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S U M M A R YWe present an algorithm to rapidly determine the moment tensor and centroid location for large earthquakes employing local and regional real-time high-rate displacement records from GPS. The algorithm extracts the coseismic offset from the displacement waveforms and uses the information to invert for the moment tensor. The Green's functions for a layered earth are obtained numerically from open source code EDGRN. To determine the centroid, multiple inversions are simultaneously performed within a grid of inversion nodes, and the node with the smallest misfit is then assigned the centroid location. We show results for two large earthquakes replayed in simulated real-time mode using recorded 1 Hz GPS displacements: the 2003 M w 8.3 Tokachi-oki and the 2010 M w 7.2 El Mayor-Cucapah earthquakes. We demonstrate that it is feasible to obtain accurate CMT solutions within the first 2-3 min after rupture initiation without any prior assumptions on fault characteristics, demonstrating an order of magnitude improvement in latency compared to existing seismic methods for the two earthquakes studied. This methodology is useful for rapid earthquake response, tsunami prediction and as a starting point for rapid finite fault modelling.

GPS instruments are noninertial and directly measure displacements with respect to a global reference frame, while inertial sensors are affected by systematic offsets—primarily tilting—that adversely impact integration to displacement. We study the magnitude scaling properties of peak ground displacement (PGD) from high‐rate GPS networks at near‐source to regional distances (~10–1000 km), from earthquakes between Mw6 and 9. We conclude that real‐time GPS seismic waveforms can be used to rapidly determine magnitude, typically within the first minute of rupture initiation and in many cases before the rupture is complete. While slower than earthquake early warning methods that rely on the first few seconds of P wave arrival, our approach does not suffer from the saturation effects experienced with seismic sensors at large magnitudes. Rapid magnitude estimation is useful for generating rapid earthquake source models, tsunami prediction, and ground motion studies that require accurate information on long‐period displacements.