The size of an earthquake is given by it's magnitude which is often referred
to as Richter Magnitude. On this scale the amplitude of shaking goes up
by a factor of 10 for each unit on the scale. Thus, at the same distance
from the earthquake, the shaking will be 10 times as large during a magnitude
5 earthquake as during a magnitude 4 earthquake. The total amount of energy
released by the earthquake, however, goes up by a factor of 32.
There are many different ways that magnitude is measured from seismograms
partially because each method only works over a limited range of magnitudes and
with different types of seismometers.
But all of the methods are designed to agree well over the range where they
overlap.
The methods used in Northern California (NC) earthquake listings include:

ML

local magnitude, the original scale defined by Richter and Gutenberg
based on the maximum amplitude of the waves.

MD

coda magnitude, based on the duration of shaking.

MW

moment magnitude, based on inverting the waveforms for the moment of the
earthquake which is equal to the rigidity of the fault times the average amount of slip
on the fault times the amount of fault area that slipped.
The methods used in Southern California (SC) earthquake listings include:

MGN

empirically calibrated local magnitude based on readings from highgain components

MLG

local magnitude based on synthetic WoodAnderson response from lowgain components

ML

local magnitude based on synthetic WoodAnderson response from TerraScope stations