Latest earthquakesWorld wide earthquake map from the USGS [For past 30 days M4.5+]

Within the studies of earth science and natural disasters or hazards,  earthquakes (also known as a quakes, tremors or temblors) are the result of  sudden release of built up tension energy in the Earth's crust that then creates seismic waves.

The seismicity, seismism or seismic activity of an area refers to the frequency, type and size of earthquakes experienced over a period of time.


The San Andreas fault visually highlights the origins of earthquakes. image: wikipedia

How earthquakes are measured and rated

Earthquakes are measured using observations from seismometers. The moment magnitude is the most common scale on which earthquakes larger than approximately 5 are reported for the entire globe. 

The more numerous earthquakes smaller than magnitude 5 reported by national seismological observatories are measured mostly on the local magnitude scale, also referred to as the Richter scale. 

These two scales are numerically similar over their range of validity. Magnitude 3 or lower earthquakes are mostly almost imperceptible or weak and magnitude 7 and over potentially cause serious damage over larger areas, depending on their depth. The largest earthquakes in historic times have been of magnitude slightly over 9, although there is no limit to the possible magnitude. 

Intensity of shaking is measured on the modified Mercalli scale. The shallower an earthquake, the more damage to structures it causes, all else being equal.

Earthquakes as general displacement of earth

At the Earth's surface, earthquakes manifest themselves by shaking and sometimes displacement of the ground. When the epicenter of a large earthquake is located offshore, the seabed may be displaced sufficiently to cause a tsunami. Earthquakes can also trigger landslides, and occasionally volcanic activity.


The general movement of tectonic plates and intersection with boundaries is the main cause of natural earthquakes. image: wikipedia

Overall locations of earthquake epicentres. image: wikipedia

In its most general sense, the word earthquake is used to describe any seismic event — whether natural or caused by humans — that generates seismic waves. 


There are 3 main types of faults that can lead to earthquakes. image: wikipedia

Most Earthquakes are caused by the rupture of geological faults, but also by other events such as volcanic activity, underground landslides, mine blasts, fracking, and nuclear tests. 

Where exactly earthquakes happen when they happen

An earthquake's point of initial rupture is called its focus or hypocenter. The epicenter is the point at ground level directly above the hypocenter.

The location of earthquake. image: wikipidia

How does the ground shake or move during an earthquake?

There are three main earthquake movements:

  • Longitudinal P-waves (shock- or pressure waves) in solid rock P-waves travel at about 6 to 7 km per second
  • Transverse S-waves (both body waves) S-waves ranges from 2–3 km/s in light sediments and 4–5 km/s in the Earth's crust up to 7 km/s in the deep mantle.
  • Surface waves — (Rayleigh and Love waves)

An animation of a p-wave movement of the ground. Notice the compression of the lines. image: wikipedia


An animation of an S-wave. image: wikipedia

Love wave. image: wikipedia/Nicoguaro 

The location of earthquakes can be precisely determined using seismometers by measuring the time taken for earthquake vibrations to be detected at least 3 different locations. 


Commentary: Can we predict earthquakes?

Some areas near tectonic plate boundaries are like to be more seismically active due to the more regular build up of pressure from tectonic plates so quakes are expected regularly. But it is hard to predict earthquakes more than perhaps a few seconds due to the uneven distribution of pressure points. These pressure points could exist in multiple areas that are hard to measure. In contrast weather is easier to predict because it deals with more homogenous bulk fluids like air and regular heating and cooling of the planet. 

The origin of tectonic plate movement is due to more continuous subterranean slow processes like convection in the mantle that itself is hard to measure due to being less homogenous. This leads to large numbers of potential fracture or weak points that push against the crust. There may be fracture points in multiple levels which are difficult to measure or predict because of lack of measurement access points. 

See also

World wide earthquake map from the USGS [For past 30 days M4.5+]