Seismic moment

Seismic moment measures an earthquake’s size from the rigidity of the rocks, the fault area that slipped, and the average slip. It is the physical quantity underlying moment magnitude.

Seismic moment tensor: A mathematical representation of an earthquake source’s strength, orientation, and pattern of deformation. It generalizes scalar seismic moment to describe the geometry and direction of fault motion.
Fault area: The area of a fault surface that slips during an earthquake. A larger ruptured area generally produces a larger seismic moment, all else equal.
Richter magnitude scale: An early logarithmic earthquake-magnitude scale based on seismic-wave amplitudes. It relies on recorded wave amplitudes, unlike moment magnitude’s basis in seismic moment.
Earthquake scaling laws: Empirical relations linking earthquake size to rupture dimensions, slip, and other source properties. Seismic moment provides a common size measure for testing how rupture properties scale.
Hiroo Kanamori: A Japanese seismologist who helped establish modern earthquake source physics and moment magnitude. His work connected seismic moment to a practical magnitude scale.
Shear modulus: A measure of a material’s resistance to deformation by shearing forces. Rock rigidity enters the seismic-moment calculation as the shear modulus.
Elastic modulus: A family of measures describing how strongly a material resists elastic deformation. Shear modulus is the specific elastic property used in the seismic-moment formula.
Radiated energy: The portion of an earthquake’s energy emitted as seismic waves. A given seismic moment does not uniquely determine how much energy an earthquake radiates.
Finite-fault model: A model of an earthquake that estimates slip across a fault divided into subfaults. Its mapped slip distribution can be integrated to estimate total seismic moment.
Thomas C. Hanks: An American seismologist who co-developed the moment magnitude scale with Hiroo Kanamori. Their 1979 formulation made seismic moment the basis of a broadly useful earthquake scale.

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