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In those maps, in the 2 posts above, the largest magnitude in the legend is shown as a 7. That's a very strong quake, 7. But there have been stronger ones in recent history. One website says there is a quake of mag 8 somewhere in the world on the average of once a year, but those that occur under the ocean are often not felt by any people, unless they are close to land. In fact, instruments are not located on the ocean floor, so some quakes might be 2,000 miles away from the closest instrument. The quake that caused the tsunami in Indonesia was centered under the ocean, and had a magnitude of about 8. Consider the damage the water caused - which was not the quake itself, for not many people felt the quake, but plenty experienced the tsunami.
The sizes of the squares are for simplicity's sake, and do not adequately show the difference between whole numbers in quake intensity measurements. For each whole number increase, a factor of 10 apples, such that a 2 quake has 10 times much ground movement as a 1 quake, and so on. Consequently, you add a zero for each whole number, and a 3 quake is 100 times "bigger" than a 1 quake, and a 7 quake is 1000 times "bigger" than a 4 quake. Obviously, to fit all that accurately on a map like those above would render the smaller quakes so tiny they would be less than one pixel on the screen (and thus not visible). This is ONLY in regards to the seismograph movements at the recording stations.
But when you consider the ground
displacement, that is, the amount of permanent shift in surface rupture occurs, as the moment magnitude scale does (and it is more difficult to compute, therefore it is not done for all the tiny quakes), the energy released by each successive whole number increases by a factor of 32 (some sources say "31").
The series is, 32, followed by
1,024
32,768
1,048,576
33,554,432
If you use a factor of 31, it's 31, followed by
961
29,791
923,521
28,629,151
In general, you can therefore estimate it as 31 or 32, followed by
1,000
30,000
1,000,000
30,000,000
Therefore a quake of moment magnitude (Mw) 4 has 32 times the energy of a 3 Mw quake.
This means that in an earthquake of 8 Mw,
thirty million times the energy is released, compared to the energy released in a quake of 3 Mw. Fortunately, the further away you are from the epicenter, the less energy you will experience from a quake.
Looking back over the record, it becomes a little confusing, because the numerical values assigned to older quakes have not had exactly the same significance as the values assigned to more recent ones. The USGS website says otherwise.
The Richter scale was developed in 1935 by Charles F. Richter of the California Institute of Technology as a mathematical device to compare the size of earthquakes. It was updated with the addition of a new scale called the moment magnitude scale, about 30 years ago.
For smaller quakes, such as 4 or less, the two scales are no different, but for larger quakes, the moment magnitude numbers tend to be a bit smaller (in my experience), such that a quake reported as 7.1 on the Richter scale might be around 6.8 on the moment magnitude scale, for example, in my experience. The USGS website claims there is no difference in the numbers from any of the various systems.
Well, if that were true, and therefore if the numbers were all the same whether it's the Richter scale (known as local magnitude) or the moment magnitude scale or the surface wave magnitude or the body wave magnitude, then why have these different names or different scales? Why bother with new designs of equipment to measure quakes?
IMHO they have different scales because the same quake is represented by different numbers in the different systems. But I can't find any website that's willing to admit this is the case. Very strange. See the video, below, if you want a better sense of this.
Moment magnitude takes into account more than simply the strength of the largest wave of the quake.
There are two or more 'waves' that quakes generate, but it's the LAST one, called the "love wave" that does all the damage, because it's the one where the surface of the earth wobbles in a ripple motion, like the surface of disturbed water.
The love wave results when the energy of the quake travels to the surface of the earth above the epicenter, and from that point, radiates out in a way similar to how ripples in a pond expand concentrically from the place where a stone is thrown into the water.
The waves that precede the love wave emanate in all directions, directly from the fracture point in the underground rocks, and that's why they are first recordable at seismograph instruments located on the ground all over the world.
From
the USGS website:
The magnitude is a number that characterizes the relative size of an earthquake. Magnitude is based on measurement of the maximum motion recorded by a seismograph. Several scales have been defined, but the most commonly used are
(1) local magnitude (ML), commonly referred to as "Richter magnitude,"
(2) surface-wave magnitude (Ms),
(3) body-wave magnitude (Mb), and
(4) moment magnitude (Mw).
Scales 1-3 have limited range and applicability and do not satisfactorily measure the size of the largest earthquakes. The moment magnitude (Mw) scale, based on the concept of
seismic moment, is uniformly applicable to all sizes of earthquakes but is more difficult to compute than the other types. All magnitude scales should yield approximately the same value for any given earthquake.
Note: When you see the word, "moment," used in this context, it has nothing to do with a period of time, such as "the ground shook for a few moments and then it stopped." The word
moment, as in moment magnitude or seismic moment, is a scientific term which signifies the effect of two combined entities: the quantity of force and the distance between two points. The force is multiplied by the distance to get the quantity known as the "moment." In websites such as USGS or SCEC, you won't see these fundamental principles explained, and so it can be rather confusing for someone who does not have any experience in the theoretical aspects of seismography or engineering.
For those interested in the technical aspects or the mathematical theory behind moment magnitude (Mw), the following video is very helpful:
[youtube]https://www.youtube.com/embed/HL3KGK5eqaw[/youtube]
Published Sept. 4, 2012, 8,058 views.
Directed by Robert Butler, University of Portland (Oregon),
Narrated by Roger Groom, Mt. Tabor Middle School, Portland.
Photographs courtesy of US Geological Survey.
In the latter part they show breaking strands of spaghetti as a device for describing what energy is involved with ground displacement during an earthquake.
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Topic: small earthquake in So. Calif. near Fontana (Read 1403 times)