| Abstract: | Four medium-aperture trilateration networks in eastern Taiwan have been surveyed three to four times since 1981. One is the Ilan network crossing an elongated active seismic zone, the others are the Hualien, Yuli and Taitung networks located at the northernmost, middle, and southernmost portions of the Longitudinal Valley, respectively. Based on changes of the observed line length, the three components of the surface strain rate tensor for each of the networks are obtained by a least squares adjustment technique. Then the principal strain rates are calculated. The Ilan network gives a principal strain rate of uniaxial extension at 2.3 μstrain/yr in the direction of N45°W. The Hualien network has a principal strain rate of 1.2 μstrain/yr extension in N62°E and 1.2 μstrain/yr contraction in N152°E. The Yuli network yields essentially a principal strain rate of uniaxial contraction at 8.4 μstrain/yr in N117°E, whereas the Taitung network has a principal strain rate of 1.5 μstrain/yr extension in N24°E and 3.9 μstrain/yr contraction in N114°E. The directions of contractionof both the Yuli and Taitung networks are consistent with the direction of the maximum compressive stress of this area. Furthermore, average velocities of the relative motion between geodetic stations in the Central Range and the Coastal Range are estimated from the average rates of changes in line length. Stations in the Hualien network show a left-lateral relative motion in a direction more or less parallel to the strike of the Longitudinal Valley, while stations in the Yuli and Taitung networks move toward each other in the direction approximately perpendicular to the trend of the Longitudinal Valley. |
