SUMMER 2003GEOHAZARDS COURSE AT UWI, MONA - page 048

Prepared and compiled by Rafi Ahmad, Unit for Disaster Studies,
Department of Geography and Geology,
University of the West Indies, Mona, Kingston 7, Jamaica

A better measure of earthquake size is seismic moment.

Mw = 2/3 log Mo – 10.7

where Mw = Moment magnitude; Mo = seismic moment ); 10.7 = a constant.
The seismic moment of an earthquake is determined by the strength of resistance of rocks to faulting (shear modulus) multiplied by the area (length times width or depth) of the fault that ruptures and by the average displacement that occurs across the fault during the earthquake.
The seismic moment determines the energy that can be radiated by an earthquake and hence the seismogram recorded by a modern seismograph.
A seismologist determines that seismic moment of an earthquake from a seismogram by using a computer to plot the seismogram's amplitude of motion as a function of period (wave length). The amplitude of the long period motions in a seismogram, when corrected for the distance from the earthquake, is a measure of the seismic moment for that earthquake.
The Moment magnitude of an earthquake is defined relative to the seismic moment for that event.

It is important to recognize that earthquake magnitude varies logarithmically with the wave amplitude or seismic moment recorded by a seismograph.
Each whole number step in magnitude represents an increase of ten times in the amplitude of the recorded seismic waves. And the energy release increases by a factor of about 31 times.
The size of the fault rupture and the fault's displacement (movement) also increase logarithmically with magnitude as shown.
Magnitude scales have no fixed maximum or minimum. Observations have placed the largest recorded earthquake (off-shore from Chile in 1960) at Moment magnitude 9.6 and the smallest at -3.
Earthquakes with magnitudes smaller than about 2 are called "microearthquakes."
Magnitudes are not used to directly estimate damage.
An earthquake in a densely populated area, which results in many deaths and considerable damage, may have the same magnitude as an earthquake that occurs in a barren, remote area that does nothing more than frighten the wildlife.

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