Earthquake Hazards Program

# Magnitude & Intensity

## Magnitude Definitions Used by the NEIC

Designator Name Formula
Mw Moment Magnitude Hanks and Kanamori formula (1979)

Mw = (2/3) log Mo - 10.7

where Mo is the scalar moment of the best double couple in dyne-cm.

Me Energy Magnitude These energy magnitudes are computed from the radiated energy using the Choy and Boatwright (1995) formula

Me = (2/3) log Es - 2.9

where Es is the radiated seismic energy in Newton-meters. Me, computed from high frequency seismic data, is a measure of the seismic potential for damage.

Ms Surface Wave Magnitude IASPEI formula

Ms = log (A/T) + 1.66 log D + 3.3

where

A
is the maximum ground amplitude in micrometers (microns) of the vertical component of the surface wave within the period range 18 <= T <= 22.
T
is the period in seconds.
D
is the distance in geocentric degrees (station to epicenter) and 20° <= D <= 160°.

No depth corrections are applied, and Ms magnitudes are not generally computed for depths greater than 50 kilometers. The Ms value published is the average of the individual station magnitudes from reported T and A data.

If the uncertainty of the computed depth is considered great enough that the depth could be less than 50 kilometers, an Ms value may still be published, computed by the IASPEI formula and NOT corrected for depth.

In general, the Ms magnitude is more reliable than the mb magnitude as a means of yielding the relative "size" of a shallow-focus earthquake.

mb Compressional Body Wave (P-wave) Magnitude mb = log (A/T) +Q(D,h)

defined by Gutenberg and Richter (1956) except that T, the period in seconds, is restricted to 0.1 <= T <= 3.0 and A, the ground amplitude in micrometers, is not necessarily the maximum in the P group. Q is a function of distance (D) and depth (h) where D >= 5°.

mbLg Body Wave Magnitude using the Lg wave mbLg = 3.75 + 0.90 log D + log (A/T)    for 0.5° <= D <= 4°

or

mbLg = 3.30 + 1.66 log D + log (A/T)     for 4° <= D <= 30°

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. D is the distance in geocentric degrees.

ML Local ("Richter") Magnitude ML = log A - log Ao

defined by Richter (1935) where A is the maximum trace amplitude in millimeters recorded on a standard short-period seismometer and log Ao is a standard value as a function of distance where distance <= 600 kilometers.

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