Role of fluids in surface deformation caused by the 1999 Chi‐Chi earthquake in Taiwan

Abstract

The 1999 Chi-Chi earthquake significantly altered the landscape of central Taiwan. Surface deformation produced by the earthquake along the trace of the Chelungpu thrust can be classified into two styles: (1) uplift without significant surface rupture, and (2) uplift accompanied by surface rupture. Here we examine areas that exhibited the first style of deformation (e.g. Wufeng). Seismic stress at the time of the main shock may have been relieved by high pore-fluid pressure in a 300-m-thick sand and gravel aquifer. Along the thrust fault, frictional heating of these sediments resulted in thermal expansion and an increase in pore-fluid pressure. High pore-fluid pressure damped seismic-wave energy and enhanced intergranular slips of unconsolidated sandy and gravel sediments, which were possibly assisted by sulphuric acid corrosion, leading to a high sulphate content in the groundwater (c. 70 mg L 1 ). These changes permitted surface folding and terracestyle uplifting to occur without significant rupture. In contrast, other areas in which the second style of deformation is dominant (e.g. Fengyuen-Shihkang) have thin (0-10 m) sand and gravel deposits and lower concentrations of sulphate (c. 30 mg L 1 ) in groundwater. In these areas, sediments were heated but not sufficiently to produce significant thermal expansion and increase in pore-fluid pressure; accumulation of stress in these locations led to rupture at the ground surface, with the formation of steep fault scarps. The areas exhibiting the first deformation style are characterized by the presence of high pore-fluid pressure, frictional heat conduction, and possibly chemical corrosion related to sulphuric acid attack and formation of sulphate, in contrast to those involving significant uplift and surface rupture. The areal distribution of these two surface deformation styles suggests that the aforementioned fluid-related subsurface processes may have altered the characteristics of sediments and caused diverse responses to the quake.