A SEISMIC MOMENT MAGNITUDE SCALE

Ranjit DAS Das, MUKAT LAL SHARMA, HANS RAJ WASON, DEEPANKAR choudhury, Gabriel Gonzalez

Theaimofobtaininga single scale for earthquake magnitudes has led many
studies in the past to either develop relationships among various existing scales or develop
an altogether new scale to represent a wide range of magnitudes on a single scale.
Although a reliable and standardized estimation of earthquake size is a basic requirement
for all tectonophysical and engineering applications, different magnitude scales estimate
different values for the same earthquake, thereby making such studies inadequate. The
moment magnitude (Mw) scale has been referred to by various researchers as the best
scale, one that matches well with the observed surface-wave magnitudes with Ms ≥ 7:5
at a global level. The formulation and validation of the Mw scale were carried out
considering the southern California region for lower and intermediate earthquakes.
In this study, an endeavor has been made to extend the moment magnitude scale to
include lower and intermediate magnitudes in a global context emphasizing the use of
body waves, particularly P waves, in which data are abundant. We first investigate the
degree of closeness of Mw values with other observed magnitudes (e.g., Ms and mb)
for smaller and intermediate magnitude ranges considering global International
Seismological Centre (ISC) and Global Centroid Moment Tensor (CMT) databases. To
improve upon the consistency of the Mw scale for a wider range, a uniform generalized
seismic moment magnitude scale Mwg logM0=1:36 − 12:68, for magnitudes ≥ 4:5,
has been developed, considering 25,708 global earthquake events having mb and M0
values from ISC and Global CMT databases, respectively, during the period 1976–2006.
The Mwg scale is also valid for 3:5 ≤ mb ≤ 7:0 because the relations between seismic
moment and the magnitudes mb and Mwg are same.
The greater accuracy of the Mwg scale over the Mw scale at different magnitudes
(i.e., mb or Ms) is found to be statistically significant in the range including smaller and
intermediate events. The similarity of the Mwg scale is also tested on 394 global seismic
radiated energy values collected from Choy and Boatwright (1995). It is observed that
76% of estimated radiated energy values obtained through the Mwg scale show closer
agreement (than with Mw) to the observed radiated energy values. Mwg is computed
from low- and high-frequency spectra, and because it is consistent for small, intermedi
ate, and large earthquake events, it will play a useful role as an earthquake magnitude
estimator for all earthquake related studies