Responses to recent great earthquakes and ensuing tsunamis in Sumatra, Chile, and Japan, with the resulting loss of life and damage to infrastructure demonstrate that our ability to ascertain the full extent of slip of catastrophic earthquakes and their tsunamigenic potential in the first minutes after the initiation of rupture is problematic. Regional GPS networks such as those in western North America and Japan are complementary to seismic networks by being able to directly measure displacements close to the source during large earthquakes in real time. We report on rapid modeling of two large earthquakes, the 2003 Mw 8.3 Tokachi-oki earthquake 100 km offshore Hokkaido Island using 356 GEONET stations and the 2010 Mw 7.2 El Mayor-Cucapah earthquake in northern Baja California using 95 CRTN stations in southern California about 75 km northwest of the epicenter. Working in a simulated real-time mode, we invert for finite fault slip in a homogeneous elastic half-space using Green's functions obtained from Okada's formulation. We compare two approaches: the first starts with a catalog of pre-defined faults, while the second uses a rapid centroid moment tensor solution to provide an initial estimate of the ruptured fault plane. In either case, we are able to characterize both earthquakes in less than two minutes, reducing the time necessary to obtain finite fault slip and moment magnitude for medium and greater earthquakes compared to traditional methods by an order of magnitude.