Magnitudes of earthquakes are measured by logarithmic magnitude scales based on the work of Dr. Charles Richter. Being logarithmic scales, the magnitude of an earthquake increases tenfold for each one point change. Note that magnitude measures the amplitude of waves on a seismogram and does not measure the strength – which is the amount of energy released. For example, a 7.0 quake is 10x bigger than a 6.0 and releases 32x the energy. An 8.0 is 100x bigger than a 6.0 and releases 1,000x the energy. Here’s a difference quake magnitude and strength calculator: https://earthquake.usgs.gov/learn/topics/calculator.php
The largest magnitude earthquake in the continental U.S. was recorded on February 7, 1812 in New Madrid, Missouri. There is some question about the exact magnitude of the quake, but it is thought to have registered around an 8. It was reported that the nearby Mississippi River ran backwards. There were two prior major quakes in the New Madrid area in the prior two months, both registering above 7.5.
The San Francisco earthquake in 1989 was a 6.9 and the Japanese earthquake in 2011 that caused a tsunami and the meltdown of the Fukushima nuclear power plant was a 9.0.
Here is a great graphic from the U.S. Geological Survey showing the number of earthquakes a year at various magnitudes as well as example earthquakes:
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