Abstract：By using forthorder RungeKutta methods, transverse displacements and the eigenperiods larger than 200 s of toroidal oscillation are calculated for PREM(Preliminary Reference Earth Model). The features of transverse displacements are analyzed to determine the effect of density and rigidity to eigenperiods at different layers. The results compared with that from Dziewonsi and Anderson show that the relative errors between calculated periods and observed periods are less than 0.25%,and the highorder fundamental modes are mostly affected by the distribution of density and rigidity of upper mantle, the overtones which the number is more than 20 are affected mostly by lower mantle.
