Analysis of Neotectonics Based on 3-D Structural Geometry and Geodetic Measurements in Hsinchu area, Northwestern Taiwan

Earthquake magnitude and coseismic deformation are strongly related to the geometry of the seismogenic fault. Consequently, the construction of a comprehensive 3-D fault geometry model for the linkage and interaction of fault systems becomes one of the essential topics in neotectonic studies. The Western Foothills of Taiwan is undergoing an active deformation of the Taiwan orogeny demonstrated by the frequent seismicity and destructive earthquake events. The study region includes several faults-and-folds systems from Hsinchu and Hsincheng faults in the north to Okungchi and Hoping faults in the south in the internal and external western Foothills in Hsinchu area. The structures in Hsinchu area show wide and gentle folds associated with NNW-SSW trending thrust faults in internal Foothills and tide and steep folds associated with the E-W trending thrust faults in external Foothills. From the seimostectonic viewpoint, the earthquakes frequently occur in internal Foothills than external Foothills. Therefore, the main purpose of this study is to investigate the linkage of subsurface structures between internal and external western Foothills in Hsinchu area. To this purpose, we restore a cross-section across Hsinchu and Hsincheng faults, and clarify the overall structural evolution of major fault systems based on this restoration. By integrating eleven CPC geological cross-sections and the profile in this study, we use GeoSec 3D software to construct 3D fault geometry model beneath Hsinchu area. Finally, we use GOCAD software to integrate 3D relocated seismicity and 3D fault model in order to explore the relationships between active structures and seismicities.
In terms of the profile reconstruction, the thickness of strata gradually increasing eastward was induced by abundant normal faults activating in early Miocene. However, Talu shale pinched out instead. According to restoration of balanced cross section, the Juanchiao fault, Chutung fault and Peipu fault are in-sequence fault systems, meanwhile the Hsinchu and Hsincheng faults are out-of-sequence fault systems. The Chingtsaohu anticline formed following the Peipu fault. After that, Hsinchu fault cut through the Chingtsaohu anticline and Hsincheng fault, bedding fault, formed in the end. The displacements along the Hsinchu and Hsincheng fauts are 1318 m and 887 m, respectively. We assumed the faults formed after the deposition of Cholan formation, therefore, the long term slip rates for Hsinchu and Hsincheng faults are 0.43 mm/yr and 0.37 mm/yr respectively. Also, short term slip rate of Hsincheng fault is larger than 0.37 mm/yr. As a result, it implies Hsincheng fault is possible a recently active structure in Hsinchu area. The average strain rate across Hsincheng fault is -0.05 μ strain/yr, and the coseismic strain drop divided by strain rate gets 600-6000 years. Therefore, we suggest the recurrence interval of Hsincheng fault is 600-6000 years.
Base on the analysis of seismicity and geological profiles, the numbers of earthquakes are larger in internal than external foothills. Clusters at the bend of Juanchiao fault at depth imply that it is the location of stress generating. According to 3D seismicity and fault model, earthquake events occur northward in 1999, 2007, 2003, 2006. The 3D fault model indicates faults at depth become gentler in internal foothills, while faults in external foothills turn into steeper. The boundary fault, Juanchiao fault, shows subsurface high angle thrust and stepped fault geometry at depth. Furthermore, the detachment becomes deeper eastward, from 6-7 km to 10 km.