The Earth may look like a seamless mass of land when seen from space, but when seen from within, so many of its mysteries become clearer.

Facts about Earthquakes

  • Earthquakes occur when friction occurs between two tectonic plates.
  • Tectonic plates are giant pieces of rock and land mass that make up the Earth’s most outer layer. These plates sit on top of the Earth’s mantle, which is a dense fluid or plasma-like substance.
  • The Richter Scale was created in 1935 by a scientist named Charles Richter. The Richter Scale measures the intensity of Earthquakes.
  • There are several different types of faults where friction can occur. These include strike-slip faults and dip-slip faults.
  • The massive Indonesian earthquake in December of 2004 registered at a 9.0 on the Richter Scale.

The Layers of the Earth

The Earth has four main layers. The outermost layer that reaches the surface of Earth is called the Lithosphere, or crust. The next inside layer is called the mantle, the topmost part of which comes together with the crust to form the outside surface of the planet itself. The third interior layer is called the core, which is made up of liquid metal. The final layer is called the inner core, which is solid.

Because some of the Earth’s layers are solid matter and some are liquid, the planet as a whole is always in motion, and not always in a way that works well for the Earth itself. Sometimes, this continual movement produces earthquakes.

What are Tectonic Plates?

The Earth’s top layer, the crust, rests on the mantle. The crust is more like a puzzle than one solid landmass. It has a number of plates that fit together, and the plates are made of rock and other solid materials. The Earth has seven major plates and dozens of minor plates. The seams in between each of these plates are called “fault lines.”

Why do Earthquakes happen?

As tectonic plates sit on top of the dense but fluid mantle, they move around. When two tectonic plates get close, one may try to slip underneath or rise above the other. Sometimes the friction caused by the two plates jockeying for position results in a great release of kinetic (movement) energy, causing an earthquake.


Faults are generally categorized as a type of boundary either within a continental plate or at the edges of two plates. Fault zones such as the San Andreas represent key areas at which earthquakes occur. Strong vibrations are generated inside the Earth as a result of earthquakes which causes the ground to shake. The various types of plate tectonics movements on which an earthquake can occur are discussed below:

Strike-Slip Faults. A strike-slip (or transform) fault results when two plates move sideways and past each other. Examples of earthquakes caused by strike-slip faults include the San Andreas and Haywards faults in California.

Dip-Slip Faults. A Dip-slip fault occurs when a plate moves vertical in respect to another plate or when two plates move vertical past each other. There are two types of Dip-Slip Faults, normal and reverse. A Normal Fault occurs when the plate on the bottom side of the fault slides down. A Reverse Fault occurs when the plate on the top side of the fault slides up. The Sierra Nevada fault, in Eastern California, is an example of a Dip-Slip Fault.

Oblique-Slip Faults. Oblique slip faults occur when two plates significantly slide both vertically and horizontally against each other.

How are Earthquakes Measured?

Scientists use a piece of equipment known as a “seismograph” to measure earthquake activity and strength. A seismograph measures three distinct pieces of information: the size (magnitude) of the earthquake, the depth of the earthquake and the location of the earthquake.

Seismographs must be quite steady themselves in order to accurately record and measure earthquake activity across the planet. The machine itself is mounted on a stable surface such as concrete. The actual measuring unit consists of a strong weight, a hanging pen and a paper drum. When an earthquake occurs, this triggers the pen to record the wave activity on the paper wrapped around the drum. Scientists can then study these markings to measure the magnitude, depth and location of the earthquake.

How are Earthquakes Ranked?

Scientists use a tool called the Richter Scale to measure the strength of an earthquake and rank each earthquake in relation to others. The Richter Scale was developed in 1935 and was named after its primary inventor, Charles F. Richter.

The Scale uses a logarithm to determine the overall size and strength of an earthquake, which is then expressed as a fraction. For example, the size and strength of a relatively minor earthquake might be expressed as a 2.3. A major earthquake, however, might be expressed as a 6.5. The scale runs from 1.0 to 9.0+.

To date, it is estimated that the vast majority of recorded annual earthquake activity falls between the 2.0 to 3.9 range. Only a relatively small minority of earthquakes register as a 4.0 and above. Over the last 47 years’ worth of measurement data, only a handful of earthquakes have ranked above an 8.0 on the Richter Scale.

What are Foreshocks and Aftershocks?

The Earth may give indications that a larger earthquake is about to occur by generating foreshocks. Foreshocks are basically smaller earthquakes.

However, to date scientists have not been able to come up with any way to determine in advance when an earthquake will happen or whether an earthquake is a foreshock or an actual earthquake. This can only be determined after the fact.

Aftershocks, as their name suggests, are the movements of the tectonic plates settling back down into a workable position post-earthquake.

Some foreshocks and aftershocks are so minor that they may not even register on a seismograph, while others may be quite noticeable.

Annual Earthquake Count

The US Geological Survey National Earthquake Information Center, or USGS for short, has estimated the annual number of earthquakes worldwide in the millions.

However, since some of these earthquakes may be too minor to be registered by a seismograph or may be located in parts of the world that are not currently monitored by a seismograph, the exact count can never be precisely determined.

Every year, new seismographs are installed to help scientists monitor more regions around the globe.

What are some of the largest Earthquakes in History?

These quakes were amongst the largest quakes ever recorded:

  • Chile: This South American quake on May 22, 1960, registered a 9.0.
  • Alaska: This North American quake on March 27, 1964, registered an 8.5.
  • Indonesia: This Asian quake on Dec. 26, 2004, registered a 9.0.