  Several pulsar timing array (PTA) experiments such as NANOGrav and PPTA
reported evidence of a gravitational wave background at the nano-Hz frequency
band recently. This signal can originate from scalar-induced gravitational
waves (SIGW) generated by the enhanced curvature perturbation. Here we perform
a joint likelihood inference on PTA datasets, and our results show that if the
PTA signals were indeed of SIGW origin, the curvature perturbations amplitude
required will produce primordial black holes (PBHs) in $[2 \times 10^{-5}, 2
\times 10^{-2}]\ m_\odot$ mass range. Mergers of these PBHs can leave a strong
gravitational wave signature in the $[10^{-3}, 10^8]$ Hz frequency range, to be
detectable at upcoming interferometers such as the Einstein Telescope, DECIGO
and BBO, etc. This offers a multi-frequency opportunity to further scrutinize
the source of the observed PTA signal.
