It is possible to make SNAP use your SLC images but the module is designed for GRD in input. Personal tests (Offset Tracking : SLC vs GRDH products) makes me very skeptical about using SLC images in SNAP.
Just for info : The way the module is implemented in SNAP will not allow you to detect very fine displacements. Pixel Offset in SNAP is good for glacier displacement, for sure, but I can’t guaranty it will work for earthquake