SEISMIC NOTES
INFORMATIVE BULLETIN OF THE PUERTO RICO SEISMIC NETWORK

No. 5 - 1998

SEISMIC TERMS GLOSSARY*

Accelerometer - Instrument to measure accelerations of the ground based on time.
Active fault - Fault throughout which there has been displacement in historical times (Holocene) or where earthquake centers have been located (fig 1).
After Shocks - Earthquakes smaller than follow the one greater one, concentrated in the zone of the main earthquake.
Astenosphere - the layer below the litósfera, characterized by low seismic speeds.
Benioff Zone - narrow zone defined by the earthquake centers, of a thickness of tens of kilometers, that descends from the surface under the terrestrial crust.
Cluster (of earthquakes) - A series of earthquakes of similar magnitude that happen in the same place.
Crust (Earth's Crust) - the most external rocky part of the Earth; its thickness is about 5 to 40 km.
Earthquakes - sudden and violent movement that originates in the crust or outer Earth mantle.
Epicenter - the point of the Earth surface directly upon the center (or hypocenter) of an earthquake.
Fault - a fracture or zone of rock fracture throughout which both sides have displaced. The total displacement can vary from centimeters to kilometers.
Focus (hypocenter) - the place where the earthquake is originated.
Fore Schocks - small Earthquakes that precede the strongest earthquake in a series, concentrated in a restricted volume of the crust.
Ground waves (of earthquake) - seismic waves that only travel the Earth's surface, with a speed lower than that of S waves. There are two types of ground waves: Rayleigh waves and Love waves (fig 2).
Intensity (of earthquakes) - a measurement of the earthquake obtained by the effects and damage caused to the human infrastructure, changes in the Earth surface and field information.
Inverted Fault - fault in which the rocks over the fault plane move upwards and on those of down, so that the oldest layers are placed on youngest (fig 1).
Liquefaction (of the ground) - Process in which the Earth and the sand behaves more like a dense fluid than like a humid solid during an earthquake.
Love Waves - superficial seismic Waves with only horizontal movement of normal shears to the direction of propagation (fig 2).
Magnitude (of earthquake) - measured as large as an earthquake, determined taking the logarithm (in base 10) from the greater movement of the ground registered during the arrival of a type of seismic wave and applying the standard correction by the distance to the epicenter. Three common types of magnitude are: Richter (or the premises) (MILILITER), wave of P(mb) volume and ground wave (Ms).
Mantle (Earth's mantle) - the most voluminous part of the Earth between the crust and the nucleus, varying from depths of about 40 kilometers to 2900 kilometers.
Microseism - weak and practically continuous seismic waves or "noise" of the Earth that can only be detected by seismographs. Often it is caused by waves of the sea, wind or human activity.
Moment (of earthquake) - measurement of an earthquake's size than is obtained by multiplying the rigidity of the rock by the area of the fault and the amount of sliding.
Normal Fault - vertical Fault in which the rocks over the fault plane have moved downwards in relation to the block that was underneath (fig 1).

Figure 1. Types of Faults (Of Bolt, 1981)

Nucleus (Earth's nucleus) - central part of the Earth found at a depth greater than 2900 kilometers that are fused on the other hand external and solid by the central part.
Oblique Fault - Fault that combines movements of tear and vertical (fig 1). Vertical fault - Fault in which the relative displacement is throughout the direction of the dip of the fault. The displacement normal or is invested (fig 1).

P Waves - the first, or fastest wave. It travels from the center through rocks and consists of a series of compressions and expansions of the material (fig 2).

Figure 2. Diagram illustrating the form of the movement of the ground near the surface in the four types of seismic waves (Of Bolt, 1981)

Plate Tectonics - theory of the movement and interaction of tectonic plates. An attempt to explain earthquakes, volcanos and formation of mountains as a result of great movements of the Earth's surface.
Rayleigh Waves - superficial seismic Waves with movement of the ground only in the vertical plane containing the direction of propagation of the wave (fig 2).
Risk (Seismic) - the relative risk is the earthquake's danger in a place compared with another place. The probabilístico risk is the probability of occurrence of an earthquake within a region in a determined interval of time.
S waves - Secondary seismic waves, they travel slower than the P waves. They consist of cross-sectional elastic vibrations on the route direction. They cannot propagate in liquids (fig 2).
Seismic Wave - elastic Earth Wave, normally generated by an earthquake or an explosion (fig 2).
Seismograph - instrument to register the movements of the Earth surface, based on time.
Seismology - the study of earthquakes, seismic sources and waves propagation through the Earth.
Seismometer - sensorial part of a seismograph, normally a suspended pendulum.
Subduction Zone - oceanic Board that descends towards the interior of the Earth moving away of the oceanic grave. Normally it is the place of intermediate and deep earthquake occurrence that defines the zone of Benioff.
Tear Fault - Fault in which the relative displacement is horizontal (fig 1).
Tectonic Earthquakes - Earthquakes that are the result of the sudden liberation of accumulated energy by Earth deformation.
Tectonic Plate - part of the litosphere of the Earth, great and relatively rigid, that moves in relation to other parts of the litósfera on deeper zones of the interior of the Earth. The boards hit in convergence zones and they separate in zones of divergence.
Tidal Wave (Tsunami) - series of waves caused generally by movements of the ocean floor in an earthquake.
Transform Fault - tear fault that connects the end of a displacement in a mid-ocean mountain range, arc of islands or in chains of arc and mountain ranges. Pairs of boards slip in relation to others throughout transform faults.
Volcanic Earthquakes - earthquakes associated with volcanic activity.
Tsunami - see tidal wave.

* Many of the deficiones were taken from Bolt, B., 1981. Earthquakes, Reverté Editorial, Barcelona, 266p.

UNIVERSITY OF PUERTO RICO AT MAYAGÜEZ
PUERTO RICO SEISMIC NETWORK
PO BOX 9017
MAYAGÜEZ, PR 00681-9017
TEL. 265-5452, FAX 265-1684
http://rmsismo.uprm.edu
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