Magnitude

General Definitions of Magnitude, Science Definitions of Magnitude

What is Magnitude in Relationship to Earthquakes

How are Magnitudes Measured? Different Methods of Measurement.

Magnitude and the Importance to the Earthquake Insurance Industry:

Magnitude has many different definitions. However as it relates to the insurance industry it is simply a measurement of an earthquake. The use of magnitudes is critical in our understanding of Earthquakes and the property damage that they do. The insurance industry depends on various measurements to derive premiums. This article is about (1)Magnitude, its (2)Earth Sciences eDfinition and (3)How to Measure it. Lastly we will explore why its use plays a part in deriving earthquake premiums,

Magnitude – Many Definitions:

Magnitude – more definitions than you thought.

Merriam-Webster defines magnitude as “the intensity of an earthquake represented by a number on an arbitrary scale.” Source. That is the fifth of five definitions. Just within that one source Other definitions have to do with a “celestial…scale,” or the “caliber of something,” or “great size,” or “a numerical … measure expressed usually as a multiple of a standard unit.” For our purposes for the QuakeCov site though Magnitude has everything to do with a measure of earthquake intensity.

According to Etymoline magnitude is from the 1400s. Originally meaning “pre-eminence, magnificance.” The word changed in the 1500s to mean “greatness of size or extent.” The word seems to be formulation of two Latin words “magnus” and “tudo.” Magnus meaning great and Tudo used as a word forming element, similar to the English phrase “ness.” In other words Magnitude means both Great and Ness.

Earth Science Definitions of Magnitude:

Two Earth Sciences Definitions:

In Earth science Dictionary.com states that Magnitude is a “measure of how strong or violent [an earthquake] is.” The word is a noun.

Brittanica calls magnitude a “a measure of the size or amplitude of the seimic waves generated by an earthquake source and recorded by seismographs.” Amplitude could be summed up with the terms movement or vibration. “In 1935 the American seismologist Charles Richter set up a magnitude scale of earthquakes as the logarithm to base 10 of the maximum seismic  amplitude recorded on a standard seismigraph  at a distance of 100 km (60 miles) from the earthquake epicentre.” [More on the Richter scale latrer]

Edaurraa says “Magnitude is the quantitative value for seismic waves formed by an earthquake.” This is a nice simple earth sciences definition.

It seems that all the earth movement related definitions of Magnitude involve “seimic waves” and “earthquakes” and often “seismographs.“

Personally QuakeCov greatly appreciates the definition of Magnitude as being a “quantitative value” of “an earthquake.” Sometimes too much science confuses many.

The USGS, perhaps the premier world government agency that deals with Earth Movement has a far more complex set of information concerning this very important earthquake term.

USGS Definition of Magnitude:

Maximum Motion.

The USGS defines magnitude as “a number that characterizes the relative size of an earthquake.” The definition goes on to explain further: “Magnitude is based on measurement of the maximum motion recorded by a seismigraph.” Source. We learn here that magnitude is not just motion, but… maximum motion. Similar to driving an auto, this is not average speed but rather the top speed.

Several Scales of Magnitude Created:

The Four Different scales of Magnitude that the USGS considers.

According to the USGS: “Several scales [of magnitude] have been defined.” And “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). “

It should be noted that the first three scales [1,2,3] ” have limited range and applicability” and do not do a terrific job measuring “the largest earthquakes.” Therefore “the moment magnitude (Mw) scale, based on the concept of seismic movement is uniformly applicable to all sizes of earthquakes but is more difficult to compute than the other types.”

• ML=Local Magnitude or Richter Scale for short
• Ms= Surface Wave Magnitude
• Mb= Body Wave Magnitude
• Mw= Moment Magnitude

The concept of Magnitude that many consumers imagine is obviously far more complex than just a single number. As an example – the Richter Scale is what many consider to the be THE method of measuring earthquakes and it is just one of the methods. The Moment Magnitude is “uniformly applicable” to all earthquakes.

Magnitude – the Richter Scale vs Moment Magnitude:

ML, Ms, Mb, and Mw, confusion under your feet.

The Richter scale is still used. But its use is best for small regional earthquakes. The Richter scale, known as ML has limitations as it is “strictly valid only for certain frequency and distance ranges.” Other scales and tools were created including Body Wave Magnitude or Mb, and Surface Wave Magnitude Ms. All three had issues and eventually the Movement Magnitude or Mw was created. This “gives the most reliable estimate of earthquake size” according to the USGS.

The exact science of why USGS and other governmental and scientific groups have moved beyond ML, Mb, and Ms is beyond the scope of this earthquake insurance site. However the Mw or Moment Magnitude is the most complete measurement tool available. That is to say that Mw is the preferred method of measuring magnitude.

While ML is more well know, it is Mw that is now the real work horse. Remember that Moment Magnitude [ML] is considered “uniformly applicable” to all the earthquakes.

*Editors note, while the second initial for most of these is lower case for some unusual reason the Richter scale initial is indeed an uppercase letter.

And ….then there is the Mercalli Scale:

Not to be confused with all the variants of measuring earthquake magnitude with instruments there is the humble Mercalli Scale. The scale “does not use scientific equipment to measure seismic waves,” according to Michigan Tech. The Mercalli scale uses “observations,… amount of damage…to estimate its intensity.” Created in 1902 and perfected in 1931 the scale is still used today in a more non scientific way.

What does Magnitude have to do with Earthquake Insurance Premiums?

Deriving quake premiums is far more difficult that you might assume.

Property insurers and specifically Earthquake insurers use past magnitude data to assist in determining the frequency and degree of future quakes. Seismic data, such as magnatude assist underwriting and software programs in determing premium per $1,000 of property insurance. All insurers price earthquake policies slightly differently. The CEA and CEA alternatives all take past quakes and faults into consideration.

All things being equal an area that has had larger magnitude events will likely see higher premiums relative to areas that have seen lower magnitude events. Seismic frequency goes hand in hand wtih this as well. The depth of earthquakes is also another very important consideration. A magnitude measurement assists in deriving earthquake premiums. Simply put.

That is not to say that Depth, Frequency, and Mw are the only considerations. Dozens of other earth sciences criteria are involved as well. This includes property location relative to known faults. Other terrestial considerations include Soil Type, Slope, and Civic Land Improvements. All of the insurers use different tools and there are numerous additional considerations having to do with the earth.

Magnitude: Definitions, Measurement Methods, and EQ Insurance:

Magnitude is the maximum motion as it relates to an earthquake. There are four main methods that the USGS uses. The older Richter Scale method is not the most accurate. Mw or Moment Motion is the most advanced and useful. Certainly new methods will be invented in the coming decades. Earthquake Insurance Underwriters use magnitude data to assist in deriving annual premiums. There are numerous other underwriting criteria used other than historical magnitude readings.