The Guerrero region of southern Mexico has accumulated more than 5 m of relative plate motion since the last major earthquake. In early 1998, a continuous GPS site in Guerrero recorded a transient displacement. Modeling indicates that anomalous fault slip propagated from east to west along-strike of the subduction megathrust. Campaign GPS and leveling data corroborate the model. The moment release was equivalent to an Mw≥6.5 earthquake. No M>5 earthquakes accompanied the event, indicating the frictional regime is velocity-strengthening at the location of slip. ¿ 2001 American Geophysical Union