Location
of active deformations and geodetic data analyses:
Jian-Cheng Lee and Jacques Angelier
(Bull.
Soc. géol. France, t. 164, n° 4, pp. 533-540)
The active surface deformation in the Longitudinal Valley Fault Zone of eastern Taiwan has been analysed by using the geodetic network data. In Taiwan area, the Longitudinal Valley has been considered as the major boundary between Eurasia and Philippine Sea plate, with a present plate motion of about 70 mm/yr in the direction N308°E. For the numerical analysis, we use two simplified models (a model of continuous-homogeneous deformation, and a model of displacements with a discontinuity between rigid blocks). The geodetic data we used in the studied area have been collected and published by Yu & Liu [1989].
The strain tensors calculated based on the first model correspond to a principal shortening along a consistent N132°E trend, with strain rates of -2.3×10-6, -3.1×10-6, and -2.1×10-6 at Juisui, Yuli, and Chihshang respectively. This result does not significantly differ from those previously obtained through heavier classical geodetic calculations. The discontinuous model involves an iterative search process with four unknowns, in order to locate of the discontinuity as well as to determine the trend and velocity of the relative motion. The calculated location of the discontinuity within the geodetic network coincides with the Longitudinal Valley Fault (LVF) line observed in the field by geological means, with an average displacement of 20-22 mm/yr in the direction N313-325°E (reverse-sinistral sense). The comparison between the calculated displacements and the observed ones shows that the discontinuous model fits the data much better. In terms of directions of shortening, the results are in close agreement with the results independently obtained from principal stress tensor analyses of focal mechanisms of earthquakes.
In the Longitudinal Valley, shortening occurs, likely in a plastic way, across a major zone of left-lateral reverse shear. The horizontal component of motion is oblique at 60° to the trend of the major fault (N20°E) which belongs to the convergent Eurasia-Philippine Sea plate boundary. Taking the recent uplift data into account, which show that the Coastal Range (belonging to the Philippine Sea plate) has an average uplift rate of 20 mm/yr relative to the Central Range of Taiwan (Eurasian plate), we reconstruct a 3-D displacement vector diagram. This active fault slip is characterized by a ratio of 0.58 between lateral (10-11 mm/yr) and transverse (17-18 mm/yr) components of horizontal motion, and by nearly equal values (20 mm/yr) of horizontal shortening and vertical offset. This left-lateral reverse fault, with a dip angle of 48-50°E, has a very similar attitude in comparison with the fault independently recognized based on analyses of earthquake foci distribution (which dips 50-50° to the east). Finally, considering the present plate motion data, it appears that the narrow Longitudinal Valley Fault zone absorbs about 1/3 of the total horizontal shortening due to Eurasia-Philippine Sea plate convergence in the Taiwan region.