Evaluation of the seismic moment of the April 20, 2013 Lushan earthquake

The April 20, 2013 Lushan earthquake which occurred in Sichuan, China had only moderate thrust. However, the computed seismic moments (M₀) for the Lushan earthquake calculated by several institutions differ significantly from 0.4 × 10¹⁹ to 1.69 × 10¹⁹ Nm, up to four times difference. We evaluate ten computed M₀s by using normal mode observations from superconducting gravimeters in Mainland China. We compute synthetic normal modes on the basis of moment tensor solutions and fit them to the observed normal modes. Comparison of our results indicates that M₀ is the main cause for some large differences between observations and synthetics. We suggest that a moment magnitude of M_w6.6, corresponding to a M₀ of 0.97-1.08 × 10¹⁹ Nm, characterizes the size and strength of the seismic source of the Lushan earthquake.