汶川地震让“地震”成为人们关注的重心之一。地震的大小是用震级来表明的，那震级是怎么测定的呢？是用测震仪、也就是地震仪。一台测震仪是怎样工作的？它如何告诉人们地壳的运动？我们所说的震级又是如何计算出来的呢？下面这篇文章就来告诉您。

    A seismograph (地震仪，测震仪) is the device that scientists use to measure earthquakes. The goal of a seismograph is to accurately record the motion of the ground during a quake. If you live in a city, you may have noticed that buildings sometimes shake when a big truck or a subway train rolls by. Good seismographs are therefore isolated and connected to bedrock to prevent this sort of "data pollution."

    (Jason Reed/Getty Images) The Richter scale is a logarithmic scale, meaning that the numbers on the scale measure factors of 10. 

    The main problem that must be solved in creating a seismograph is that when the ground shakes, so does the instrument. Therefore, most seismographs involve a large mass of some sort. You could make a very simple seismograph by hanging a large weight from a rope over a table. By attaching a pen to the weight and taping a piece of paper to the table so that the pen can draw on the paper, you could record tremors in the Earth's crust (earthquakes). If you used a roll of paper and a motor that slowly pulled the paper across the table, you would be able to record tremors over time. However, it would take a pretty large tremor for you to see anything. In a real seismograph, levers or electronics are used to magnify the signal so that very small tremors are detectable. A big mechanical seismograph may have a weight attached that weighs 1,000 pounds (450 kg) or more, and it drives a set of levers that significantly magnify the pen's motion.

    The Richter scale (里氏震级) is a standard scale used to compare earthquakes. It is a logarithmic scale (对数标度), meaning that the numbers on the scale measure factors of 10. So, for example, an earthquake that measures 4.0 on the Richter scale is 10 times larger than one that measures 3.0. On the Richter scale, anything below 2.0 is undetectable to a normal person and is called a microquake. Microquakes occur constantly. Moderate earthquakes measure less than 6.0 or so on the Richter scale. Earthquakes measuring more than 6.0 can cause significant damage. The maximum quake rating ever measured is about 8.9.