Although it is possible to have a magnitude smaller than 1, and there is technically no upper limit, this is the range commonly used when reporting earthquakes to the public. Both the Richter scale and the Moment Magnitude scale use the numbers 1 - 10 to measure the amount of energy released. Hanks and Hiroo Kanamori developed the Moment Magnitude scale, abbreviated M w. The Richter scale is somewhat limited, however, because it cannot accurately measure or compare large earthquakes. The first scale used to measure magnitude was the Richter scale, which measures the amplitude of a seismic wave at a defined distance from the earthquake. Magnitude (M) is the measure of the energy released by the earthquake, whereas the intensity is what people actually experience. There are two ways to measure earthquakes: magnitude and intensity. Although the San Francisco earthquake was quite large, most of the damage to the city was actually caused by a fire, which is a common secondary hazard of earthquakes. Looking at the photo, one can visualize how the foreground moved to the left, while the background moved to the right. The fence was once connected, but today it is separated by a 6-meter (20-foot) gap. The Earthquake Trail, which begins at the Bear Valley Visitor Center, follows the trace of the 1906 San Francisco earthquake to a picket fence along the fault ( Figure 10.3). An excellent example of this kind of offset can be found along the San Andreas Fault at Point Reyes National Seashore. The plane defined by the rupture is known as a fault, and the rock layers become offset along it. The rocks then snap apart, releasing energy in the form of seismic waves ( Figure 10.2). According to the elastic rebound theory, rocks can bend elastically up to a point, until they finally break. The upshot is that seismologists have found that using 10^1.5 as the base for the modern scale fits the original Richter scale well, and 10^1.5 is about 31.62.Earthquakes occur when a critical amount of stress is applied to the crust. For example, software people might use that log base 2 of 64 is 6 (this is 100% equivalent to saying 2^6 = 64). It might be helpful when reading that article to keep in mind that although e and 10 are the most common logarithmic bases, you can define a logarithm with any base. UPDATE: Although the main wikipedia titled "Richter magnitude scale" isn't very helpful, it turns out there is another article titled "Moment magnitude scale" that has the quantitive explanation. To see people trying the empirical best fit and arriving at a base of about 31, you need to dig into the scholarly literature. This is so widely misquoted on the web that it is becoming sort of urban legend that the base of the logarithm is 10. Each point piles on another factor of (empirically about) 31. Since 31**2 is close to 1000, two points on the Richter scale e.g., from 4 to 6 or from 5 to 7, corresponds to about 1000 times the energy release. In terms of energy released, the best fit for the base turns out to be about 31. The base of the Richter scale is empirical. In short: you are half-right (one unit of magnitude it is 10 times bigger on the seismogram, not in energy), and the book is right (two units of magnitude are 1000 times stronger). While one unit of magnitude is 10 times the amplitude on a seismogram, one unit of magnitude represents $10^$ times the energy, and so on. The magnitude scale is really comparing amplitudes of waves on a seismogram, not the STRENGTH (energy) of the earthquakes. Braile's collection of earthquake hazard information, that explains it quite nicely:
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