One could come up with a lot of issues, but I'll focus on three:

1. If we take an extremely simplistic view, faults are large but extremely narrow, i.e., effectively single fracture planes extending tens of kilometers into the crust. As such, the logistics of somehow injecting material to depths along these planes (which typically don't have an aperture in a normal sense) significantly below the deepest we've ever drilled are challenging (to put it mildly).

2. Faults represent locations where sufficient stress existed to break rocks and then to cause slip along the fracture plane (i.e., the fault). Generally it requires less total/differential stress to have movement along an existing fracture than forming a fracture, so if we sidestep the logistical impossibility from above, if we were able to do this, at best you've increased the stress necessary to reform the fault (compared to have slip on the fault) that you sealed, but if the stress exists, the rocks will just break again eventually. This also largely ignores the existence of extensive damage zones around faults (e.g., Kim et al., 2004). In detail, the fault planes themselves are surrounded by halos of "damaged" i.e., fractured and otherwise weakened, rock with the width of the damage zone being proportional to the amount displacement accommodated by a fault. If we're considering a large, plate boundary scale fault, the damage zone might be hundreds of meters to several kilometers wide. So in some hypothetical where we were able to "seal" the fault plane itself, the fault would likely just reorganize within the damage zone as this would be weaker than the "sealed" fault or intact rock outside the damage zone, assuming the stress still exists. Which brings us to our final point.

3. Faults are manifestations of differential motion between portions of the crust and upper mantle, driven by differential stress resultant from plate motion. If we assume that we could somehow overcome the logistical challenges in the first point, and even if we were able to maybe somehow "seal" the entire damage zone discussed in the second point, none of this changes the driving stresses. As a simple analogy, if you tear a piece of paper and then "seal" the rupture with some tape and then try to tear the paper again, it's not as though the paper won't just rip again (even if you do a great job with your tape and it doesn't reoccupy the same tear, some other part of the paper will tear instead). As long as a stress is being applied that overcomes the strength of a portion of the material, a fracture will form to accommodate the differential stress and motion. Thus, if we assumed we could "seal" faults, this wouldn't really do anything unless you somehow also stopped plate tectonics.

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The earthquake comes from an entire contential plate ramming into each other. ‘Sealing’ that fault (which is already mostly rock to rock) would do exactly nothing.

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