HR: 1340h
AN: T43B-1344    [Abstracts]
TI: Fluid Mechanical Interactions In The Active Creeping Chihshang Fault Zone In Eastern Taiwan
AU: * Dong, J
EM: jjdong@geo.ncu.edu.tw
AF: 1Institute of Applied Geology, National Central University, No. 300, Jungda Rd., Jungli City, 320, Taiwan
AU: Mu, C
EM: momo@geo.ncu.edu.tw
AF: 1Institute of Applied Geology, National Central University, No. 300, Jungda Rd., Jungli City, 320, Taiwan
AU: Lee, J
EM: jclee@earth.sinica.edu.tw
AF: Institute of Earth Sciences, Academia Sinica, P.O. Box 1-55, Nankang, Taipei, 115, Taiwan
AU: Guglielmi, Y
EM: guglielmi@geoazur.unice.fr
AF: Géosciences Azur Laboratory (UMR 6526), 250 rue Albert Einstein, Sophia Antipolis, Valbonne, 06560, France
AU: Angelier, J
EM: angelier@geoazur.obs-vlfr.fr
AF: Géosciences Azur, Observatoire Océanologique de Villefranche, BP 48, La Darse, Villefranche-sur-Mer, 06235, France
AU: Lin, C
EM: cplin@mail.nctu.edu.tw
AF: Department of Civil Engineering, National Chiao Tung University, 1001 Ta Hsueh Road, Hsinchu, 300, Taiwan
AB: The Chihshang thrust fault is one of the most active segments of the Longitudinal Valley fault (LVF) situated along the plate suture between the Philippine Sea plate and the Eurasian plate in eastern Taiwan. During the past two decades, different surface monitoring efforts have been undertaken across the Chihshang fault at different spatial and temporal scales. Some interesting phenomena were observed, revealing a close interactions between fluids, creep in the shallow fault segment and seismogenic zone at depth : (1) seasonal variation clearly influence Chihshang fault creep rate; (2) coseismic slip propagation was attenuated at shallow depth during the 2003 Mw=6.5 Chengkung earthquake and followed by a dramatic postseismic creep; (3) a decreasing creep rate was observed before the 2003 earthquake and the total crust shortening after Chengkung earthquake compensated the deficit of surface creep about 3-4 years before the earthquake. To better understand the role of fluids in the creeping of Chihchang fault, subsurface geophysical investigation, in-situ hydro-mechanical tests and long-term pressure, deformation and seismic monitoring were initiated within the frame of an integrated project called Chihshang Fault Monitoring Observatory.. Eight boreholes were drilled at a depth of 30-100 m through the Chihshang fault fault zone at the Chinyuan site. Pore pressure variations in hydraulic observation wells induced by artificial pumping and injections and natural seasonal variation were monitored, together with surface electrical 4D tomography. Creeping was monitored with both creepmeters and tiltmeters set in surface and TDRs set in boreholes. First results show a complex fault zone affecting several hydrogeological sedimentary units with a high variability of hydraulic properties, 6 10-4 to 2 10-8 of storativity values and 10-4 to 4 10-6 m2/s transmissivity values respectively. Groundwater flow is parallel to the deformation zone direction that is characterized by relatively impervious materials and act as a low-permeability barrier. Indeed, the fault zone bounds a confined aquifer in the foot wall block and a free aquifer in the hanging wall. Complex connections exist between the two aquifers, the foot wall aquifer being recharged by rainfall infiltration through the hanging wall aquifer. When recharge occurs, the fault movement is accelerated in relation with induced overpressures in the foot wall block. The size of the foot wall aquifer estimated of a few hundreds of meters, we suspect that rainfall-induced-effective-stress variations in that aquifer could influence the poro-elasto-plastic behaviour of the fault zone shallow segment.
DE: 7230 Seismicity and tectonics (1207, 1217, 1240, 1242)
SC: Tectonophysics [T]
MN: 2007 Fall Meeting