How is the amount of energy released during an earthquake measured?
We can determine how big an earthquake is by measuring the size of the signal directly from the seismogram. a seismograph is an instrument used by seismologist to measure any earth tremor.
you need to also know how far away is the source of the tremor (btw it could be caused by earthquake or other thing like underground nuclear blast). This is because the amplitude of the seismic waves recorded on the seismograph decreases with distance. so a correction factor is required To correct for and yes to pin-point the epicenter.
There several scales in use - Richter scale, Momentum magnitude scale etc. the one you would hear in the news after an occurrence is the former. Hope this helped...