Magnitude of an Earthquake

An earthquake is the result of a sudden release of energy in the earth’s crust that creates vibration radiating all directions with a decreasing rate from the centre of disturbance. When an earthquake occurs, it is important to know where the seismic event took place, how intense it was, and what its impact was on the built environment. The more we know about earthquakes and how and when they occur, the more we can do to lessen their effects on our communities.

Measure of Energy

Whenever there is an earthquake, it indicates the some of the strain energy held within the body of the earth has been released in the form of rupture. The released energy travels around in the form of earthquake waves, causing displacement of the ground surface. In a low-intensity earthquake this displacement is small, while in a high intensity earthquake it is large. Thus, the displacement is directly a function of the amount of energy released during an earthquake.

If ‘ a’ is the maximum displacement recorded, then magnitude ‘M’ is given by the relationship

M = log₁₀ a +c

According to Richter the relation of magnitude with the energy released ( E in ergs) is given by the equation

log₁₀ (E) =5.8+2.4M.

Details of Scale

In this scale, one can measure size of earthquakes based on recording of ground motion by instruments. The Richter scale value is calculated by measuring the maximum recorded amplitude of a wave. This measurement quantities i.e., the ground motion and the energy released at the source of an earthquake, which is referred to as its magnitude. The Richter scale is open ended and logarithmic (base 10) and accommodates the wide range of ground motions, appeared during arrival of a p-wave or surface waves and measured applying a standard correction for distance to the epicenter, that earthquake can caused. This means that there are no upper and lower limits to the scale and that every time the magnitude goes up by one unit, the amount of energy this represents increase thirty times. Each unit of the Richter Scale represents a 10 times increases in wave amplitude. Richter magnitude is expressed as an Arabic number, which helps to distinguish it from the Marcalli scale.

It is observed that a magnitude less than 5.0 is of no engineering interest as it does not have any adverse effect at all.

Energy in Relation to Magnitude

An earthquake of magnitude 4.0 on Richter scale would release energy equivalent to 6 tons of TNT, or about as much energy as a small atom bomb. A magnitude of 3.0 is equivalent to only 397 pounds of TNT. It should be noticed that 397 pounds is 30 times smaller than 6 tons. In the following table the different magnitude equivalent to TNT energy are stated clearly:

Historical Development of Scale

The first magnitude scale was devised by Charles Richter in 1935. Scales today are based on various aspects of the seismograms as told by the following names:

      a) Body-waves magnitude
b) Duration magnitude
c) Moment magnitude

Jamaica uses the duration magnitude and moment magnitude. Moment magnitude is the most true indication of the size of an earthquake because it is based on the amount of movement of fault. 

Moment Magnitude

Moment magnitude is the measure of total energy released by an earthquake. It is based on the area of the fault that ruptured in the quake. It is calculated in part by multiplying the area of the fault’s rupture surface by the distance the earth moves along the fault. 

Richter Scale in Relation to Mercalli Intensity Scale

 

The Mecalli scale describe the effect of the energy, while the Richter scale describes the amount of energy associated with an earthquake. So both the scales are used to measure earthquake.

Example:

In 1989 an earthquake struck Loma Prieta which had a magnitude of 7.1 on Richter Scale and VII on the Mercalli Scale. This quake produce moderate loss of life (67 people died) and the built environment suffered relatively little damage. Most of the building affected in the Loma Prieta earthquake had been built with seismic provisions to reduce damage.

In contrast, the 1988 Armenian earthquake measured almost the same magnitude (6.9) on the Richter, but its Mercalli measured intensity was XI. More than 50,000 people lost their lives, and the built environment was almost totally destroyed. 

The damage done by the Armenian earthquake was greater than that done by the 1989 Loma Prieta earthquake, although the magnitude was almost the same.

  

As the quality of the built environment is a big factor in the number of lives lost and the amount of damage done in an earthquake and intensity is a function of loss and destruction. So the magnitude of an earthquake does not always correlated with the intensity or impact. The local codes also keeps focus on the intensity rather than magnitude.