Why did initial Herlong earthquake magnitudes differ?

Why did magnitudes differ in initial reports of an earthquake near Herlong on Wednesday night?
• Short answer: Initial reports are calculated automatically and can look at different measurements, delivering estimates that often disagree. Additional analysis is used to determine a more accurate "moment magnitude."

Full question

RGJ.com reported a 4.9 earthquake hit Herlong at 9:55 p.m. based on information from the Nevada Seismology Laboratory at the University of Nevada, Reno. A reader questioned why the U.S. Geological Survey was reporting 4.7 magnitude.

Full reply

Graham Kent, director of UNR’s seismology lab, explained that an automated process looks at bumps and wiggles from seismometers that measure shakes in the earth. When it detects a number of similar spikes in about the same range, it determines there has been an earthquake and sends out an alert.
A quick and dirty “local magnitude” or ML reading is put out.
“There’s a utility in the first 5 or 10 minutes to have something close to the right answer because a 6.5 is different than a 4.5,” Kent said.

RENO GAZETTE JOURNAL

Swarm of earthquakes rattles Herlong

This initial estimate will be revised as further analysis is done using different readings.
Kent said people generally don’t understand that there a number of different magnitudes: local magnitude, surface-wave magnitude, body-wave magnitude and moment magnitude.
Moment magnitude — called MW in seismology circles — is generally considered the most accurate. It looks at the amount of energy released at the source, rather than relying on the effects of that energy at some distance from the source.
“At the end of the day, our moment magnitude is most reliable but it is the slowest to calculate,” Kent said. “That’s what goes in the record books.”
For what it’s worth, the USGS uses data from UNR’s seismology lab.
Regarding the largest of the quakes Wednesday night in Herlong, Kent said it had a moment magnitude of 4.5.
“The USGS and the seismology lab, using totally different techniques, came to the same number,” he said.

http://www.rgj.com/story/news/2016/08/04/why-did-initial-herlong-earthquake-magnitudes-differ/88204152/