Techniques for Calculating Earthquake Magnitude
ML (Richter Scale): It was developed by Charles Richter in 1930 and is defined as the logarithm of the wave amplitude. All other scales were developed based on the Richter scale.
Ml: is calculated according to Richter's original relation. It is used for shallow, nearby and small earthquakes (Local magnitude).
Mb: It is calculated based on the amplitude of P and S waves (Body-wave magnitude).
Mb = log10(A/T) + Q(D,h)
Here;
A : Amplitude of grain vibrations (ground motion) (micron);
T: Period (second);
Q(D,h) correction factor, function of the distance between the epicenter and the recorder (D -degrees) and the focal depth (h -kilometer).
Md :It is calculated using the duration of very small and nearby earthquakes (Duration magnitude).
MS :It is calculated based on the amplitude of surface waves (Surface-wave magnitude).
MS = log10 (A/T) + 1.66 log10 (D) + 3.30
Mw :From the seismic moment of the released energy (Moment magnitude).
Seismic moment; is a measure of the magnitude of the earthquake-producing source and is calculated as follows;
Mo=mAu
Here m is the resistance of the rocks to shear stress, A is the broken (i.e. sliding) section of the fault level, and u is the average displacement, i.e. slip, that occurs along the fault. A very important point here is that there is a relationship between u and the fault length. The higher the u, the longer the fault should be.
The moment magnitude is calculated using the following formula:
Mw = 2/3 log10(MO) – 10.7
Me :The amount of energy released by the earthquake, the measure of the damage potential in structures (Here, the energy unit is erg.)
Me =2/3 log10E-9.9
For every 1 unit increase in magnitude, the amount of seismic energy increases approximately 32 times.
After any earthquake, different values are given for the magnitude of the earthquake and differences between countries or institutions are generally due to the different calculation techniques mentioned above.
