Magnitude & Intensity

Mw = (2/3) log Mo - 10.7

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

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 = 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.

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 = 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.

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.