Active faulting and earthquake hazard: The case study of the Chihshang Fault, Taiwan

The Longitudinal Valley Fault Zone of eastern Taiwan is the present-day pla te boundary between the Philippine Sea Plate and the South China block of E urasia, Repeated surveys of active deformation were carried out at five sit es along its most active segment, the Chihshang Fault. Annual surveys durin g the period 1990-1997 reveal a rather constant slip velocity of 2.2 cm/yr in a N40 degrees W direction, involving both a thrust component with horizo ntal shortening of nearly 1.7 cm/yr and a left-lateral component of nearly 1.3 cm/yr. The fault trends N18 degrees E and dips 39-45 degrees to the eas t. The vertical displacement velocity is about 1.3 cm/yr and the actual obl ique offset of the fault increases at a rate of 2.6 cm/yr. Comparison with GPS data suggests that some additional deformation occurs on the edge of th e Valley. Active faulting of the Chihshang Fault and of the entire Longitud inal,Valley Fault Zone accounts for 24% and 37% (respectively) of the total shortening across the Taiwan collision in the N54 degrees W direction of r elative motion between the Philippine Sea Plate and the South China shelf. This distribution of relative displacements illustrates the major role play ed by this boundary, as a zone of mechanical weakness where tectonic partit ioning occurs. Permanent surveying of the displacement on the Chihshang Fau lt has the potential to detect significant decrease in slip rates, and henc e to predict forthcoming locking stages, which would increase earthquake ha zard. (C) 1999 Elsevier Science Ltd. All rights reserved.