The short‐period seismometer‐based magnitude saturation problem, especially for events with magnitude > 8, can be improved by a real‐time Global Navigation Satellite System (GNSS) positioning technique, which has enabled rapid estimation of a finite fault model for a large earthquake without any saturation. A new real‐time fault modeling system based on the GNSS Earth Observation Network (GEONET) is developed and is under experimental operation in Japan. In this paper, we present the newly developed system REGARD (the Real‐time GEONET Analysis system for Rapid Deformation monitoring), which consists of real‐time GNSS positioning, automatic detection of coseismic displacement by the event, and quasi real‐time finite fault model inversion routines. The performance of the automatic event detection system is tested through experimental real‐time operation based on GEONET data for 2 months. Furthermore, we also test the reliability of the finite fault model inversion routines using real raw GNSS data observed for past large earthquakes: the 2003 Tokachi‐oki earthquake (moment magnitude (Mw) 8.3), the 2011 Tohoku earthquake (Mw 9.0), and the 2011 off‐Ibaraki earthquake (Mw 7.7). A simulated 1707 Hoei‐type Nankai Trough earthquake (Mw 8.7) is also tested. The real‐time experimental operation shows that real‐time GNSS positioning is precise enough to detect all the tested earthquakes, and the inversion results demonstrate that the REGARD can reliably estimate the earthquake size and its extent within 3 min after the origin time. These results suggest that the REGARD system will complement the seismometer‐based magnitude determination system.

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