Groundwater level variations and special geological phenomena caused by dramatic changes of pore water pressure in central Taiwan during the 1999 Chi-Chi earthquake

• Main Authors: Shih-Wei Huang, 黃世偉
• Other Authors: Tzong-Yeang Lee
• Format: Others
• Language: zh-TW
• Published: 2006
• Online Access: http://ndltd.ncl.edu.tw/handle/80643967404307046111

博士 === 國立成功大學 === 地球科學系碩博士班 === 94 === The 1999 Chi-Chi earthquake on September 21, 1999 (MW=7.3) was caused by the intraplate’s Chelungpu fault rupture, while the 2002 Haulien earthquake on March 31, 2002 (MW=6.8), and the 2003 Chengkung earthquake on December 10, 2003 (MW=6.8), were caused by the dislocation of collision and subduction zone between the Eurasian and Philippine Sea plates. The different earthquake locations, and depths helped us study the effect of an earthquake on the densely distributed groundwater level observation net in the Choshui river alluvial fan of central Taiwan. The scope and extent of the coseismic groundwater level change caused by the 1999 Chi-Chi earthquake was based on the observed groundwater level data for the alluvial fan are more extensive and remarkable than those of the 2002 Hualien and 2003 Chengkung earthquakes. The significantly high amplitude of coseismic surge-type groundwater level changes appeared in the 1999 Chi-Chi earthquake, but did not appear in the 2002 Hualien’s and the 2003 Chengkung’s. A few of the coseismic rise-type groundwater level changes and post-seismic groundwater level changes were seen in the 2002 Hualien’s and 2003 Chengkung’s earthquakes, but were not found in the 1999 Chi-Chi earthquake. Such differences might be caused by distinct earthquake mechanisms and seismic wave propagation path. All of the different groundwater level changes which were observed in the present study suggest that the key parameters, from high to low correlations for Layer 2-1 (a confined to partially unconfined aquifer) in the Choshui river alluvial fan, are the distance from the observation well to the epicenter in the 1999 Chi-Chi earthquake (r=-0.77, p<<0.05, n=37), vertical direction peak ground acceleration (z-PGA, r=0.75, p<<0.05, n=37), and logarithmic hydraulic conductivity (logK, r=-0.35, p=0.036<0.05, n=37). Meanwhile, in general, the coseismic groundwater level change and the arithmetic average of hydraulic conductivity for a well in different depths of aquifers in the alluvial fan have a tendency to decrease from the proximal to distal fans. Our results also revealed that the rate of change in tectonic stress and strain is faster than that of the coseismic groundwater level. In the 1999 Chi-Chi earthquake, unlike a common landslide a particular landslide occurring at Chiu-Fen-Erh-Shan was associated with contemporaneous formation of tektite induced by high frictional heat and large-scale eruptions of rock formations in the adjoining region. The occurrence of pseudotachylyte suggests a low water content in rocks and a high ratio of slip distance to slip-zone thickness so that high heat can be produced to initiate the formation of glassy materials. Evidence for a large-scale rock eruption was observed in the nearby region of the landslide area. Three possible causes of rock eruption are proposed here including: (1) transmission of seismic waves gave rise to relative displacement and high frictional heating that caused vaporization of pore water in association with high seismic acceleration and created fractures and adjoining rock eruption; (2) propagation of seismic waves did not produce relative motion along fractures but built up pressure forcing eruption of pore fluid and rocks; (3) high frictional heat produced by the processes of large-scale landslide increased the temperature and pressure of pore water. The country rocks were uplifted and erupted when the uplift force exceeded the gravity and cementation forces during the period of strong seismic motion. There were different surface deformed styles occurred in the Wufeng and Fengyuen-Shihkang areas, in which the former one is an uplift without significant surface rupture and the latter one is an uplift accompanied with surface rupture. The areas exhibiting different deformations are characterized by different sediment depth, pore pressure and trishear of seismic propagation.