Earthquake
The sudden movement of the Earth caused by the abrupt release of accumulated strain along a fault in the
interior. The released energy passes through the Earth as seismic waves (low-frequency sound waves), which
cause the shaking. Seismic waves continue to travel through the Earth after the fault motion has stopped.
Recordings of earthquakes, called seismograms, illustrate that such motion is recorded all over the Earth for
hours, and even days, after an earthquake.
Earthquake Engineering
The branch of engineering concerned with reducing earthquake or seismic risk to structures. Because strong
earthquakes are rare events, building codes have traditionally allowed a significant degree of damage. Even
high-seismic regions, such as San Francisco or Tokyo, typically experience a strong earthquake only once in
many decades. If an earthquake occurs, buildings and other structures are designed such that most will be
damaged (but should not collapse), and will have costs for repairs, business interruption, and potentially
casualties.
Intraplate Earthquakes
Intraplate (within the plate) earthquakes occur far removed from the plate boundaries. Although nearly two-thirds
of the Earth's continental crust is stable interior crust, less than 10% of all earthquakes are intraplate earthquakes. The causes of intraplate
earthquakes are poorly understood, but it is likely that they relate to the driving forces of plate tectonics. As the plates collide,
separate, or slide past one another, some plates gain and other plates lose material at their edges, and the plate boundaries
change over geologic time. Weakened former plate boundaries become part of the interiors of plates. Stresses originating at the
edges of the plates or in the deeper crust may be localized (concentrated) by these weaker structures, causing intraplate
earthquakes. Other, more localized possible causes of intraplate earthquakes include growing or shrinking glaciers, impoundment
or drawdown of reservoirs, and upwelling of mantle plumes.
Paleoseismology
The study of geological evidence for past earthquakes. This is a scientific discipline that has contributed
greatly to modern understanding of the nature of earthquakes. The patterns of earthquakes, in both space
and time, evolve over centuries and millennia and cannot be discovered by modern instruments. Knowledge
of these patterns is important for understanding the physics of earthquakes and for forecasting future
destructive earthquakes.
Plate Tectonics
Plate tectonics theory provides an explanation for the present-day tectonic behavior of the Earth, particularly the global distribution of mountain building, earthquake activity, and volcanism in a series of linear belts. Numerous other geological phenomena such as lateral variations in surface heat flow, the physiography and geology of ocean basins, and various associations of igneous, metamorphic, and sedimentary rocks can also be logically related by plate tectonics theory.
Seismic Risk
The probability that social or economic consequences of earthquakes will equal or exceed specified values at
a site, at several sites, or in an area, during a specified exposure time. Historically the term seismic risk has
been used to describe an assortment of earthquake effects that range from ground shaking, surface faulting,
and earthquake-induced landsliding to economic loss and casualties. As more quantitative methods for
estimating the effects of earthquakes have been developed, terminology has become more precise. Although
the term seismic risk is still sometimes used in a general sense to mean the potential for both the occurrence
of natural phenomena and the economic and life loss associated with earthquakes, it is useful to differentiate
between the concepts of seismic hazard and seismic risk. Seismic hazard may be defined as any physical
phenomena (for example, ground shaking or ground failure) that are associated with an earthquake and that
may produce adverse effects on human activities.
Seismographic Instrumentation
Various devices or systems of devices for measuring movement in the Earth. Ground motion is generally the
result of passing seismic waves, gravitational tides, atmospheric processes, and tectonic processes.
Seismographic instrumentation typically consists of a sensing element (seismometer), a signal-conditioning
element or elements (galvanometer, mechanical or electronic amplifier, filters, analog-to-digital conversion
circuitry, telemetry, and so on), and a recording element (analog visible or direct, frequency modulation, or
digital magnetic tape or disk). Seismographs are used for earthquake studies, investigations of the Earth's
gravity field, nuclear explosion monitoring, petroleum exploration, and industrial vibration measurement.
