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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. Various scales have been developed to describe the intensity of an earthquake. The scale we hear about most is the Richter scale. Beno Gutenberg and Charles Francis Richter developed the Richter scale in 1935. The scale ranks earthquake shock waves by the severity of effects at the ground surface. The Richter scale is logarithmic. An earthquake with a ranking of 3.0 is ten times stronger than a shock wave with a ranking of 2.0. But a 4.0 quake is one hundred times stronger than a 2.0 quake. It's also possible to measure quakes with negative values on the Richter scale simply because seismographs now are far more sensitive than those used by Richter and Gutenberg in 1935. |
| 2.0-2.9 | Perceived only by sensitive seismographs. |
| 3.0-3.9 | Slight vibrations; hanging objects swing. |
| 4.0-4.9 | Vibration; crockery rattles; small objects displaced. |
| 5.0-5.9 | Furniture moves; masonry cracks and falls; waves on ponds. |
| 6.0-6.9 | Difficulty standing; walls and chimneys partly collapse. |
| 7.0-7.9 | Buildings collapse; cracks in ground; landslides. |
| 8.0-8.9 | Damage to underground structures; masses of rock displaced. |
| 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. |