  As spiral waves driven by a planet in a gaseous disk steepen into a shock,
they deposit angular momentum, opening a gap in the disk. This has been well
studied using both linear theory and numerical simulations, but so far, only
for the primary spiral arm -- the one directly attached to the planet. Using
two-dimensional hydrodynamic simulations, we show that the secondary and
tertiary arms driven by a planet can also open gaps as they steepen into
shocks. The depths of the secondary/tertiary gaps in surface density grow with
time in a low viscosity disk ($\alpha = 5\times10^{-5}$), so even low-mass
planets (e.g., super-Earth or mini-Neptune-mass) embedded in the disk can open
multiple observable gaps, provided that sufficient time has passed. Applying
our results to the HL Tau disk, we show that a single 30 Earth-mass planet
embedded in the ring at 68.8~au (B5) can reasonably well reproduce the
positions of the two major gaps at 13.2 and 32.3 au (D1 and D2), and roughly
reproduce two other major gaps at 64.2 and 74.7 au (D5 and D6) seen in the mm
continuum. The positions of secondary/tertiary gaps are found to be sensitive
to the planetary mass and the disk temperature profile, so with accurate
observational measurements of the temperature structure the positions of
multiple gaps can be used to constrain the mass of the planet. We also comment
on the gaps seen in the TW Hya and HD~163296 disk.
