards and see what walls had the most items falling over or off the shelves. This might give you a rough direction of the impulse.
When you're a historical earthquake detective, it is important to remember that all of these environmental clues give you evidence but none of them are exact. Therefore, you want to get the same ideas or information from a number of sources before you come to conclusions. Keep a tally sheet as you collect your evidence.