台灣西部麓山帶安裝的十二具井下地殼應變儀已證實可量測微小至10-9 的應變量，理論上應變儀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 年迄今共有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。再者，台灣地區年降雨量高，且明顯有乾、濕季之分，除了地表降雨量本身之影響外，地下水位面因而變動也會造成地層內體應變與剪應變之變化。要保存井下型應變儀訊號中單純屬於來自地體構造運動之訊號(tectonic strain)，除了要濾除地潮、氣壓等影響外，仍有必要釐清水文變化對應變儀之影響並濾除之。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 under the auspices of Geological Survey of Taiwan since the end of 2003. The instrument clusters installed are also intended to supplement the earthquake-monitoring and crust deformation observations from the arrays of seismometers, continuous GPS and InSAR as part of a more integrated program for the observation of plate boundary tectonics of this region. 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. If the rock surrounding the borehole is inhomogeneous and anisotropic, the pore pressure change effect in the shear strain components could also be significant. 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. In addition to the threat of frequent earthquake shaking, Taiwan also suffered from serried typhoon attack during the seasons of summer and autumn each year. 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. The atmospheric admittance of GTSM strainmeter is documented in about 0.5 nanostrain per millibar. Groundwater level change initiated by rainfall seems to be the major cause that is responsible for the observed strain anomalies. Although the co-site piezometer will enhance the risk of the more expensive strainmeter’s fail due to a lightening strike, this program has deployed a piezometer at tens of meters above the co-site strainmeter at last 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.
