As a part of the earthquake prediction program in the
Tokai district, the Geographical Survey Institute (GSI)
have performed levelling surveys repeatedly in the
Omaezaki peninsula (Fig. 2). It has been observed that the
peninsula is subsiding for the past 100 years (e.g., GSI,
1978). The subsidence is considered to be due to subduction
of the Philippine Sea plate at the Suruga Trough. In 1974,
seismic activity in adjacent area, Izu peninsula, increased
suddenly with enhanced rate of subsidence at Omaezaki.
This change in the mode of deformation drew wide
attention (GSI, 1977). In order to monitor further
development, levelling has been performed four times a
year since 1981. This frequent repetition of levelling in
Omaezaki indicates that there is obvious seasonal crustal
motion (Fig. 3) in this area (e.g., Tanaka and Gomi, 1989;
GSI, 1998). It raises the doubt that, for least-squares
network adjustment, the velocity model in which rates of
vertical movements are linear functions of time (El-Fiky
et al., 1996; El-Fiky, 1998) is no longer valid.