The first magnitude scale was developed in 1935 by the physicist and seismologist Charles Richter, and even today earthquakes are commonly classified in units on the Richter scale (local magnitude). However, over time it became apparent that the Richter scale was only suitable for earthquakes of certain magnitudes and occurring within specific distances because it does not accurately reflect the quantity of energy released by very large or extremely distant earthquakes. For this reason, other magnitude scales were developed.

Local magnitude ML (the Richter scale)

Use Local magnitude, abbreviated ML (though the Swiss Seismological Service (SED) often uses the acronym MLh), is determined for earthquakes that occur relatively close to recording stations, normally for distances of up to a few hundred kilometres between the station and the earthquake.

Determining parameters The local magnitude of an earthquake depends on its maximum amplitude as recorded by a Wood-Anderson seismometer (see the question "What is amplitude?"). Since this type of seismometer is barely still in use today, the vibrations measured by modern equipment are converted into artificial Wood-Anderson seismograms.

Advantages Local magnitudes are very quick and easy to calculate. Furthermore, Wood-Anderson seismometers are sensitive within a frequency range very similar to smallish buildings (see the question "What is frequency?"). This makes the estimated local magnitude a good basis for predicting potential damage to properties.

Disadvantages If an earthquake has a magnitude higher than around 6, the ML 'tops out', i.e. does not rise significantly even if the earthquake was larger. What is more, the ML is less meaningful for earthquakes with a magnitude of less than around 2 or which occurred further than about 600 km from the measuring station.

 

Local magnitude MLhc

Use In 2020, the SED revised the previously used local magnitude and changed it from "MLh" to "MLhc". The "c" stands for "corrected". Since the last time the local magnitude scale was changed, the seismic network in Switzerland has very significantly densified. The revised local magnitude MLhc calibrated for Switzerland allows the SED to take advantage of this change. For smaller earthquakes, MLhc enables to accurately estimate magnitudes since it is correctly calibrated for seismic stations very close to the earthquake (within 15 to 20 km). In addition, the SED can use all seismic stations across Switzerland to determine MLhc, including those in urban areas, since it includes site amplification factors.

Determining parameters The local magnitude of an earthquake is determined by taking the mean value of local magnitudes as determined at each seismic station. The local magnitude at each station depends on the maximum amplitude as recorded by a Wood-Anderson seismometer (see the question "What is amplitude?"); the distance of this station from the earthquake, and a site specific amplification factor. The original Wood Anderson seismometer has been replaced decades ago by modern broadband seismometers that have much larger amplitude and frequency ranges. In order to be consistent with the original magnitude scale, the vibrations measured by modern equipment are first converted into artificial Wood-Anderson seismograms.

Advantages The calculation of MLhc is based on a larger data set compared to the one previously used for the calculation of the local magnitude, using all earthquakes recorded back to 2000. Recordings from almost all stations can now be used, even from those located within less than 15-20 kilometres from the earthquake focus in the ground (hypocentre). The procedure for calculating MLhc takes into account physics-based site corrections that are routinely calculated and updated by the SED.

Disadvantages As any local magnitude definition, MLhc exhibits saturation for moderate-to-large magnitude events. This becomes apparent for events greater than magnitude 6.

 

Body wave magnitude mb

Use Body wave magnitude is normally used for earthquakes that occurred more than 2,000 km away from a measuring station.

Advantages and determining parameters The mb for such distant earthquakes can be estimated relatively quickly because it is derived directly from the amplitudes of P waves, compression waves that are transmitted through the Earth and are the first signals to reach a seismic station (see the question "What are P, S, Love and Rayleigh waves?").

Disadvantages If an earthquake has a magnitude higher than around 6, the mb tops out, making it unable to distinguish between, say, magnitude 6 or 7.5 earthquakes.

 

Surface wave magnitude MS

Use Surface wave magnitude (MS) is used to estimate the energy released by strong and/or distant earthquakes.

Determining parameters and disadvantages The MS is calculated from surface wave data. The velocity of surface waves (S waves) is far lower than that of P waves, which pass through the Earth (see the question "What are P, S, Love and Rayleigh waves?"). The slow propagation of surface waves explains why, after an earthquake occurs, seismologists cannot estimate immediately whether it was strong or very strong. Moreover, earthquakes occurring very deep underground may generate few surface waves or even none at all.

Advantages The MS only tops out in the event of very large earthquakes with a magnitude higher than around 8. Earthquakes that occur close to the surface (within the topmost 30 kilometres) generate stronger surface waves than deeper quakes of the same magnitude. If the MS value of an earthquake is higher than its mb value, this is an indication that the quake occurred near the surface and can be expected to cause greater damage, assuming its epicentre is close to a densely populated region. The MS:mb ratio is also used to distinguish earthquakes from (nuclear) explosions, which have a smaller source volume than an earthquake of similar magnitude. In addition, explosions generate fewer shear waves, which play a major role in forming surface waves. Accordingly, explosions tend to be associated with far lower MS values. This makes the mb:MS ratio a good criterion for distinguishing between shallow earthquakes and explosions: if the value is high, the event was probably caused by an explosion.

 

Moment magnitude Mw

Use

Moment magnitude (Mw) is the most revealing type of magnitude because it is the only type of magnitude that does not saturate, regardless of how powerful the earthquake is. The classical determination of Mw was based on long-period (low-frequency) parts of the seismograms at stations not too close to the earthquake, which meant that the determination was only possible reliably for earthquakes with magnitude 4 and above. In the meantime, Mw can also be determined for smaller quakes and even for micro-earthquakes with magnitudes in the minus range if the number and quality of the seismic stations are good enough (see the question "What does a negative magnitude mean?").

Determining parameters

The moment magnitude Mw (‘w’ stands for ‘work’) is the only type of magnitude which has a physical meaning. Mw was derived from the seismic moment M0 based on theoretical studies. M0 is the product of the size of the fault multiplied by the average displacement at the location of the fault multiplied by the shear strength of the rock. There are several ways to determine Mw. Often synthetic seismograms are matched to actual observations. This entails varying the size of the fault, the mean vertical displacement und the fault's orientation until the synthetic seismograms sufficiently match their naturally recorded equivalents. In many cases, the amplitude spectrum of the earthquake signal is used, whose long-period plateau value is a measure of the seismic moment M0.

In order to correct the influence of the station’s local soil conditions, the geometry of the earthquake focus and the noise, it is necessary to assess M0 at a sufficient number of stations, from which an average value is then determined. Especially for the determination of Mw for micro-earthquakes, the seismic stations have to record the signals with sufficient high sampling to determine the plateau value of the spectrum.

Advantages

Mw directly indicates the energy released during an earthquake and does not saturate even during the largest earthquakes. Furthermore, Mw is globally comparable.

Disadvantages

Estimating Mw is more time-consuming than calculating other types of magnitude. When larger earthquakes occur, it can take a several hours to produce an initial estimate.

 

M

If you find magnitude designated simply as 'M' in one of our lists, this indicates that the seismic observatory that estimated it did not specify which type it used. This is often the case for observations made by the US Geological Survey. In such instances it is assumed that a type of magnitude that does not 'top out' was used. For strong earthquakes, this often means Mw.