Continuous monitoring of an active fault in a plate suture zone: a creepmeter study of the Chihshang Fault, eastern Taiwan

Data from continuously monitored creepmeters across the active Chihshang fault in eastern Taiwan are presented. The Chihshang fault is an active segment of the Longitudinal Valley fault, the main suture between the converging Philippine and Eurasian plates in Taiwan. Since the 1951 earthquake (Mw = 7.0), no earthquake larger than magnitude 6.0 has occurred in the Chihshang area. At least during the last 20 years, the Chihshang fault underwent a steady creep movement, resulting in numerous fractures at the surface. Five creepmeters were installed in 1998 at two sites, Tapo and Chinyuan, within the Chihshang active fault zone. One-year results (from August 1998 to July 1999) show a horizontal shortening of 19.4 plus/minus 0.3 mm and 17.3 plus/minus 0.7 mm, at Tapo and Chinyuan, respectively. These annual shortening rates are in good agreement with other estimates of strain rate independently obtained from geodetic measurements and geological site investigation. The creepmeter measurements were made on a daily basis, providing accurate information on the previously unknown evolution of creep during the year. The records of fault creep at the Tapo site thus revealed close seasonal correlation with average rainfall.