Abstract: The observation of short-baseline and short-leveling has been continuing for more than 20 years since 1984 at Tangshan station. And it is clear from the leveling data that there are three periods: 1984—1989, 1990—1998, and 1998 till now. Here the Seasonal-Trend on Loess (STL) method is used to decompose the observation data into season variation, trend and residual. From the seasonal series, the yearly period can be clearly found in all observation and there seem to be more than one factor which has the period of one year. From the trend series, the synchro-changes have been shown and the synchro-changes occurred at the wide-range leveling observation. At the same time, there are some earthquakes: Datong earthquakes (M6.1) in 1989 & 1991, Tangshan earthquake (M5.0) in 1995, Zhangbei earthquake (M6.2) in 1998, and Tangshan earthquake (M4.9) in 2004. It seems that the synchro-changes in the trend series of the short-leveling and short-baseline data can be explained by these earthquake preparations.

Key words: Tangshan station, Short-leveling, Short-baseline, STL