- Shaking Measured by Seismographs
- Earthquakes are felt as a shaking, or rolling, movement of the earth's surface. The strength of this shaking can cause damage to buildings which are relatively inflexible so they may crack, break apart, and even collapse
if the shaking is strong enough. Some geologists estimate the Earth has more than a million quakes every year but only about 1500 of these are strong enough to be felt by most of us. The vast number of earthquakes are recorded by sensitive instruments, called seismographs, and never reported in the news.
- Rocks Under Pressure
- Rocks deep with the Earth are subject to great pressures pushing them in various directions. Rocks can become elastic and respond to these pressures by bending, or stretching, but only to a certain extent. Pressures can be so great, and so persistent, that the bending reaches its limit and the rock suddenly snaps. In some cases, the rocks then rebound back into their original positions although they are now broken.
- Could It Happen Here?
- The Earth's surface is composed of a series of sections called "plates." These plates move toward or away from one another and some move sideways, sliding past an adjoining plate. Wherever these plates meet, earthquakes and volcanoes are more prominent. Recent earthquakes in Turkey, Greece, Taiwan, and Mexico were all due to movements near plate boundaries. An earthquake could happen almost anywhere. In fact, small earthquakes do happen on a regular basis, and we don't even know about them. The probability of a serious earthquake away from a plate boundary, however, is low compared to regions near plate boundaries.
- An Easy Demonstration
- A classic demonstration is to use a thin wood lath, or ruler, as an example of rock under pressure. (Use safety glasses when you do this demonstration.) Hold the lath, or ruler, firmly in both hands and push the ends in opposite
directions. As you slowly apply more pressure, the wood will begin to bend noticeably. Rocks can exist in this stressed state for many years. Continue adding pressure and, at some point, the wood will suddenly snap and both ends will straighten (rebound). Earthquake! This breaking sends shock waves throughout the surrounding rock layers. In this demo, shock waves radiated through the air and actually struck your ears, which recorded it as the "sound" of the breaking.
- Try Your Hand at This
- Another easy illustration of the slipping of rocks under slow pressure uses nothing but your hands. Hold your hands together, palm-to-palm. Push one hand tightly against the other. Now, while pushing quite hard, also try to slide one hand forward against the other. Usually, with the right combination of pressures, you can feel the sliding pressure build up in your palm while friction between the palms holds your hands in place. Finally, the sliding force overcomes friction and some sliding occurs but only a fraction of an inch. Earthquake! Keep applying pressure and the process will repeat. Each little slide moves the hands further from their original position. Parts of the Earth have been doing this for hundreds of millions of years!
Thanks to Bill Einsig of SEE Flash for this explanation. The Science, Environment and Ecology (SEE) Flash is an email newsletter sponsored by Keystone Outdoors magazine and the Pennsylvania Department of Education with support of the Science Teacher's Resource Center at Penn State York. Its purpose is to provide an exchange of information and instructional ideas for the outdoor community as well as teachers of elementary and secondary education. Questions, comments, and requests for changes to the mailing list should be sent to wxe1@psu.edu. There no fees and new subscribers are always welcome.
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