code: def _get_mechanism(self, rup, C):
          
          U, SS, NS, RS = self._get_fault_type_dummy_variables(rup)
  
          return C['f1'] * NS + C['f2'] * RS + C['f3'] * SS
docs: Compute the fifth term of the equation 1 described on paragraph :
          Get fault type dummy variables, see Table 1