Earthquake Definitions

Aftershocks are earthquakes in the same region of the central shock (generally within a few rupture length) but of smaller magnitude and which occur with a pattern that follows Omori's law. Omori's law, or more correctly the modified Omori's law, is an empirical relation for the temporal decay of aftershock rates. Omori published his work on the aftershocks of earthquakes, in which he stated that aftershock frequency decreases by roughly the reciprocal of time after the main shock, in 1894.

Aftershocks are dangerous because they are usually unpredictable, can be of a large magnitude, and can collapse buildings that are damaged from the mainshock. Bigger earthquakes have more and larger aftershocks and the sequences can last for years or even longer especially when a large event occurs in a seismically quiet area, see New Madrid Seismic Zone where events still follow Omori's law from the mainshocks in 1811/1812. An Aftershock sequence is deemed to be over when the rate of seismicity drops back to a background level i.e. no further decay in the number of events with time can be detected.
The epicenter or epicentre is the point on the Earth's surface that is directly above the hypocenter or focus, the point where an earthquake or other underground explosion originates. Seismic waves propagate spherically out from the hypocenter. After an earthquake has occurred, geologists can find out where the epicenter was located by triangulation of seismographic data from three or more different locations.

The epicenter is usually the location of greatest damage. However, in some cases the epicenter is above the start of a much larger event. In these cases, damage may be spread across a larger area with the greatest damage possibly occurring some distance from the epicenter. For example, in the magnitude 7.9 2002 Denali Fault Earthquake in Alaska, the epicenter was at the western end of the rupture, but the greatest damage occurred about 330 km away at the eastern end of the rupture zone.
In geology, a fault or fault line is a planar rock fracture, which shows evidence of relative movement. Large faults within the Earth's crust are the result of shear motion and active fault zones are the causal locations of most earthquakes. Earthquakes are caused by energy release during rapid slippage along faults. The largest examples are at tectonic plate boundaries but many faults occur far from active plate boundaries. Since faults do not usually consist of a single, clean fracture, the term fault zone is used when referring to the zone of complex deformation that is associated with the fault plane.
In physics, intensity is a measure of the time-averaged energy flux. To find the intensity, take the energy density (that is, the energy per unit volume) and multiply it by the velocity at which the energy is moving. The resulting vector has the units of power divided by area. It is possible to define the intensity of the water coming from a garden sprinkler, but intensity is used most frequently with waves (i.e. sound or light).
Intraplate Earthquakes
Intraplate earthquakes are earthquakes that occur within tectonic plates, whereas interplate earthquakes occur at plate boundaries. Although the theory of plate tectonics well describes the mechanisms for interplate earthquakes, very large intraplate earthquakes can inflict heavy damage on towns and cities.
The lithosphere is the solid outermost shell of a rocky planet. On the Earth, the lithosphere includes the crust and the uppermost mantle which is joined to the crust across the Mohorovicic discontinuity. Lithosphere is underlain by asthenosphere, the weaker, hotter, and deeper part of the upper mantle. The base of the lithosphere-asthenosphere boundary corresponds approximately to the depth of the melting temperature in the mantle. As the conductively cooling surface layer of the Earth's convection system, the lithosphere thickens over time. It is fragmented into tectonic plates, which move independently relative to one another. This movement of lithospheric plates is described as plate tectonics.
Magnitude (Richter Magnitude Scale)
The Richter magnitude of an earthquake is determined from the logarithm of the amplitude of waves recorded by seismographs (adjustments are included to compensate for the variation in the distance between the various seismographs and the epicenter of the earthquake). Because of the logarithmic basis of the scale, each whole number increase in magnitude represents a tenfold increase in measured amplitude; in terms of energy, each whole number increase corresponds to an increase of about 32 times the amount of energy released. Events with magnitudes of about 4.6 or greater are strong enough to be recorded by any of the seismographs in the world.
New Madrid Seismic Zone
The New Madrid Seismic Zone, also known as the Reelfoot Rift or the New Madrid Fault Line, is a major seismic zone in the Southern United States and Midwestern United States stretching to the southwest from New Madrid, Louisiana Territory (now Missouri).

The New Madrid fault system was responsible for the 1812 New Madrid Earthquake and has the potential to produce damaging earthquakes on an average of every 300 to 500 years. Since 1812 frequent smaller intraplate earthquakes (earthquakes within a tectonic plate) were recorded for the area.

The seismic zone covers parts of five U.S. states: Illinois, Missouri, Arkansas, Kentucky and Tennessee.
Plate Tectonics
Plate tectonics is a theory of geology that has been developed to explain the observed evidence for large scale motions of the Earth's lithosphere. The theory encompassed and superseded the older theory of continental drift from the first half of the 20th century and the concept of seafloor spreading developed during the 1960s.

The lithosphere is broken up into what are called tectonic plates—in the case of Earth, there are seven major and many minor plates. The lithospheric plates ride on the asthenosphere. These plates move in relation to one another at one of three types of plate boundaries: convergent or collision boundaries, divergent or spreading boundaries, and transform boundaries. Earthquakes, volcanic activity, mountain-building, and oceanic trench formation occur along plate boundaries. The lateral movement of the plates is typically at speeds of 0.65 to 8.50 centimeters per year (the speed at which human nails grow).
Seiches are often imperceptible to the naked eye, and observers in boats on the surface may not notice that a seiche is occurring due to the extremely long wavelengths. The effect is caused by resonances in a body of water that has been disturbed by one or more of a number of factors, most often meteorological effects (wind and atmospheric pressure variations), seismic activity or by tsunamis. Gravity always seeks to restore the horizontal surface of a body of liquid water, as this represents the configuration in which the water is in hydrostatic equilibrium. Vertical harmonic motion results, producing an impulse that travels the length of the basin at a velocity that depends on the depth of the water. The impulse is reflected back from the end of the basin, generating interference. Repeated reflections produce standing waves with one or more nodes, or points, that experience no vertical motion. The frequency of the oscillation is determined by the size of the basin, its depth and contours, and the water temperature. The length of the lake is an exact multiple of the distance between nodes.
Seismic Wave
A seismic wave is a wave that travels through the Earth, most often as the result of a tectonic earthquake, sometimes from an explosion. Seismic waves are also continually excited by the pounding of ocean waves and the wind. Seismic waves are studied by seismologists, and measured by a seismograph, which records the output of a seismometer, or geophone.
Seismometers are used by seismologists to measure and record the size and force of seismic waves. By studying seismic waves, geologists can map the interior of the Earth, and measure and locate earthquakes and other ground motions. Seismograph is often interchangeable with seismometer.
Stress is a measure of force per unit area within a body. It is a body's internal distribution of force per area that reacts to external applied loads. Stress is often broken down into its shear and normal components as these have unique physical significance. In short, stress is to force as strain is to elongation. Term normal stress has synonym in rheology - extensional stress. Term normal stress has synonym in acoustics - longitudinal stress.

Solids, liquids and gases have stress fields. Static fluids support normal stress (hydrostatic pressure) but will flow under shear stress. Moving viscous fluids can support shear stress (dynamic pressure). Solids can support both shear and normal stress, with ductile materials failing under shear and brittle materials failing under normal stress. All materials have temperature dependent variations in stress related properties, and non-newtonian materials have rate-dependent variations.
A tsunami is a series of waves created when a body of water, such as an ocean, is rapidly displaced. Earthquakes, mass movements above or below water, volcanic eruptions and other underwater explosions, landslides, large meteorite impacts comet impacts and testing with nuclear weapons at sea all have the potential to generate a tsunami.

A tsunami has a much smaller amplitude (wave height) offshore, and a very long wavelength (often hundreds of kilometers long), which is why they generally pass unnoticed at sea, forming only a passing "hump" in the ocean. Tsunamis have been historically referred to tidal waves because as they approach land, they take on the characteristics of a violent onrushing tide rather than the sort of cresting waves that are formed by wind action upon the ocean (with which people are more familiar). Since they are not actually related to tides the term is considered misleading and its usage is discouraged by oceanographers.

Definitions obtained from