code: def _get_site_amplification(self, sites, imt, C):
          
          Ss, Sa = self._get_site_type_dummy_variables(sites)
          return (C['b7'] * Ss) + (C['b8'] * Sa)
docs: Compute the third term of the equation described on p. 199:
  
          ``b7 * Ss + b8 * Sa``