Geodetic measurements from a network of permanent GPS stations along the Pacific coast of Mexico reveal a large silent earthquake along the segment of the Cocos-North American plate interface identified as the Guerrero seismic gap. The event began in October of 2001 and lasted for 6--7 months. Average slip of ~10 cm produced measurable displacements over an area of ~550 ¿ 250 km2. The equivalent moment magnitude of the event was Mw ~ 7.5. Recognition of this and previous slow event here indicate that the seismogenic portion of the plate interface is not loading steadily, as hitherto believed, but is rather partitioning the stress buildup with episodic, as opposed to steady-state or periodic, slip downdip of the seismogenic zone. This process increases the stress at the base of the seismogenic zone, bringing it closer to failure. These results call for a reassessment of the seismic potential of Guerrero and other seismic gaps in Mexico.