Abstract Prior studies have cataloged thousands of icequakes at Ross Ice Shelf (RIS) Western Rift 4 (WR4) and linked them to tidal cycles, thermal stress, and temperature fluctuations. Building on this foundation, we apply a full moment tensor inversion using Green’s functions for a layered firn, ice, water and sediment model. A prominent extensional Lamb wave propagating at 2.7–3.2 km/s is identified in both observed and synthetic waveforms. Moment tensor solutions reveal crack‐opening, crack‐closing, and double‐couple mechanisms oriented subparallel or oblique to the rift. Crack‐opening and shear motions are more common than crack‐closing events. Low corner frequencies imply long, narrow rupture geometry with stress drops of ∼1 MPa for the largest events, consistent with shallow firn fracturing. These results provide the first quantitative constraints on brittle deformation at the WR4 rift and reveal diverse source mechanisms not clearly controlled by tidal or thermal forcing alone.