# Factors That Determine the Impact of an Earthquake

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Instructor: David Wood

David has taught Honors Physics, AP Physics, IB Physics and general science courses. He has a Masters in Education, and a Bachelors in Physics.

Ever wonder why some earthquakes are literally earth-shattering, while others go unnoticed? This lesson will highlight the contributing factors that determine whether earthquakes register at all or are catastrophic occurrences.

## What Is an Earthquake?

An earthquake is a violent shaking of the ground caused by motions of the Earth's crust. The Earth's surface might look solid and stationary, but it really isn't. The Earth's crust isn't a single, unbroken surface. Rather, it's broken down into a lot of sections, like cracks in a smashed window. We call these tectonic plates, the pieces that make up the whole of the Earth's crust. What's more, those parts are always moving. The Earth's plates shift toward each other, away from each other, or alongside each other, depending on which plates we're talking about. And these motions can cause violent destructive events, including earthquakes and volcanoes.

Earthquakes happen either when two plates collide with each other or when two plates slide alongside each other. This is why California has so many earthquakes: there is a plate boundary called the San Andreas Fault that lies right along the coast. The sliding motion of the fault creates friction along it, resulting in a lot of tectonic movement and, as a result, earthquakes.

But why do California's earthquakes rarely cause huge destruction? Why do so few people die in American earthquakes, but so many do in other countries, like Nepal? Today, we're going to talk about the factors that affect the impact of an earthquake.

## Factors that Affect the Impact of an Earthquake

There are seven main factors that affect the impact of an earthquake:

#### Severity

Some earthquakes are just bigger than others. In fact, earthquakes hit the United States every single day, but most are too small to notice. We know they occur because of seismographs, sensitive devices that detect tremors in the ground, and the application of the Richter scale, which rates earthquakes on a scale of 1 to 10. The most severe earthquake ever recorded was in Valdivia, Chile, in 1960 and registered at 9.5 on the Richter scale, though, strictly speaking, the scale doesn't have an end point.

#### Distance

Earthquake intensity is affected by both the distance along the surface of the Earth and how deep the earthquake is below the Earth. There have been earthquakes that were very severe but didn't cause much damage, because they occurred a long way from places that humans live. Earthquakes can also hit anywhere from the surface to about 450 miles below the surface. By the time a deep earthquake has reached us, its energy has often dissipated through the ground, leaving little left to do damage.

#### Population Density

Another big deal is population density. If the earthquake hits in an area where people are spread out, the impact will be much smaller, since fewer people will be affected. However, if it occurs in a dense metropolitan area, a larger number of people will be affected.

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