We analyze single-frequency GPS measurements taken before and after the July 16, 1990, Ms 7.8 Luzon earthquake to constrain the faulting parameters of this major strike-slip event. The geodetic data imply fairly uniform left-lateral slip of 5.5-6.5 m along the well-mapped part of the fault trace, in agreement with field mapping of the rupture and in general accord with seismological estimates of the fault parameters. In addition, we find that substantial slip of about 4-5 m magnitude continued about 40-50 km northward or northwestward into the Cordillera Central beyond the end of the mapped rupture. This fault segment includes a substantial thrust component in addition to left-lateral slip. The geodetic data favor a faulting depth of at least 20 km along the central part of the rupture, unusually deep for a major continental strike-slip earthquake.