Body wave magnitude

[3] Use of mB  as originally defined has been largely abandoned,[4] now replaced by the standardized mBBB  scale.

[6] It was introduced in the 1960s with the establishment of the World-Wide Standardized Seismograph Network (WWSSN); the short period improves detection of smaller events, and better discriminates between tectonic earthquakes and underground nuclear explosions.

[8] As originally defined by Gutenberg (1945c) mb was based on the maximum amplitude of waves in the first 10 seconds or more.

The ML scale was developed in southern California, which lies on blocks of oceanic crust, typically basalt or sedimentary rock, which have been accreted to the continent.

In this area the ML scale gives anomalous results for earthquakes which by other measures seemed equivalent to quakes in California.

Differences in the crust underlying North America east of the Rocky Mountains makes that area more sensitive to earthquakes. Shown here: the 1895 New Madrid earthquake, M ~6, was felt through most of the central U.S., while the 1994 Northridge quake , though almost ten times stronger at M 6.7, was felt only in southern California. From USGS Fact Sheet 017–03.