Moment magnitude scale

A logarithmic scale that estimates earthquake size from seismic moment, providing consistent magnitude measurements across earthquakes of different sizes.

Seismic moment: A measure of an earthquake’s size based on fault area, average slip, and rock rigidity. It is the physical quantity from which moment magnitude is calculated.
Richter scale: A local-magnitude scale developed by Charles Richter for measuring nearby earthquakes from seismogram amplitudes. Its saturation for great earthquakes motivated wider use of moment magnitude.
Earthquake early warning: Systems that detect an earthquake’s initial waves and warn locations before stronger shaking arrives. Rapid magnitude estimates help determine warning areas and expected shaking.
Thomas C. Hanks: An American seismologist whose work helped establish moment magnitude for earthquake measurement. He co-developed the scale’s influential formulation with Hiroo Kanamori.
Moment magnitude equation: The logarithmic equation that converts seismic moment into moment magnitude. It shows how seismic moment is translated into the scale’s reported value.
Surface-wave magnitude: An earthquake magnitude estimated from the amplitudes of long-period surface waves. It can saturate for large earthquakes, unlike moment magnitude.
Seismic hazard: The probability and expected severity of earthquake shaking at a location over a stated period. Magnitude distributions help quantify the earthquakes that contribute to regional hazard.
Hiroo Kanamori: A Japanese seismologist known for foundational work on earthquake source physics and magnitude measurement. His collaboration helped introduce moment magnitude as a consistent measure.
Fault rupture: The sudden slip of a fault that releases stored elastic strain in an earthquake. Rupture dimensions and slip determine the seismic moment behind a magnitude estimate.
Body-wave magnitude: An earthquake magnitude estimated from amplitudes of seismic body waves. Its limited wave periods cause it to underestimate the largest events.

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