code: def _get_mechanism(self, sites, rup, imt, C):
          
          Fn, Fr = self._get_fault_type_dummy_variables(sites, rup, imt)
          return (C['b9'] * Fn) + (C['b10'] * Fr)
docs: Compute the fourth term of the equation described on p. 199:
  
          ``b9 * Fn + b10 * Fr``