Saturation 01 (Of A Magnitude Scale)
Saturation 01 (Of A Magnitude Scale) : Above a certain magnitude level, the magnitude determined from the amplitudes or amplitude/period ratios of specified seismic waves recorded by a specific seismograph increases only slowly or does not further increase as the physical size of the earthquake (such as measured by the moment magnitude MW) increases. This behavior, sometimes called saturation, is due to the shape of the displacement spectrum of a typical seismic signal, which is characterized by a corner frequency above which the displacement amplitude decreases rapidly. As the size of the earthquake increases, the corner frequency moves to lower frequencies. When the frequency at which a specific type of magnitude is calculated falls above this corner frequency, that magnitude scale tends to underestimate systematically the event magnitude. Saturation sets in at smaller magnitudes for scales based on records from short-period seismographs. Accordingly, magnitudes mb >6.5, Ml >7.0, mB >7.5, or Ms >8.0 are rarely reported. The new IASPEI (2005 and 2011) standards delay magnitude saturation for mb and mB to values above 7.0 and 8.0, respectively. The new IASPEI (2005 and 2011) standards delay magnitude saturation for mb and mB to values above 7.0 and 8.0, respectively. No saturation occurs for Mw or mantle magnitude Mm, because these scales (if properly applied) always use frequencies well below the corner frequency. See Chapter 3 of this Manual as well as Bormann et al. (2009) and Bormann (2011)
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