Well, not zero, but about as close as you can get without it being zero. That recent comet flyby had more gravitational influence than Mars. Gravity is pretty weak.
I didn’t say it was caused by gravity. It could be an electromagnetic phenomenon.
The basic case for this is we know that the Sun is electromagnetically connected to the Earth and it logically follows that it would have similar connections to other planets as well. If either alignment or proximity causes a significant fluctuation in these electrical flows, it is possible that may be evidenced in terrestrial effects.
Seismo-electromagnetics is a known phenomenon as well. It is not beyond the realm of possibility that electromagnetic interactions between planets could affect the balance of forces in the Earth’s crust.