First (dotted line) and second (solid line) overtones of the ℓ = 2 crustal torsional oscillations for a neutron star model with (M, R) = (1.4M_⊙, 14km) and N_s/N_d = 0 as functions of ς. For reference, we plot the lowest observed QPO frequency (red shaded area) in GRB 200415A, i.e., 835.9 Hz (Castro-Tirado et al. 2021). The ranges for ς (dark shaded area) and ς_QPO (the area between the dashed lines) are also drawn for comparison.