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Illustrated Earthquake Glossary

acceleration

Earthquakes
Generally, acceleration is increase of speed or velocity. During earthquakes, the shaking of ground is a swinging motion involving both acceleration and deceleration. Its maximum value is the largest increase in velocity recorded at a particular point. It largely influences how damaging an earthquake is in this area.

aftershock

Earthquakes
Aftershocks following the M6.5 quake in Idaho on 31 Mar 2020
Aftershocks following the M6.5 quake in Idaho on 31 Mar 2020
Aftershocks are smaller quakes that follow a usually larger earthquake in the same rupture zone. The can occur up to weeks, months or even years after, depending on the size of the main shock.
Aftershocks are smaller than the mainshock and occur within 1-2 rupture lengths distance from the mainshock.

In general, the larger the mainshock, the larger and more numerous the aftershocks, and the longer they will continue. Large earthquakes can be followed by thousands of aftershocks.

The larger aftershocks following an earthquake can be particularly dangerous if many structures have been ... -> See whole entry

Related keywords (2):

foreshock - mainshock

crust

Earthquakes
Continental and oceanic crust (image: USGS)
Continental and oceanic crust (image: USGS)
The earth's crust is the outermost major layer of the earth, which forms its varied surface both above and under water that we live on. Its thickness ranges from about 10 to 65 km worldwide. The uppermost 15-35 km of crust is brittle enough to produce earthquakes.
The crust is composed by magmatic, sedimentary and metamorphic rocks. There are two major types of crust: oceanic and continental crust.

Oceanic crust is usually less than 10 km thick and primarily composed of volcanic rocks rich in iron and magnesium (called: mafic rocks). Oceanic crust is much thinner and heavier than continental crust, having a mean density of approx. 3.0 grams per cub... -> See whole entry

earthquake

Earthquakes
An earthquake (also called quake, tremor or temblor, rumbling etc) is the shaking of the surface of the earth resulting from a sudden release of energy in the Earth's interior, usually by sudden movements along fault lines. This movement releases energy that propagates as seismic waves.
The size of earthquakes is determined by the amount of energy released in this process. It can range in size between many orders of magnitude and is commonly given as a number on the logarithmic Richter scale. It ranges from so tiny that they are not felt and can be only detected by the most sensitive modern instruments (seismometers) to so strong that entire regions are shaking violently.

-> See whole entry

epicenter

Earthquakes
The point on the Earth’s surface vertically above the point (focus or hypocenter) in the crust where an earthquake occurs, i.e. where the seismic rupture nucleates.

Related keywords (1):

hypocenter

fault

Earthquakes
Illustration of the main types of tectonic faults (source: USGS)
Illustration of the main types of tectonic faults (source: USGS)
A fault is a fracture in the rigid crust of the earth, along which the two blocks on either side have moved relative to each other and often may do so again in the future. When blocks move along a fault, tectonic earthquakes are generated.
Depending on the movement, there are strike-slip faults (side-wards movement), normal faults (caused by extension), and reverse or thrust faults (caused by compression). A reverse fault with a small dip angle is called a thrust fault. -> See whole entry

focusSynonym of: hypocenter

Earthquakes
Epicenter and hypocenter of an earthquake (Source: USGS earthquake glossary)
Epicenter and hypocenter of an earthquake (Source: USGS earthquake glossary)

foreshock

Earthquakes
Foreshocks are smaller earthquakes that precede the largest earthquake in a series, which is termed the mainshock. Not all earthquakes arre preceded by foreshocks.

Related keywords (2):

aftershock - mainshock

hypocenter

Earthquakes
Epicenter and hypocenter of an earthquake (Source: USGS earthquake glossary)
Epicenter and hypocenter of an earthquake (Source: USGS earthquake glossary)
When talking about earthquakes, the hypocenter or focus is the point inside the earth where an earthquake rupture starts. It is often confused with epicenter, which is the is the point directly above it at the surface of the Earth, where the quake is usually felt most strongly.

Related keywords (1):

epicenter

magnitude

Earthquakes
A number that characterizes the relative size of an earthquake. Different scales are used, but the most common is the logarithmic Richter scale. where most quakes are between 0 (tiny) and 9 (extremely large quake). Each step on this scale indicates a 10 fold energy increase of an earthquake. Felt quakes are normally above magnitude 2.5 on this scale.
Magnitude is based on measurement of the maximum motion recorded by a seismograph(sometimes for earthquake waves of a particular frequency), corrected for attenuation to a standardized distance.

Several scales have been defined, but the most commonly used are (1) local magnitude (ML), commonly referred to as œRichter magnitude, (2) surface-wave magnitude (Ms), (3) body-wave magnitude (Mb), and (... -> See whole entry

mainshock

Earthquakes
A mainshock is the largest earthquake in a sequence, sometimes preceded by one or more foreshocks, and almost always followed by many aftershocks.

Related keywords (2):

aftershock - foreshock

P-wave

Earthquakes
Illustration of a P-wave (image: USGS)
Illustration of a P-wave (image: USGS)
The P wave (short for primary wave, also called pressure or compressional wave) is the seismic wave that shakes the rocks forth and back in the same and opposite direction of the propagation direction (longitudinal).
The primary (P) waves of earthquakes are the same type of waves as the more familiar sound waves. Both are waves propagating spherically away from its source through a compressible medium (rocks, fluids and gasses) where particles are swinging in the same and opposite direction of the wave propagation.

Typical speeds of P-waves from earthquakes are between 1450m/s in water and 5000m/s in granite.... -> See whole entry

primary waveSynonym of: P-wave

Earthquakes
Illustration of a P-wave (image: USGS)
Illustration of a P-wave (image: USGS)

seismograph

Earthquakes
A seismograph at a volcano observatory in Indonesia
A seismograph at a volcano observatory in Indonesia
Seismic recording on a drum
Seismic recording on a drum
An instrument that displays vibrations and movements in the Earth = the recording of a seismometer.
Traditional seismographs transfer the recorded shaking to a precise pencil that writes a continuous shake line onto a sheet of paper wrapped around a turning drum.

The drum slowly turns at steady speed and at the same time, the pencil is slowly moved horizontally, so that the line forms a continuous spiral line on the paper until it is filled. Each turn of the drum (or each line on the unwrapped... -> See whole entry

tectonic

Earthquakes
The adjective "tectonic" refers to processes, properties, or resulting features related to the deformation (breaking and slow, plastic deformation) of rocks over large sections of the upper mantle and crust (lithosphere). It is usually applied to large-scale features and processes, in the km scale and app. Smaller miniature tectonic processes are sometimes called micro-tectonic.

tectonic faultSynonym of: fault

Earthquakes
Illustration of the main types of tectonic faults (source: USGS)
Illustration of the main types of tectonic faults (source: USGS)

 

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