2009 年 61 巻 Supplement 号 p. 479-487
Monitoring of crustal movements with a dense continuous GPS network called GEONET started in 1990’s in the Japanese islands and drastically changed observational studies of crustal movements. This paper briefly summarizes new findings and knowledge about seismotectonics of the Japanese islands obtained from observation of crustal movements since 1990’s. High precision (∼1 mm/yr), good spatio-temporal resolution (25 km and 1 day), and dramatic increase of data (>1200 daily coordinate solutions) due to continuous observation have enabled us to monitor minute crustal movements related to tectonic loading processes at subduction zones as well as inland active faults in a quantitative manner. On the other hand, continuous GPS data cover only a rather short time period and comparison of these data with those from conventional surveys and geological investigation show both similarities and dissimilarities. Integrated analysis of observation data obtained from different time scales is an important key to understand the whole tectonic processes. As for seismic deformation, we can separate postseismic transient motion from coseismic displacement with continuous observation. Now coseismic deformation can be resolved very quickly and precisely, facilitating the estimation of a fault model. Postseismic deformations were studied in detail using GPS network data, and their significance in seismic cycle has been clarified. Spatio-temporal distribution of afterslip provides information about frictional property on the fault plane and its heterogeneity, which is important in understanding seismogenic processes. Episodic slow slip events not preceded by major seismic events was first discovered by the Japanese continuous GPS network, and it has been shown slow slip is a common phenomenon in various subduction zones although their physical mechanism and tectonic implications have not been elucidated yet. Continuous GPS observation has significantly widened our scope about active deformation processes of the Japanese islands and provided a new opportunity to make comprehensive understanding as well as a quantitative forecast of crustal activities.