Magnitude is a measure of the amount of energy released during an earthquake; seismologists use a Magnitude scale to express the energy released by the quake.
Every earthquake has a unique amount of energy and the magnitude values given by different seismological observatories can sometimes seem confusing. Values may vary because several different methods are used to estimate magnitude. So as seismologists obtain and analyse the data that comes in, they might revise magnitude estimates.
Seismologists will consider estimates received from seismograph stations that record the earthquake then average them. Earthquake magnitude measured by the Richter magnitude scale is based on the height of the waves on a seismogram. Since each lab is averaging from the different stations they have access to, magnitude units reported can be different and it may take several days before a consensus is agreed on the best magnitude estimate.
The more recently developed moment magnitude scale is independent of all this in that it comes from the actual seismic moment. Earthquakes are caused by internal torques (a force that changes the angular momentum of a system which is defined by the force times the distance from the centre of rotation.) The moment magnitude scale takes into account the amount and nature of fault slippage through a mathematical formula which provides a more constant measure.
So whereas the Richter test scale assigns a single number to quantify the size of the earthquake’s seismic waves, the Moment magnitude scale is a rating system that calculates the the torque of an earthquake. This involves a formulae based on the energy released at the moment of the earthquake multiplied by the average amount of slip on the fault and the size of the area slipped.
The Moment Magnitude Scale ~
The moment magnitude scale was introduced by Thomas Hanks and Hiroo Kanamori in 1979 and is used by scientists, seismologists and geologists to compare the size of earthquakes where the Richter scale is not so accurate.
Moment magnitude which is more precise than the Richter scale is not based on instrumental recordings of the earthquake but on the area of the fault that ruptured at the moment of the quake. The moment magnitude scale is based on the physical quantity of the moment and it is calculated by multiplying the area of the fault’s rupture surface by the distance the earth moves along the fault times the rigidity of the rock.
A further equation is used to convert the seismic moment to a magnitude. This equation is: Magnitude = 2/3 x[log(Moment) – 16.1] and a calculator for solving the earthquake magnitude using seismic moment method can be found here.
According to UpSeis, an educational site for budding seismologists, there are an estimated 900,000 earthquakes with magnitude 2.5 or less each year. These are quakes that are usually not felt but can be recorded by a seismograph using the Richter scale. Approximately 500 earthquakes a year are magnitude 5 to 6.0; these can cause slight damage to buildings and other structures while once every five to ten years there is a Great earthquake of magnitude 8.0 or more which can totally destroy communities near the epicentre.
Moment magnitude scale has advantages over other magnitude scales and these include: it remains an accurate indicator of the magnitude of very large earthquakes and can be used, with confidence, for earthquakes of any size. It is generally the preferred magnitude scale used by seismologists today.
Sources
Earthquake Measurement Scales
Earthquake Magnitudes
A Moment Magnitude Scale
