What is an earthquake?

Ask most people what an earthquake is and they will respond with what they have seen on TV or from a movie. Many believe an earthquake is when the earth shakes a back and forth. While this is a good start, it’s only a small part and it doesn't explain WHY the ground is shaking.

So, what causes the earth to quake? For most quakes, we look to the faults.

It’s the fault

Simply defined an earthquake is the sudden slip on a fault and the resultant ground shaking and radiated seismic energy. Earthquakes can also be caused by volcanic or magmatic activity, or they can be caused by sudden stress changes in the earth. Most people think of the earth moving in a horizontal motion (side to side) during an earthquake, but the earth movement can be vertical or horizontal or both.

A fault is where two tectonic plates meet. You can think of it as a fracture along which blocks of the earth’s crust move relative to one another, parallel to the fracture. The fault plane angles down into the earth along the tectonic plates. It is along this fault plane were slipping can occur. This slipping along the plates is where the seismic rupture (the earthquake) begins.

In an earthquake, the point on the fault plane where this slipping is centered (where the rupture begins) is called the hypocenter, or focus, of the earthquake. Directly above the hypocenter (usually miles above), on the earth’s surface, is the epicenter of the earthquake.

Types of faults

There are three general types of faults. From these three types others are defined, such as which direction the plates move relative to an observation point. For the purposes of simplicity we will stick to the general types.

In a strike-slip fault, the blocks of crust are vertical, or almost vertical. When movement along the strike-slip fault occurs, the blocks of crust move sideways (horizontally) along the fault line.

The next two types of faults are categorized as dip-slip faults, where the blocks of crust are inclined and fault movement is mostly vertical. These two faults reference the blocks of crust as one being above the inclined fault, and the other below the incline. Think of it as when you are walking on a hill you are above the incline. Similarly, the crust above the incline (on top) is the referenced rock mass.

If the rock mass above the incline moves down, where the top crust drops along the fault plane, it is referred to as a normal fault.

The other dip-slip fault is called a reverse fault. In the reverse fault the top crust (the rock mass above the incline) moves upwards along the fault.

If a reverse fault has a dip of 45 degrees or less it is called a thrust fault.

A thrust fault that has not ruptured to the surface, so there is no evidence of the fault that we can see, is called a blind thrust fault.

Oblique-slip faults which have components of different slip styles.

Most of earthquakes are single fault ruptures. However, multi fault of earthquakes are possible.

Most earthquakes are, in part, described by the type of fault where the rupture occurred, such as a thrust fault. Many of the largest earthquakes are megathrust earthquakes.

Megathrust earthquakes happen where plate boundaries converge. This is where one plate if forced under another, and area referred to as a subduction zone. In the process of one plate going under, the other plate is pushed upwards, which is called a thrust.

For more information you can check the USGS site

USGS webpage with animations for earthquake terms and concepts.