  We show that heavy axion domain walls induce domain walls of the QCD axion
through a mixing between the heavy axion and the QCD axion, even when the
pre-inflationary initial condition is assumed for the QCD axion. The induced
domain walls arise because the effective $\theta$ parameter changes across the
heavy axion domain walls, shifting the potential minimum of the QCD axion. When
the heavy axion domain walls collapse, the induced QCD axion domain walls
collapse as well. This novel mechanism for producing the QCD axions can explain
dark matter even with the axion decay constant as small as ${\cal O}(10^{9})$
GeV. In particular, this scenario requires domain wall collapse near the QCD
crossover, potentially accounting for the stochastic gravitational wave
background suggested by recent pulsar timing array observations, including
NANOGrav. Using this mechanism, it is also possible to easily create induced
domain walls for string axions or axions with a large decay constant, which
would otherwise be challenging. We also comment on the implications for cosmic
birefringence using induced axion domain walls.
