Regional relationships among earthquake magnitude scales∗

Tectonophysics, 93

(I 983) 249-250

Elsevier Scientific

Publishing

REGIONAL SCALES

249

Company,

Amsterdam

RELATIONSHIPS

- Printed

AMONG

in The Netherlands

EARTHQUAKE

MAGNITUDE

*

D.H. CHUNG

and D.L. BERNREUTER

Lawrence

Livermore National Laboratory University of California, Livermore, Calif: (U.S.A.)

(Received

September

23, 1982)

ABSTRACT

Various magnitude scales commonly are reviewed. It is shown that problems

used in the United necessary

States. When

to determine

information

used and the interrelationships exist with all of the magnitude

using regional

how the reported

is not available,

although

catalogs,

magnitudes

the potential

Both the MS and mb scales were designed

among them scales being

for example,

it is often

were determined.

Often

such

scales. However,

both

errors are quite large.

to be universal

Ms and mb magnitudes are often determined beyond the applicable range of the equations used to define the two scales. Furthermore, the MS magnitudes are not generally available for moderate to small earthquakes in most earthquake catalogs. The mb magnitudes

are more generally

also much greater variation in the mb scale

change

established. significant saturation

This change effect

care

than MS values;

occurred

in the

in instrumentation

on estimated

magnitudes

early

must

than can be recorded be taken

when

1960s when

(post-1960

with the WWSSN

selecting

however,

In particular,

values

m,, values

are lower)

instruments

was had

and

recorded

instruments.

the m,, magnitudes

there is

a significant

the WWSSN

used to determine

level of the m,, scale. The older, longer-period

mb magnitudes

great

available

in the way mb is determined.

a the

larger

In addition,

of western

US.

earthquakes, because the values are often in considerable error due to the fact that they were determined at distances less than 25” and were not properly corrected for attenuation in the upper mantle or asthenosphere. The seismic body-wave magnitude mb of an earthquake is strongly affected by regional variations in the Q structure, composition, and physical state within the earth. Therefore, because of differences in attenuation of P-waves between the western and eastern United States, a problem arises when comparing m,, values for

* The full text of this paper has been published in Reviews of Geophysics and Space Physics, Vol. 19. No. 4, pp. 649-663, November 1981, the American Geophysical Union (Paper No. lR1067).

the two regions. A regional earthquake

as large as one-third of an eastern

given ground

in the western

U.S. earthquake

by M, = 0.57 + 0.92 motion

earthquake

magnitude

bias exists which. depending

unit. There is also a significant

values for earthquakes mb

mh

occurs and where the P-waves are recorded,

between

records

United

difference

result

eastern

earthquakes.

errors

nzh and M,

link between

the

western earthquake

is important

the two regions and for choosing

to represent

between

States. An empirical

and the M, of an equivalent

(n~,,)~~~~. This

on where the

can lead to magnitude

when

is

comparing

a set of real western

U.S.