由於井下型地殼應變儀之直徑約僅十公分，可能對地下水文之變化會比較敏感；文獻
也顯示觀測冰島火山活動的井下型應變儀，有受地下水文變化之干擾。因此可藉由應變儀
與地下水位聯合觀測，以釐清含水層水文變化引發地殼應變之可能性。經由清晰的地潮訊
號顯示台灣西部麓山帶安裝的十二具井下地殼應變儀確實可量測微小至10-9 的地殼應變
量，理論上應變儀應是理想的地震前兆觀測儀器，但是儀器的長期穩定性則是另一項關鍵
因素。在實驗台上儀器的長期穩定性證實每年小於10-9；在地殼活動性相對低且雨量不高
的澳洲大陸，井下實測值則為每年約10-7。這個數值不致影響觀測板塊邊界地殼累積應變
與時間之關係；應可充分觀察累積的地殼應變與地震觸發之時空相關性。然而台灣每年夏、
秋兩季是暴雨及颱風季節，自2004 年初至2008 年中共有29 個颱風侵襲或掠過台灣。颱風
影響期間，地殼應變儀觀測資料明顯受氣象因素影響，但是因為雨量最大時與氣壓最低時
往往同時發生，故不易釐清何者才是主要的干擾因素。由泰興(TAIS)與中興(TSUN)兩觀測
站共站的地下水位資料顯示，地殼應變明顯受地下水位變動之影響，而地下水又受地表水
(包括降雨)之影響，這些氣象波動皆非線性的。本研究正積極釐清地殼應變觀測資料中之
地潮、氣壓、雨量等影響成分，俾便排除這些非地殼變動因素成分，以獲得台灣西部麓山
帶單純與板塊運動有關之地殼應變。 GTSM strainmeters have been deployed at 12 sites in western Taiwan since the end of 2003. The
earth tidal signals useful for in-situ calibration of the instrument response are clearly seen in all
channels at each site. This fulfills the first requirement concerning instrument performance, i.e.,
with high sensitivity to the crustal strain, when strainmeter was selected as one of the major
components in the integrated project.The long-term stability of the instrument is the second
requirement concerned. In the laboratory, the GTSM strainmeter has long-term stability of better
than one nanostrain per year if the distressing is properly performed. In the field, one example of
installation of a standard instrument in a relatively arid and stable tectonic region in Australia has
demonstrated that a long-term stability of much better than 100 nanostrain per year, so that
observation of long-term strain rates in tectonic regions larger than this will be robust. However,
there is evident that dilatometers in Iceland have been strongly influenced by aquifer pore
pressure change. Another meteorological effect to be considered is the influence of atmospheric
pressure on measured strain. Effects from change of either pore pressure or atmospheric pressure
must be understood if the tectonic strain is to be understood. There have been 29 recorded
typhoons attacked or swept this island since the first GTSM strainmeter was deployed in the end
of 2003. Typhoons were usually accompanied by heavy rainfall and significant atmospheric
depression which had left their effects on the continuously recorded strain data at each site.
Groundwater level change initiated by rainfall seems to be the major cause that is responsible for
the observed strain anomalies. This crustal strain measurement project has deployed a piezometer
at tens of meters above the co-site strainmeter at nine sites, this will be able to use the pore
pressure measurements to define repeatable strain responses to pore pressure change that can be
used to correct the strain data.
