mbLg Magnitude (0.5°< D < 4°, T~1 s)

Calculation for the AS-1 Seismograph

Fig. (a) AmaSeis 24-hr record showing the June 18, 2002, mbLg 5.3 Evansville earthquake (event at right center). Fig. (b) Extracted seismogram showing the Lg waves (largest). Fig. (c) Close-up of the Lg wave arrival. Click on the image for an enlarged view and an example of the measuring the amplitude and period.

Supply the first three inputs in the Calculator and click on Calculate to find the mbLg Magnitude.

EARTHQUAKE MAGNITUDE

Amplitude

Digital Units

form AS-1/AmaSeis

Distance

Degrees

Period

Seconds

Magnitude

mbLg

mbLg = 3.75 + 0.90 log D + log (A/T) for 0.5° <= D <= 4° (Body Wave Magnitude using the Lg wave) as proposed by Nuttli (1973) where A is the ground amplitude in micrometers and T is the period in seconds calculated from the vertical component 1-second Lg waves. Use only the following values of T : 1, 1.5, 2, 3 seconds. D is the distance in geocentric degrees. The mbLg magnitude formula is primarily used for earthquakes at relatively small distance in intraplate regions such as central and eastern North America.

Developed by Lawrence W. Braile & Saptarshi Dasgupta, Purdue University, May, 2003

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