Sky wave magnitude scale

The body wave magnitude (m _B , of English body wave magnitude ) is a method for measuring earthquake - magnitudes . It is based on the investigation of the movement of the surface by earthquake waves that do not move on the surface, but in the interior of the earth.

In 1945, Beno Gutenberg developed the following dimensionless number for teleseismic space waves ( body waves , such as P, PP and S waves) for periods T from 0.5 to 12 seconds:

{\ displaystyle m _ {\ mathrm {B}} = \ log _ {10} \ left ({A \ over T} \ right) _ {\ max} + Q \ left (\ Delta, h \ right)}

A is the movement of the surface through the space waves, analogous to the method of determining the local magnitude at the maximum amplitude on the seismogram, measured in micrometers , T is the period of the wave and is a correction factor that depends on the distance of the measurement from the epicenter in degrees , the depth of the earthquake ( hypocenter ) and the instrument used. ${\ displaystyle Q \ left (\ Delta, h \ right)}$ ${\ displaystyle \ Delta}$ ${\ displaystyle h}$

Short-period sky wave magnitude scale

With the introduction of the 1-second seismometer of the Worldwide Standardized Seismograph Network , it became customary at the National Earthquake Information Center to only consider the vertical components of the P-waves (PV) and incorporate them into the correction factor. In addition, the greatest amplitude was only determined within the first cycles instead of over the entire wave train (e.g. to distinguish earthquakes from underground nuclear weapons tests). This variant is referred to as the short-period sky wave magnitude scale (m _b , body-wave magnitude, short period ).

The determination of the magnitude using the short-period sky wave magnitude scale underestimates the strength of earthquakes at a magnitude of around 6.5, since the energy of the earthquake waves emitted by large earthquakes shifts to long-period ranges and is therefore not included in the determination.

source

Peter Bormann: 3.2 Magnitude of seismic events . In: Peter Bormann (Ed.): New Manual of Seismological Observatory Practice NMSOP . revised edition. Deutsches GeoForschungsZentrum GFZ, Potsdam 2009, ISBN 3-9808780-0-7 , p. 33-34, 48 , doi : 10.2312 / GFZ.NMSOP_r1_ch3 .