There are sometimes confusion about the magnitude of Earthquakes. What scales, other than Richter's, are used to measure the magnitude of an earthquake?
Is there a scale where magnitude 9.0 is regarded as small and 13-14 regarded as strong?
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2$\begingroup$ Moment magnitude is better. $\endgroup$– staliApr 21, 2015 at 13:41
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4 Answers
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Earthquakes are measured for intensity and magnitude. Magnitude and intensity are related but measure very different properties of the event. Magnitude is the energy released of the earthquake. It is determined from measurements. Intensity is determined from effects on buildings, landscape and people. Naturally an earthquake with high magnitude would also generate high intensity, but the intensity depends on distance from the hypocenter and the local geological conditions.
Modified Mercalli Intensity scale is a way to rate the intensity of an earthquake, but the scale ends at XII (Damage total), not at 13-14 (XIII-XIV), as mentioned. China seismic intensity scale (CSIS), Medvedev–Sponheuer–Karnik scale and European macroseismic scale can reach values above ten, but not 13-14. XII on the Medvedev–Sponheuer–Karnik scale is Very catastrophic and IX in Destructive. Medvedev–Sponheuer–Karnik scale is used in some Asian and European countries, Russia, India, Israel etc. CSIS is used in mainland China.
Magnitude is also measured in different scales. Related to the Richter's scale is the Moment magnitude scale, it's an updated better way to measure, but it usually produce similar values as the old Richter's scale and the two scales are often confused in media. There are few other scales, Body wave magnitude, surface wave magnitude, but to my knowledge they also stay under 10.
Some of the confusion might come from that the Richter scale is logarithmic. A difference in magnitude of 2.0 is equivalent to a factor of 1000. An earthquake measured to 9.0 is one million times stronger than an earthquake at 5.0.
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3$\begingroup$ Keep in mind that these are different concepts. Mercalli is a intensity scale, i.e. for the effects of the earthquake at a specific location. OTOH, Richter is a magnitude scale, i.e. energy released at the hypocenter/focus. $\endgroup$ Apr 22, 2015 at 15:54
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$\begingroup$ Yes, should probably clarify the answer and also discuss how intensity depends on geological and geometric setting. $\endgroup$– user2821Apr 22, 2015 at 18:50
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The problem with most scales is that they become saturated after a certain magnitude.
3 scales can be named which fall in this category
Local Magnitude/Richter (ML):
From Wikipedia:
The Richter magnitude of an earthquake is determined from the logarithm of the amplitude of waves recorded by seismographs (adjustments are included to compensate for the variation in the distance between the various seismographs and the epicenter of the earthquake). The original formula is: $$M_\mathrm{L} = \log_{10} A - \log_{10} A_\mathrm{0}(\delta) = \log_{10} [A / A_\mathrm{0}(\delta)],$$ where $A$ is the maximum excursion of the Wood-Anderson seismograph, the empirical function $A_0$ depends only on the epicentral distance of the station, $\delta$. In practice, readings from all observing stations are averaged after adjustment with station-specific corrections to obtain the $M_\text{L}$ value.
Technically this scale is valid only for earthquakes in the California region.
Body wave Magnitude ($M_b$)
From Wikipedia:
Body wave magnitude ($m_b$) is a way of determining the size of an earthquake, using the amplitude of the initial P-wave to calculate the magnitude. The P-wave is a type of body wave that is capable of traveling through the earth at a velocity of around 5 to 8 km/s, and is the first wave from an earthquake to reach a seismometer. Because of this, calculating the body wave magnitude can be the quickest method of determining the size of an earthquake that is of a large distance from the seismometer.
Limitations in the calculation method mean that body wave magnitude saturates at around 6-6.5 $m_b$, with the figure staying the same even when the moment magnitude may be higher.
Surface wave Magnitude ($M_s$)
From Wikipedia:
The formula to calculate surface wave magnitude is: $$M = \log_{10}\left(\frac{A}{T}\right)_{\text{max}} + \sigma(\Delta)$$ where $A$ is the maximum particle displacement in surface waves (vector sum of the two horizontal displacements) in μm, $T$ is the corresponding period in s, Δ is the epicentral distance in °, and $$ \sigma(\Delta) = 1.66\cdot\log_{10}(\Delta) + 3.5$$
This scale gets saturated at around $M_s$=8.4
The advantage of using Moment Magnitude scale(MW) is that it does not saturate.
From Wikipedia:
The symbol for the moment magnitude scale is $M_\mathrm{w}$, with the subscript $\mathrm{w}$ meaning mechanical work accomplished. The moment magnitude $M_\mathrm{w}$ is a dimensionless number defined by $$M_\mathrm{w} = {\frac{2}{3}}\log_{10}(M_0) - 6,$$ where $M_0$ is the seismic moment in N⋅m ($10^7$ dyne⋅cm).
From Wikipedia:
The magnitude($M_0$) is based on the seismic moment of the earthquake, which is equal to the rigidity of the Earth multiplied by the average amount of slip on the fault and the size of the area that slipped.
This scale is more useful in a sense that it provides some insight into the fault plane geometry of the earthquake based on parameter $M_0$.
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1$\begingroup$ Please don't copy/paste straight from Wikipedia without attribution: you're welcome to summarize the relevant bits though. Additionally, mathjax is activated for this site so you can write the equation down directly (between single
$for inline equations or double$$for single-line equations) instead of linking to a picture of the equation. $\endgroup$ Apr 23, 2015 at 13:15 -
2$\begingroup$ Content copied to Stack Exchange sites must conform to the licensing of the source material. Wikipedia licenses its content under CC-Attribution-Share Alike-3.0 license, which requires attribution. I have marked all of your copied text as quotes and added links back to wikipedia to attribute the work. In the future, please take care to do this yourself when using content cut and pasted from another site. Also, please make sure that the site you are copying from allows that activity, otherwise copied content will be removed. $\endgroup$– caseyApr 23, 2015 at 13:42
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I think Wikipedia provides a decent answer to this question: http://en.wikipedia.org/wiki/Seismic_scale First of all, a source of confusion is the difference between magnitude and intensity. Intensity (expressed in Roman numerals) estimates the potential for damage of an earthquake on the surface (the effects we see: people, structures...). Meanwhile, magnitude (Arabic numerals) indirectly measures the energy released by an earthquake. There is a list of different scales (intensity and magnitude) in the Wikipedia article.
The more common magnitude scales are the Richter scale (a quantitative logarithmic scale that has problems capturing the overall power of the source above magnitudes around 6) developed by Charles F. Richter in 1934 and the moment magnitude scale. Today, the moment scale is preferred because it works over a wider range of earthquake sizes and is applicable globally. Also, it was developed (at least partially) by a guy called Tom Hanks!
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1$\begingroup$ Clearly, this answer made a bit more sense before the edits to the question clarified the intensity vs magnitude confusion. I think it is still valid. $\endgroup$– arkaiaApr 21, 2015 at 16:28
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Earthquakes are measured by their energy which is in magnitude. Richter scale was a numerical calculation and used before moment magnitude scale is used. Nowadays we use magnitude rather than richter scale. How measuring earthquakes.
