Pore pressure in aquifers confined below a cryosphere will increase as Mars cools and the cryosphere thickens. The increase in pore pressure decreases the effective stress and hence will promote seismicity. We calculate the rate of pore pressure change from cooling of the Martian interior and the modulation of pore pressure from solar and Phobos tides and barometric loading. Using the time-varying pressure and tidal stresses, we compute Coulomb stress changes and the expected seismicity rate from a rate-and-state friction model. Seismicity rate will vary by several 10s of percent to two orders of magnitude if the mean pore pressure is within 0.2 MPa and 0.01 MPa of lithostatic, respectively. Seismic events promoted by high pore pressure may be tremor-like. Documenting (or not) tidally-modulated shallow seismicity would provide evidence for (or against) water-filled confined aquifers, that pore pressure is high, and that the state of stress is close to failure — with implications for processes that can deliver of water to the Martian surface.