<P> Measurement of mb has changed several times . As originally defined by Gutenberg (1945c) m was based on the maximum amplitude of waves in the first 10 seconds or more . However, the length of the period influences the magnitude obtained . Early USGS / NEIC practice was to measure mb on the first second (just the first few P - waves), but since 1978 they measure the first twenty seconds . The modern practice is to measure short - period mb scale at less than three seconds, while the broadband mB scale is measured at periods of up to 30 seconds . </P> <P> The regional mb scale--also denoted mb_Lg, mbLg, MLg (USGS), Mn, and m--was developed by Nuttli (1973) for a problem the original M scale could not handle: all of North America east of the Rocky Mountains . The M 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 . East of the Rockies the continent is a craton, a thick and largely stable mass of continental crust that is largely granite, a harder rock with different seismic characteristics . In this area the M scale gives anomalous results for earthquakes which by other measures seemed equivalent to quakes in California . </P> <P> Nuttli resolved this by measuring the amplitude of short - period (~ 1 sec .) Lg waves, a complex form of the Love wave which, although a surface wave, he found provided a result more closely related the mb scale than the M scale . Lg waves attenuate quickly along any oceanic path, but propagate well through the granitic continental crust, and Mb is often used in areas of stable continental crust; it is especially useful for detecting underground nuclear explosions . </P> <P> Surface waves propagate along the Earth's surface, and are principally either Rayleigh waves or Love waves . For shallow earthquakes the surface waves carry most of the energy of the earthquake, and are the most destructive . Deeper earthquakes, having less interaction with the surface, produce weaker surface waves . </P>

Who developed procedure used to measure size of an earthquake