High-Definition Strain-Rate Mapping of Japan from a Public–Private GNSS Network

The interseismic crustal strain-rate distribution in Japan has traditionally been estimated from coordinate time series derived from GEONET, the nationwide GNSS network operated by the Geospatial Information Authority of Japan (GSI). Beginning with Sagiya et al. (2000) and subsequent studies, these analyses have revealed the existence and broad extent of inland strain-concentration zones. However, because the average spacing of GEONET stations is ~20 km, its ability to resolve highly localized deformation, such as strain accumulation associated with individual active faults, has remained limited.

In contrast, SoftBank Corp. (hereafter SoftBank), a Japanese telecommunications company, has operated an independent nationwide GNSS network of more than 3,300 stations since late 2019, nearly three times the number of GEONET stations. The suitability of SoftBank stations for crustal deformation monitoring was demonstrated by Ohta and Ohzono (2022).

By integrating GNSS data from GEONET and SoftBank, we constructed an unprecedentedly dense observation network and estimated interseismic crustal strain-rate fields at substantially higher spatial resolution. The integrated network achieves an effective station spacing of <10 km, enabling us to resolve localized strain features that are not captured by GEONET-only solutions. For example, our results suggest that the Niigata–Kobe Tectonic Zone, previously interpreted as a continuous belt, may instead comprise a series of smaller, spatially localized strain-concentration zones.

Moreover, the improved resolution enables a more direct comparison between the strain-rate field and the spatial distribution of earthquake epicenters. We find that seismicity tends to be more active along the margins of strain-concentration zones rather than directly above their cores. This pattern is consistent with the interpretation of Hasegawa et al. (2004), which proposes that stress preferentially accumulates at boundaries between regions undergoing rapid inelastic deformation and surrounding regions deforming more slowly, thereby promoting earthquake occurrence along the edges of strain-concentration zones. 

Acknowledgments: The SoftBank's GNSS observation data used in this study was provided by SoftBank Corp. and ALES Corp. through the framework of the "Consortium to utilize the SoftBank original reference sites for Earth and Space Science".