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 Fa ult in eastern Taiwan are presented. The Chihshang Fault is an active segme nt 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 are a. At least during the last 20 years, the Chihshang Fault underwent a stead y creep movement, resulting in numerous fractures at the surface. Five cree pmeters were installed in 1998 at two sites, Tape and Chinyuan, within the Chihshang active fault zone. One-year results (from August 1998 to July 199 9) show a horizontal shortening of 19.4 +/- 0.3 mm and 17.3 +/- 0.7 mm, at Tape and Chinyuan, respectively. These annual shortening rates are in a goo d agreement with other estimates of strain rate independently obtained from geodetic measurements and geological site investigation. The creepmeter me asurements were made on a daily basis, providing accurate information on th e previously unknown evolution of creep during the year. The records of fau lt creep at the Tape site thus revealed close seasonal correlation with ave rage 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 com parison with the Tape site, the creep behaviour as a function of time is co mplex at the Chinyuan site. Possible factors of irregularity are under inve stigation (thermal effect acting on the concrete basement of the creepmeter s, earth tide effect, water table variations in a nearby rice field, and ra infall). The comparison between GPS measurements across the Longitudinal Va lley (31 mm/year of horizontal displacement) and the creepmeter measurement across the Chihshang Fault zone (17-19 mm/year 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. (C) 2001 Elsevier Science B.V. All rights reserved.