The study of coseismic geohydrological changes and its effects on soil liquefaction potential in Taiwan

    Earthquake-induced geohydrological changes have been monitored and investigated in the last fifty years. However, most of the previous studies focused on the effects of a single earthquake event on different observations or multiple-independent events on many different data sources which arising uncertainties from different mechanisms and site effects. The quantitative analysis of earthquake-induced geohydrological changes and their effects on soil liquefaction remains a challenge. In order to complete the soil liquefaction potential map in Taiwan and improve the accuracy of the analysis and evaluation, the Central Geological Survey of the Ministry of Economic Affairs conducts a six-year plan from 2018 to 2023. The geological and geohydrological data thoroughly collected by previous projects serve as a solid foundation for this 4-year project to systematically probe the coseismic geohydrological changes and their effects on soil liquefaction.
     In the four year of this project, four primary tasks had been finished including (1) database establishment of the long-term groundwater level observations in the study areas, (2) three-dimensional hydrogeological structures construction of the study areas,, (3) case study of induced groundwater level changes and liquification in three catastrophic earthquakes (4) methodology development and establishment of the coseismic geohydrological changes and their effects on soil liquefaction potential. 
     The establishment and management of the long-term groundwater level observations in the study areas were done in this year. The statistical program was merged into processing procedures for data analysis. Also, external observational data produced by the Water Resources Agency and Central Weather Bureau was integrated. The three-dimensional hydrogeological models were established based on the models constructed in T-PROGS (Transition Probability Geostatistical Software). With limited drilling data, the three-dimensional hydrogeological models could be applied to estimate and build an underground database for those areas with no data. In addition, the geological zoning after geological research and judgment could serve as a reasonable geological basis for subsequent interpolation of soil liquefaction.
    The hourly and secondly data of groundwater level variations and the hourly river discharge variations triggered by the 1999 Chi-Chi earthquake, 2016 Meinong earthquake and 2018 Hulien earthquake are checked and analyzed to investigate the responses under different hydrogeological conditions in west and south regions of epicenter. The increased groundwater levels are shown to consistent with the horizontal peak ground velocity (PGV), which imply that the increased groundwater levels might result from the buildup pore water pressure induced by shear strain, like the liquefaction mechanism.
    Uncertainties associated with groundwater level measurement or incorrect representation of regional groundwater level could easily lead to erroneous assessments of liquefaction potential in regional areas. These uncertainties result from spatial and temporal groundwater level variability and/or measurement error. Natural variability also makes it difficult to correctly identify the groundwater depth. The groundwater level fluctuates in response to recharge and discharge. In this project, an alternative methodology was adopted in order to overcome these inherent uncertainties.