The Earth make look like a seamless mass of land when seen from space, but when seen from within, so many of its mysteries become clearer. For example, the surface of the Earth is actually more like a puzzle than a solid landmass. It has a number of plates that fit together. All together, the Earth has seven major plates and many dozens of minor plates. The seams in between each of these plates are called "fault lines."
When two of the plates get too 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.
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.
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. Dip-slip faults
How Earthquakes are 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 Earthquakes are 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.
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.
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.