# Calculating the displacement of a fault

In the calculation of scalar moment magnitude of an earthquake we have the formula

$$M_0=\mu AD$$

where:

• $\mu$ is the shear modulus of the rocks involved in the earthquake (in Pa)
• $A$ is the area of the rupture along the geologic fault where the earthquake occurred (in m2), and
• $D$ is the average displacement on $A$ (in m).

In many cases the displacement can occur in the subsurface.In order to predict the moment magnitude of such earthquakes the value of $D$ must be estimated. How is this process carried out ?