| dc.description.abstract | The Ilan Plain in northeast Taiwan is located at the southwestern tip of the Okinawa Trough, which extends westward into the Taiwan orogeny. The Ilan Plain covered by thick sediments is clipped by the Hsuehshan Range in the northwest side and the Central Range in the south side. High geothermal gradients with plenteous hot springs of this area may result from magmatism associated with the back-arc spreading of the Okinawa Trough. In this study, we use geophysical methods to explore underground structures in the whole Ilan Plain, especially around the LangYang river and the SanShin area. We aim to find high geothermal potential spots in the plain area.
The research is divided into three parts. First, we use explosion sources with 119 Texan instruments to detect velocity structures by tomography mapping. Second, we use two mini-vibrators to investigate the area along the LangYang river by reflection seismics. Third, we explore a significant fault by refraction seismics and semi-reflection seismics with the impactor source. Finally, we combine with other geophysical and drilling data to analyze, compare and explain the high geothermal gradients in this area.
The shallow part of Ilan Plain can be divided into three layer: 800 m/s for alluvial deposit, 3000 m/s for Kanko formation, 5700 m/s for Szeleng sandstone. The thickness of each layer are 100~800m, 800m, and 800m, respectively. From seismic survey, there is a Zailian fault with an offset of at least 1000m, along the north bank of LangYang river. After comparing with the drilling data around Sansing area, we find that the Taiyaqiao anticline in Hsuehshan Range has entered the Sansing area and is bounded by the Zhuoshui fault (south) and the Zailian fault (north). According to the report of the HCL-1 well, the formations predicted by seismic method agree well with the drill results. However, the temperature in HCL-1 well is only 70.7℃ at the 2200m deep. The low temperature may be due to the Szeleng sandstone which has very limited fractures. The geothermal reservoir system might be controlled by faults, instead of lithology. The heat sources from the magmatism formed in the upper mantle/ lower crust may come from the Ryukyu backarc system. The magma migrates into the Ilan Plain along the Lanyang river. Meteoric water flowing underground from the Hsuehshan Range to the Ilan Plain gets its temperature heated by the intruded igneous hot rock. The fault systems provide the paths of heat migrating into the shallow parts of the Ilan Plain, which may cause the high geothermal gradients in this plain area. | en_US |