How strong is a 4.8 earthquake? Quake magnitudes explained.

The 4.8 magnitude earthquake that struck New Jersey 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. Click here to see the USDA shake map for the most recent earthquakes.

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 around 63 earthquakes of between magnitude 5.0 and 5.9 each year, according to USGS, about five between 6.0 and 6.9 and less than one between 7.0 and 7.9.

A major 7.4-magnitude earthquake hit Taiwan on Wednesday morning, killing 9 people and injuring over 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 US Geological Survey. Instead, it’s because there are more measurement devices, called seismometers that record vibrations, which 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-sized slabs of rock that glide over the planet’s mantle, constantly but very slowly reshaping the 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 ten 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 totally 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. A 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 senation 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 a 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.

While you might have heard the term "the Richter Scale" used to describe earthquakes, 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 what kind of soil it occurs in.

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 actually 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, 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 in the movies

Though movies like San Andreas and Earthquake show enormous entire coastlines dropping off into the ocean, it's not going to happen.

According to the US 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 magnitude depends in part on how long the geological fault it occurs on is. 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 USGS.

The largest earthquake in US history was the 1964 Good Friday quake in Alaska, a 9.2 magnitude quake which killed 131 people. It lasted four and a half minutes and ran along the Aleutian fault.

What's the chance of an aftershock?

Any time there's an earthquake, one concern is that it was merely a foreshock to a bigger quake that's coming.

Globally, the probability one earthquake will be followed by a large earthquake within 3 days is somewhere just over 6%, according to USGS.

That means whenever there's an earthquake, there's about a 94% chance that it wasn't a foreshock and there won't be any more quakes linked to it.

The numbers are a little different for California, which is very seismically active. There, about half of the biggest earthquakes were preceded by foreshocks, according to USGS. But that means half were not.

Unfortunately, it's impossible to know if any one quake is a foreshock.

Aftershocks are defined as smaller earthquakes that happen in days after a larger quake. As USGS puts it, they represent "minor readjustments along the portion of a fault that slipped at the time of the mainshock."