Continuous Monitoring of an Active Fault in a Plate Suture Zone: a Creepmeter Study of the Chihshang Fault, Eastern Taiwan

 Jian-Cheng Lee^a,  Jacques Angelier^b,  Hao-Tsu Chu^c,  Jyr-Ching Hu^d,  and Fu-Shu Jen^e

^aInstitute of Earth Sciences, Academia Sinica, P.O. Box 1-55, Nankang, Taipei, R.O.C., Taiwan

^bTectonique Quantitative, Département de Géotectonique and ESA 7072, Université P.-&-M. Curie, Paris, France

^cCentral Geological Survey, P.O. Box 968, Taipei, R.O.C., Taiwan

^dThe Institute for Secondary School Teacher of Taiwan, Taichung, R.O.C., Taiwan

^eDepartment of Civil Engineering, National Taiwan University, Taipei, R.O.C., Taiwan

 (Tectonophysics, 333, pp. 219-240, 2001)  

Abstract.  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 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±0.3 mm and 17.3±0.7 mm, at Tapo and Chinyuan respectively. These annual shortening rates are in a 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: the period of high creep rate coincides with the wet season, whereas that of low creep rate coincides with the dry season. Also, in comparison with the Tapo site, the creep behaviour as a function of time is complex at the Chinyuan site. Possible factors of irregularity are under investigation (thermal effect acting on the concrete basement of the creepmeters, earth tide effect, water table variations in nearby rice-field, and rainfall). The comparison between GPS measurements across the Longitudinal Valley (31 mm/yr of horizontal displacement) and the creepmeter measurement across the Chihshang Fault zone (17-19 mm/yr of horizontal displacement) suggests that there exists other shortening deformation across the active fault zone in addition to those we have measured from the creepmeters.