Earthquakes’ magnitudes, by the numbers
The 4.8 magnitude earthquake that struck New Jersey on Friday generated shaking that could be felt from Washington, D.C., to north of Boston.
About 55 earthquakes a day – 20,000 a year – are recorded by the National Earthquake Information Center. Most are tiny and barely noticed by people living where they happen.
Worldwide, there are on average about 16 major earthquakes in any given year, 15 in the magnitude 7.0 range and one 8.0 or greater, according to records going back to 1900.
The United States typically has about 63 earthquakes between magnitude 5.0 and 5.9 each year, according to the U.S. Geological Survey, about five between 6.0 and 6.9 and fewer than one between 7.0 and 7.9.
A major 7.4-magnitude earthquake hit Taiwan on Wednesday morning, killing 12 people and injuring more than 1,000. The strongest earthquake there in a quarter century was followed by a series of aftershocks, reaching up to 6.4 magnitude.
More earthquakes are being recorded around the globe, but that doesn’t mean there are more earthquakes happening, according to the U.S. Geological Survey. Instead, it’s because there are more measurement devices, called seismometers, that record vibrations and they have been deployed in more places.
Earthquakes are a natural part of life on Earth, a geologically active planet with seven major tectonic plates. These are continent-size slabs of rock that glide over the planet’s mantle, constantly but very slowly reshaping Earth’s landscape.
What does magnitude mean in an earthquake?
Magnitude is a measurement of the strength of an earthquake. Officially it’s called the moment magnitude scale. It’s a logarithmic scale, meaning each number is 10 times as strong as the one before it. So a 5.2 earthquake is moderate, while a 6.2 is strong.
The magnitude and effect of an earthquake, according to Michigan Technological University:
⬤ Below 2.5: Generally not felt
⬤ 2.5 to 5.4: Minor or no damage
⬤ 5.5 to 6.0: Slight damage to buildings
⬤ 6.1 to 6.9: Serious damage
⬤ 8.0 or greater: Massive damage, can destroy communities
Intensity scales, measured in Roman numerals, are used to describe how strong the earthquake felt to people in the area.
According to the California Earthquake Authority, an intensity of I is typically felt only under especially favorable conditions. An intensity of IV, which leads to light shaking, is felt indoors by many, but not typically outdoors. It might awaken some people at night and lead to a sensation like a truck striking a building. A parked car would rock. Intensities VI and above would be strong, frightening and felt by all, with the damage increasing up to an X, where the shaking would be violent. Some well-built wooden structures would be destroyed and most masonry and frame structures along with their foundations would be ruined.
You might have heard the term “the Richter Scale” used to describe earthquakes, but it is no longer commonly used because it was only valid for certain earthquake frequencies and distance ranges.
Why does an earthquake’s depth matter?
Magnitude measures how strong a quake is. That said, how much the ground shakes depends on an earthquake’s intensity, which in turn depends on two things: how far away the actual site of the temblor was and the kind of soil it which it occurs.
This is why an earthquake’s depth is often given. The epicenter of the Northridge earthquake in Los Angeles in 1994 was in the San Fernando Valley, the point where the quake was strongest. But the hypocenter, the location where the quake occurred, was more than 11 miles underground. If it had been closer to the surface, the 6.7 magnitude quake would have been even more devastating.
The type of ground matters, too. Bedrock shakes least, sand and gravel as much as two times more, and mud and landfill as much as five times more. This was a major factor in why some areas of San Francisco suffered more than others in the Loma Prieta quake of 1989.
Why real earthquakes aren’t like those in the movies
Though movies such as “San Andreas” and “Earthquake” show entire coastlines dropping into the ocean, it’s not going to happen.
According to the U.S. Geological Service, while a “mega-quake” with a magnitude of 10 or larger is “theoretically possible,” it’s very unlikely.
The magnitude of an earthquake depends in part on the length of the geological fault on which the quake occurs. Longer faults result in stronger earthquakes.
There are no known faults capable of generating a magnitude 10 or larger. The San Andreas fault of movie fame couldn’t produce a quake larger than about 8.3 given its length, according to the USGS.
The largest earthquake in U.S. history was the 1964 Good Friday quake in Alaska, a 9.2 magnitude temblor that killed 131 people. It lasted four and a half minutes and ran along the Aleutian fault.