Tidal strain observations in Chu-Chie, Taiwan

In Chu-Chie, Taiwan, tidal strain observation with a borehole strainmeter system has been carried out since 2001. The Chu-Chie station (N23°31'39″, E120°35'59″, h = 300 m) is located in the central part of Taiwan and is about 40 km distant from the epicenter of the destructive Chi-Chi earthquake ( M_L = 7.3) which occurred on September 21, 1999. The strainmeter system detects three components of horizontal strains in 82°, 202° and 322° measured from the North in clockwise directions using differential transformers. We analyzed tidal strains and determined amplitudes and phase lags of 13 major constituents by applying the tidal-analysis program BAYTAP-G to 1-year strain data of 2003. The "observed" tidal strain amplitudes were compared with the "theoretically expected" amplitudes that were obtained from the GOTIC2 program. As a result, the ratio of the observed amplitudes and the theoretically expected amplitudes ranges between 0.7 and 3.3. Some parts of these discrepancies may be caused by uncertainty of determination of scale factors of instruments. Other reasons to be considered are complicated geological and topographic effects around the observation site.