Moment magnitude scale of formula_33 are the same for all earthquakes, one can consider as a measure of the potential energy change Δ"W" caused by earthquakes. Similarly, if one assumes formula_34 is the same for all earthquakes, one can consider as a measure of the energy "E" radiated by earthquakes. Under these assumptions, the following formula, obtained by solving for the equation defining , allows one to assess the ratio formula_35 of energy release (potential or radiated) between two earthquakes of different moment magnitudes, formula_36 and formula_37: As with the Richter scale, an increase of one step on the logarithmic scale of moment
Seismic magnitude scales times. As originally defined by 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 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 scale at less than three seconds, while the broadband scale is measured at periods of up to 30 seconds. The regional mb scale – also denoted mb_Lg, mbLg, MLg (USGS), Mn, and m – was developed by for