Current Crustal Deformation of Ilan Plain Based onPS Measurements
Date Issued
2008
Date
2008
Author(s)
Chiu, Yung-Tien
Abstract
The northeast Taiwan is undergoing the transtensional deformation in a transition zone due to the collision and subduction between the Philippine Sea Plate and the Eurasian Plate. The Ilan Plain is a triangular-shape flat alluvial plain located on the southwest end of the Okinawa Though which behind the Ryukyu trench-arc system is an active marginal back-arc basin. It is generally considered that the active crustal deformation is deeply influenced by the westward opening of the Okinawa Trough. Moreover, focal mechanisms of a doublet (ML 5.9 and 6.0) occurred on 5 March 2005 showed that the strike-slip faulting with minor normal component are dominated in the southern part of plain. According to seismic reflection profile, the fault systems in the Ilan Plain were preliminary proposed. However, how these faults accommodate the deformation is a crucial topic for the characteristics of crustal deformation at the transition zone between subduction and collision. Therefore, we use the recent GPS measurements to better understand the current deformation pattern in study area. We analyzed the GPS data from 33 stations observed by the Central Geological Survey, 12 continuous GPS stations of the Central Weather Bureau and 2 continuous GPS stations of the National Ilan University to understand the crustal deformation and the spatial distribution of active structures in the Ilan Plain of NE Taiwan. The crustal deformation of the Ilan area is affected by the 2005 Ilan earthquake doublet, therefore, we analyze time series of continuous GPS stations and evaluate co-seismic displacement of the Ilan earthquake doublet by using dislocation model. Thus the secular velocity field related to a stable continental margin station, S01R is recalculated. The horizontal station velocities relative to the Chinese continental margin show that the Ilan area is moving southeastward（the station velocities between 0.9 mm/yr and 37.3 mm/yr in azimuths between 15° and 340°）and the fastest station is near the Suao. The tendency of the Ilan area is that the stations velocities increase from north to south and from west to east. The station velocities at the northern flank of the Ilan Plain are insignificant（the station velocities between 0.9 mm/yr and 9.5 mm/yr）. On the contrary, the magnitudes of station velocities at the southern flank（the station velocities between 3.1 mm/yr and 37.3 mm/yr）are larger than those of the north part. Thus, the opening of Okinawa trough is asymmetric, where the extension rate in the southern part of the trough is faster than the northern one. In addition, the station velocities rotate clockwise along the direction from 26° to 148° in the southernmost margin of the Ilan Plain. The principal extension strain rate axes in the Ilan plain mostly trend NW-SE directions. The maximum principle strain rate is 0.15-2.32 μstrain/yr and the minimum principle strain rate is -2.24-0.65 μstrain/yr. The extension directions rotate clockwise in 43° form western part to eastern part of Ilan Plain. The western plain was affected by both the compressional force by plate’s collision and the tensile force by Ryukyu trench retreat. Thus, the transtensional regime is predicted by the strain rate field. Based on the discontinuous velocity vectors, we supposed there are four faults in the Ilan plain, from north to south, the Niutou fault, the Kengssu fault, the Choushui fault and the Sanhsing fault, respectively. Due to the results of the velocity profiles and shear strain rate field, the faults in northern Ilan plain are dominant with normal fault systems and sinistral fault systems in southern plain. The southward opening of the Okinawa trough is a main driving force in the northeastern Taiwan and results in bending the north Taiwan mountain belt. The left-lateral fault system appears at southern plain was caused by the bending of the mountain belt.
Subjects
GPS
Ilan Plain
crustal deformation
Type
thesis
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