Why do seismologists change the magnitude of an earthquake?
Obtaining an accurate preliminary magnitude can be difficult due not only to the complex processes that occur deep within the Earth, but because there are over a dozen techniques of for calculating the magnitude of an earthquake. In this animation we look at two magnitude scales, and use two examples of magnitude changing.

Animation and graphics by Jenda Johnson, geologist, Earth Sciences Animated
Narrated by: Dr. Wendy Bohon, Informal Education Specialist, IRIS
Written by Jenda Johnson and Dr. Wendy Bohon,
Reviewed by: Dr. Anne Trehu, seismologist, Oregon State University and Dr. Danielle Sumy, Project Associate, IRIS
ShakeMap intensity maps from the United States Geological Survey
NEIC earthquakes animation from Pacific Tsunami Warning Center (NOAA)
“Blind Thrust” fault model by Brad Aagaard
“The Loma Prieta Earthquake” footage from California Highway Patrol, Multidisciplinary Accident Investigation Team (https://archive.org)
Photographs from the United States Geological Survey
“Global Resonance” by Knights of Fire (Burnt Promises album)
Maps from Google Earth