The northwesternmost part of the Sagami trough, a part of the Philippine Sea--Eurasian plate boundary, was ruptured during the great South Kanto earthquake in 1923. Very extensive and frequent geodetic measurements of crustal deformation have been made in the South Kanto district since the 1890's, and these constitute the most complete data set on crustal movements in the world. These data were reanalyzed and interpreted and according to our interpretation indicate the following sequence of events. The coseismic movements were due to oblique thrust and right lateral slip of about 8 m on a fault outcropping at the base of the Sagami trough. This was followed by postseismic deformation resulting from reversed afterslip of 20--60 cm that occurred at an exponentially decaying rate in time. The interseismic deformation is produced by steady subduction at a rate of about 1.8 cm yr-1. During subduction the top 10--15 km of the plate boundary is apparently locked, while deeper parts slip aseismically at an irregular rate. No significant precursory deformation was observed. The recurrence time for 1923 type earthquakes is 200--300 years. The Boso and Miura peninsulas are broken into a series of fault-bound blocks that move semi-independently of the sourrounding region. The subduction zone itself, where it is exposed on land, is shown to be a wide zone encompassing several faults that are active at different times.