[MUSIC] Let's think a little bit about the size or the meaning of the numbers on a magnitude scale. Often, the numbers of earthquakes that are recorded are usually three, four, five or larger. That's because earthquakes with magnitudes that are smaller than about three or four are almost inconsequential. You may feel them if you're near the epicenter as a small vibration, but they really don't have any significant consequence. When you start getting earthquakes four, five, and larger, they have consequences, and may be newsworthy. Now, one of the things about the magnitude scale is that it's logarithmic. That means that each number represents and increase of a factor of 10 over the number before it. Now remember, we were saying that a magnitude is a measure of the amplitude or is based on a measure of the amplitude of the waves recorded by a seismograph. So that means that if we have an earthquake of magnitude 5, the vibration is about a factor of 10 larger than that of a magnitude 4. So here's the record for a day as recorded by a seismometer in Japan. And you can see that for much of the day, hardly anything was happening and the lines are fairly smooth. They're not perfectly straight, because remember, they're also recording so called noise vibrations due to the environment around the seismometer. But every now and then you see a little blip where there's been a bigger vibration and that's an earthquake. And you an see where the smaller blips occur, that's a smaller earthquake and where the larger blips occur, that's a larger earthquake. So let's now return to the question of how to compare the size of earthquakes. This time, using the magnitude scale. Seismologists distinguish among earthquakes based on the magnitude. Another way to look at the size of an earthquake is to consider the amount of energy released by an earthquake. Now when we were talking about the size of the vibration, we said that each increase of an integer in the magnitude scale represented an increase in a factor of 10 in the size of the vibration. But, it turns out at an increase of one integer in a magnitude scale represents an increase in a factor of 32 in the amount of energy released by an earthquake. That means that an earthquake of magnitude 6 releases about a million times more energy than does an earthquake of magnitude 2. There's a huge energy difference between different size earthquakes. In fact, one great earthquake, one earthquake greater than magnitude 8 can release more energy in a matter of minutes than all the other earthquakes that happen on Earth in a given year. Because we can think of earthquakes as being sources of energy, we can compare the size of an earthquake to other sources of energy. So for example, we can create the scale here where we represent on a vertical scale the magnitude and then plot on that different events and compare them to more familiar events. So for example, a large lightning bolt is about the equivalent of magnitude 3 earthquake [SOUND]. Tornado is the equivalent of about a magnitude 4.6 earthquake. The Hiroshima atomic bomb is the equivalent of about a 6.2 earthquake. The largest nuclear test ever recorded on Earth was the equivalent of a magnitude 8. And the Krakatoa explosion or the explosion of Krakatoa in 1883 that created sound that was heard around the world was equivalent of about an 8.5. The largest earthquake that has ever been recorded on Earth is about a magnitude 9.5. It was a earthquake that happened in 1960 in Chile. Earthquakes larger than about 9.5 probably can't happen because there really isn't a fault big enough that could have enough slip over a small interval of time that can generate that size earthquake. So 9.5 is about as big as earthquakes get on the planet. The last thing we'll mention here, and we are not going to into detail during the lecture part here, is how you can measure the location of an epicenter of an earthquake. That's done by measuring the earthquake at three different seismograph stations. And what we can determine is the distance between that seismograph station and the epicenter. And if we do that for three different earthquakes and draw a circle around the seismograph. Where those circles intersect represents the location of the earthquake. So just to summarize, we've seen today that not all earthquakes are the same size. And that we can describe the differences between different size earthquakes using one of two scales. The Mercalli Intensity scale, which is a subjective measurement of the amount of damage cause by an earthquake. And the magnitude scale, which is a objective measurement of the size of the vibration generated by the earthquake. [MUSIC]
