An episodic creep-slip event detected by precise levelling surveys in the central part of the Longitudinal Valley Fault, eastern Taiwan, in 2011-2012

摘要

Precise levelling surveys were conducted across the central Longitudinal Valley Fault, eastern Taiwan, to understand the deformation of the transition zone between the stable fault creep area and the asperity area. In order to investigate the surface relationship between the fault creep area and the geological condition of the transition zone, we established levelling routes in the Yuli, Chike-san, and Reishuei areas. The Yuli area forms the geological boundary of the Lichi Melange Formation, which is composed of chaotic mudstones containing numerous exotic blocks of various sizes and lithologies. Along the Yuli route, located on the Lichi Melange, an uplift rate of 30. mm/yr was detected during the period 2010-2012, suggesting that aseismic fault creep might be continuing with long-term stability. Along the Chike-san route, a vertical deformation rate of 8. mm/yr was detected in the period 2010-2011. However, a large deformation with an uplift rate of 40. mm/yr was detected in the period 2011-2012. Along the Reisuei route, we detected a deformation of 8. mm/yr in the period 2011-2012.A two-dimensional single-fault model was developed to discuss the slip distributions in the periods 2010-2011 and 2011-2012. Relatively large slip rates were estimated at two parts of the fault plane-one at a depth of ~. 1.5. km and another at a depth of ~. 4. km-in both periods. Because both parts of the fault plane show approximately the same slip distribution, we believe that the detected deformation resulted from an episodic acceleration event of creeping slip.The northern limit of the stable creep area may be the Yuli area. The episodic creep event occurred in the transition zone between the stable fault creep area and the asperity area. The boundary between the stable creep area and the episodic creep area is consistent with the geological boundary of the Lichi Melange Formation.