Earthquakes

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, an earthquake (also known as a quake, tremor or temblor) is the result of a sudden release of energy in the Earth's crust that 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.

Falla_de_San_Andrés

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.

Global_plate_motion_2008-04-17

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. 

Fault_types

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, landslides, mine blasts, 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)
Onde_compression_impulsion_1d_30_petit

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

Onde_cisaillement_impulsion_1d_30_petit

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. 

sourcehttp://en.wikipedia.org/w/index.php?title=Earthquake&oldid=603632640


See also

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