遠近地震所引起地下水位變化之研究

Study of Water Level Fluctuations Induced by Local and Teleseismic Earthquakes at Two Wells in Hualien, Eastern Taiwan

劉政(Chen Liu)

指導教授 ： 顏宏元

國立中央大學/地球科學學院/地球物理研究所/博士(2006年)

摘要

導致地下水位變化的原因有許多種，其中包括來自於自然和人為現象的「非構造因子」，及來自於地質活動和板塊運動的「構造因子」。在非構造因子中的自然現象，包括：降雨、大氣壓力、溫度、地下含水層分部及附近水系的伏流、地潮、海潮、引力、季節變化等；非構造因子中的人為現象，包括：人為的抽取與補注等，甚至於地下含水層附近地表負載及震動都可能造成地下水位在井裏的振盪。在構造因子的現象中，則包括：來自於地球組成與大地構造的現象所造成的火山活動與地震事件及板塊運動作用等。
地下水位產生變化在非構造因子中，以降雨為最大的影響因子，其次為大氣壓力與潮汐變化。但這些影響因子對地下水位變化的影響，降雨會因區域性地質現象及含水層分布，導致降雨入滲到含水層時，產生遲延的現象。而大氣壓力與潮汐雖能直接影響水位的變化的，但都屬長週期的影響。
地下水位產生變化在構造因子中，以地震事件最為明顯，影響範圍較大，火山活動屬區域性，影響範圍較小。地震的發生多來自於板塊運動後淺部地殼受到應力的擠壓與大地應力轉移與釋放所造成。在大地應力藉由地震事件釋放時，是短暫且劇烈的，其影響的範圍也較廣。地震造成的地震動是短暫而瞬間的，但一個較大地震發生後可能因餘震的持續發生使得整個地震延續較長的時間。
地震事件所產生的地震動現象，是造成地下水位產生瞬間上升或下降的主要影響因子。這種水位瞬移的現象，有可能來自於地殼在地震發生時因應力傳遞造成地下含水層的變化，或因地殼在地震發生瞬間產生含水層的破裂或地下水流的阻塞，造成地下水位的變化。本研究中將地震事件分成近震地震與遠震地震分別討論。在地震中，地震波以彈性波的模式傳遞，造成區域性地層的質點非一致性的位移現象，也就是應變的改變，此應變的變化值在各個方向上根據地震波的特性而改變；以直角座標(Cartersian)系統下，假設在短暫的時間內(dT)，U為質點的位移量，則隨著地層之正應變與剪切應變的影響將會使得地下水位面隨著地層應變的變化而有上升或是下降的動態變化；此地下水位面之震盪與地震波波傳至之含水地層之內涵頻率有極大之關係性。在遠場地震中，大地應力的傳遞及造成區域地殼發生應變的現象，對地下水位變動的直接影響較小，但地震所產生的能量釋放確能將地震波傳播到遙遠的地方；本研究中，因遠場地震所造成的地下水位變動與地動之S波和表面波的波相有明顯的相關性。將水井之水位變化與地震站之記錄做一比對，以探討水位變化與地震波之相關性，地震波之內涵頻率與水位震盪之內涵頻率之比較。另外探討水位震盪的最大值與地震波最大速度值(PGV)以及最大位移值(PGD)之行為模式是否與近震地震所造成的現象一致。

關鍵字

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並列摘要

Fluctuations of Water level in wells often are in response to changes of volumetric strains caused by tectonic deformation or by passage of seismic waves. Continuous monitoring of water level fluctuations in two wells, coded HLC-05 and HLC-03, in Hualien area of eastern Taiwan has been made since 2002 by digital recorders with high sampling rates at 1- and 6-second intervals, respectively. The data thus far show that fluctuations of water level are often induced by earthquakes. The results can be summarized as follows: 1) Observations of water level fluctuations can be induced by earthquakes of magnitude M³0.43+2.39log10D, where D is the hypocenter distance from the well; 2) The peak water level fluctuation (PWL) is linearly proportional to either the peak ground velocity (PGV) or peak ground displacement (PGD) on logarithmic scales, and the correlation coefficients of the former one are larger than those of the latter one. On the contrary, no clear trends with the peak ground acceleration (PGA) are found.
A well, coded HLC-05, with high sampling rate of 1 Hz for monitoring the water level changes has been installed since May 2002 in Hualien, eastern Taiwan. The changes of tectonic strains induced by earthquakes have led to changes of water level in the well, including the static (steps) and dynamic behaviors (fluctuations). Static water level changes, with which water level raises to a step persistently for about 30 seconds and then decreases gradually to its background trend which is in response to the long period effects (i.e., earth tides and rainfalls), induced by three events (ML4.5, 5.8, and 6.5) are well recorded so far. Based on a dislocation model of a finite fault plane, we try to describe the correlation between volumetric strain changes and water level steps. Due to lack of pumping tests, an one-dimensional diffusion equation (Crank, 1975; Brodsky et al., 2003) applied to a blockage system can provide a good estimate of stratum parameter (i.e., hydraulic diffusivity) from the continuous drops of water level as function of lapse time. From the results of observation from the three events, we have determined three values of hydraulic diffusivity at 0.21, 0.17, and 0.17 m2/sec, correspondently, with an average value of 0.18 m2/sec. The results can be verified by the future events or by pumping tests in the well.

並列關鍵字

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參考文獻

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Brodsky, E. E., E. Roeloffs, D. Woodcock, I. Gall, and M. Manga, 2003. A mechanism for sustained groundwater pressure changes induced by distant earthquakes, J. Geophys. Res., 108, B8, 2390.

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Crank, J., 1975. The Mathematics of Diffusion, Oxford Sci., Oxford, UK.

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