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Using a brand‐new nationwide continuous GPS array, we monitored coseismic displacements from the October 4, 1994 Hokkaido‐Toho‐Oki (Kurile islands) earthquake (MJMA=8.1). Based on 2‐week time series of site coordinates of 21 GPS stations, we present a coseismic deformation field of whole Hokkaido within 1 cm precision. For example, the station at Nemuro, 170 km west of the epicenter, moved 44 cm to the east and subsided 10 cm. Even stations in southern Hokkaido, 600 km apart from the epicenter, moved a few cm horizontally toward the epicenter. We compare the GPS result with displacements calculated from two preliminary models, assuming fault planes parallel or perpendicular to the Kurile trench. Since the observed were far‐field displacements on land, both models can explain the GPS result.

The Geospatial Information Authority of Japan operates one of the largest GNSS Continuously Operating Reference Station networks in the world, GEONET, consisting of more than 1,300 GNSS-based control points in Japan. GEONET has become a social infrastructure for surveying and mapping, monitoring crustal deformation, and mitigating natural disasters. It is also used for precise positioning to guide construction and farm machines, and even for weather forecasting. Here we review its history and outlook, focusing on its application to disaster mitigation. As Japan is surrounded by four tectonic plates with interactions that lead to frequent earthquakes and volcanic activities, it must maintain such a geodetic infrastructure for monitoring crustal deformations in as near to real-time for possible disaster mitigation.