Seismic magnitude scales waves travel through. Determination of an earthquake's magnitude generally involves identifying specific kinds of these waves on a seismogram, and then measuring one or more characteristics of a wave, such as its timing, orientation, amplitude, frequency, or duration. Additional adjustments are made for distance, kind of crust, and the characteristics of the seismograph that recorded the seismogram. The various magnitude scales represent different ways of deriving magnitude from such information as is available. All magnitude scales retain the logarithmic scale as devised by Charles Richter, and are adjusted so the mid-range approximately correlates with the original "Richter" scale. Since 2005
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