Earthquake Magnitudes

From the RedPuma site. …

I have no idea what Seismologists do to arrive at their data stats. And with all due respect, since I am of the opinion the root causes of earthquakes are so completely different to the accepted understanding I might be talking about a different phenomenon on a different planet in a different solar system.

Originally posted by RedPuma:

Earthquake size is expressed by its magnitude.

Magnitudes usually are measured from the amplitude and period of seismic signals as they arrive and are recorded at a seismic station. For a given earthquake, the amplitude decreases with increasing distance (due to attenuation of the signals) and a distance dependent correction is applied when computing magnitude to result in one magnitude value for each station.

Earthquake size does not depend on where an earthquake was recorded, this is contrary to felt effects – the intensity – which decreases with distance from the earthquake source.

Several methods exist on how to compute magnitude – in principal, all methods provide the same or a similar value. However, there are fundamental differences on how these magnitudes are computed (sometimes resulting differing magnitudes). Here is a short summary, describing various magnitude types:

The local magnitude ML is computed for earthquakes, which occurred relatively close to the recording stations. Typically this is done for earthquakes within a few hundred kilometers between the earthquake and the recording station. The first magnitude scale developed 1935 by Richter (the 'Richter-Magnitude') is such a local magnitude; even today earthquake size is commonly given as 'Richter-Magnitude'.

The body-wave magnitude mb is typically recorded for earthquakes that occurred more than about 2000 kilometers away from the recording station. It can be computed relatively fast, because its value relies on the amplitude of the so-called P-phase of an earthquake. P-phases are waves travelling through the body of the earth's interior and are the first signal that reaches a seismic station.

For large earthquakes (magnitude larger than 6), mb 'saturates', meaning that even if the actual size of the earthquake is larger, the value of mb does not increase any more. In such cases, seismologists have to rely on other magnitude types.

The surface wave magnitude MS is measured from surface waves. These waves travel along the surface of the earth with a velocity much slower than P-waves travel through the earth. Therefore, one has to wait a longer time, until these waves arrive at a distant station and MS cannot be computed as rapidly as mb. Depending on distance, it may take up to 1 or 2 hours until surface waves arrive, compared to a maximum of 20 minutes of P-waves.

MS is measured from 20 s period waves (compared to 1 s for mb) and 'saturation' begins only for very large (magnitude larger than 8) earthquakes. The slow surface wave speed is the reason, why seismologists cannot distinguish quickly between a strong and very strong (magnitude > 6) earthquake.

Earthquakes close to the earth's surface (say, the upper 30 kilometers) generate large surface waves compared to a same-size earthquake at larger depth (this has to do with how surface waves are generated).

Shallow earthquakes are more prone to cause damage than deep ones; a high MS-value compared to the mb-magnitude thus indicates that strong damage might have occurred for an earthquake close to a major urban area.

The ratio between MS- and mb-magnitude is also a good measure to distinguish earthquakes from (nuclear) explosions. Explosions have a much smaller source-volume than similar sized earthquakes and explosions typically cause less shearing motion (which mainly generate surface waves) than earthquakes.

Explosion MS-values are thus typically much smaller than for an earthquake of the same size. For shallow seismic events, the mb/MS ratio is thus a good discriminant (large ratios pointing to an explosion).

The moment magnitude Mw is the only magnitude that is directly related to the physics at the earthquake source. Mw is derived (based on theoretical considerations) from the seismic moment M0, which is the product of the fault area times average displacement at the fault times material rigidity.

In theory, Mw does not saturate since M0 includes the complete earthquake rupture.

Several ways exist to determine Mw; often Mw is obtained by fitting seismic waveforms or spectral amplitudes by scaling synthetic seismograms to match observed seismogram amplitudes. The procedures are (a bit) more time consuming than simple seismogram amplitude measurements (ML, mb, MS) and Mw for larger events globally are currently available several hours after an earthquake.

Whenever the magnitude type in one of our lists is given as 'M', this means, that the seismological observatory reporting the specific magnitude did not specify how the magnitude was computed. Often, these are magnitude values from the NEIC.

You may assume, that such a magnitude value represents 'their best effort', and for strong earthquakes such magnitudes often are magnitudes of the type Mw.

So I prefer to use the far less specific M when considering magnitudes.

The only one that I can go with is the Richter scale which is location bound, thus can't be catered for earth wide when quakes occur in uninhabited areas.

And of course building methods and standards differ earth wide too. And that's not counting corruption which is endemic in all countries from Britain to China.


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