Since 2002, the United States Geological Society has used moment magnitude as its official earthquake rating system. Enter the “ moment magnitude” scale ( Mw).ĭesigned to adhere closely to local magnitude ratings, the moment magnitude scale uses the earthquake’s “moment”–a calculation involving the ratio of shear stress to shear strain, the average amount of slip on the fault, and the total affected area of the quake. The Richter scale worked well for earthquakes up to 7.0, but became saturated at that point, failing to accurately differentiate major and “great” earthquakes. Earthquakes Larger Than 7.0 – Moment Magnitude Scale However, the energy released by a seismic wave is 101.5 (or about 31.6x) the amount of its amplitude, meaning that a 7.0 quake releases 31.6 times more energy than a 6.0 quake, or 1,000 times more energy than a 5.0 quake. a 7.0 earthquake is 10 times stronger than a 6.0 earthquake, and 100 times stronger than a 5.0 earthquake. The Richter Scale (more accurately referred to now as the “local magnitude” scale or ML), like all other magnitude scales to follow, is logarithmic, meaning each unit up on the scale equals a 10-fold increase in amplitude–e.g. Aiming to assess the strength of earthquakes in Southern California, Richter used measurements of seismic waves combined with their distance from the earthquake epicenter to create a magnitude rating system. Most Americans still associate the concept of earthquake magnitude with the Richter Scale, published by Charles Richter in 1935 (PDF link). Defining Earthquake Magnitude – Types of Scales Richter Scale Scientists rate earthquakes on two common factors: magnitude, or the amount of seismic energy released and intensity, or how the earthquake felt to humans and its effect on structures. The magnitude 9.0 earthquake in Japan in 2011 killed 16,000 people and damaged or destroyed more than 400,000 buildings the magnitude 7.0 quake in Haiti in 2010 resulted in a staggering 316,000 deaths the 2008 quake of magnitude 7.9 in China’s Sichuan province injured almost 400,000 people and left 4.8 million homeless.
Most of us are familiar with the magnitude ratings used to quantify the strength of earthquakes, but what do those numbers really mean? Learn about the systems scientists use to measure earthquake strength beyond the Richter scale.
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