Abrupt fluid emissions from shallow marine sediments pose a threat to
seafloor installations like wind farms and offshore cables. Quantifying
such fluid emissions and linking pockmarks, the seafloor manifestations
of fluid escape, to flow in the sub-seafloor remains notoriously
difficult due to an incomplete understanding of the underlying physical
processes. Here, using a compositional multi-phase flow model, we test
plausible gas sources for pockmarks in the south-eastern North Sea,
which recent observations suggest have formed in response to major
storms. We find that the presence of free gas in the subsurface
effectively damps storm wave-induced pressure changes due to its high
compressibility, so that the mobilization of pre-existing gas pockets is
unlikely. Rather, our results point to spontaneous appearance of a free
gas phase via storm-induced gas exsolution from pore fluids. This
mechanism is primarily driven by the pressure-sensitivity of gas
solubility. We show that in highly permeable sediments containing
gas-rich pore fluids, wave-induced pressure changes result in the
appearance of a persistent gas phase. This suggests that seafloor fluid
escape structures are not always proxies for overpressured shallow gas
and that periodic seafloor pressure changes can induce persistent free
gas phase to spontaneously appear.