Seismic moment
Encyclopedia
Seismic moment is a quantity used by earthquake
seismologists to measure the size of an earthquake. The scalar seismic moment
is defined by the equation
, where
thus has dimensions of energy, measured in dyne centimeters.
The seismic moment of an earthquake is typically estimated using whatever information is available to constrain its factors. For modern earthquakes, moment is usually estimated from ground motion recordings of earthquakes known as seismogram
s. For earthquakes that occurred in times before modern instruments were available, moment may be estimated from geologic estimates of the size of the fault rupture and the displacement.
Seismic moment is the basis of the moment magnitude scale
introduced by Hiroo Kanamori
, which is often used to compare the size of different earthquakes and is especially useful for comparing the sizes of especially large (great) earthquakes.
Earthquake
An earthquake 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...
seismologists to measure the size of an earthquake. The scalar seismic moment
is defined by the equation, where
is the shear modulus of the rocks involved in the earthquake (in dyneDyneIn physics, the dyne is a unit of force specified in the centimetre-gram-second system of units, a predecessor of the modern SI. One dyne is equal to exactly 10 µN...
/ cm2)
is the area of the rupture along the geologic fault where the earthquake occurred (in cm2), and
is the average displacement on 
(in cm).
thus has dimensions of energy, measured in dyne centimeters.The seismic moment of an earthquake is typically estimated using whatever information is available to constrain its factors. For modern earthquakes, moment is usually estimated from ground motion recordings of earthquakes known as seismogram
Seismogram
A seismogram is a graph output by a seismograph. It is a record of the ground motion at a measuring station as a function of time. Seismograms typically record motions in three cartesian axes , with the z axis perpendicular to the Earth's surface and the x- and y- axes parallel to the surface...
s. For earthquakes that occurred in times before modern instruments were available, moment may be estimated from geologic estimates of the size of the fault rupture and the displacement.
Seismic moment is the basis of the moment magnitude scale
Moment magnitude scale
The moment magnitude scale is used by seismologists to measure the size of earthquakes in terms of the energy released. The magnitude is based on the seismic moment of the earthquake, which is equal to the rigidity of the Earth multiplied by the average amount of slip on the fault and the size of...
introduced by Hiroo Kanamori
Hiroo Kanamori
is a Japanese American seismologist who has made fundamental contributions to understanding the physics of earthquakes and the tectonic processes that cause them....
, which is often used to compare the size of different earthquakes and is especially useful for comparing the sizes of especially large (great) earthquakes.