Surface Creeping Analysis of the Fengshan Fault in SW Taiwan from GPS observations and PSInSAR

• Main Authors: Chuan-MinChao, 趙荃敏
• Other Authors: Ruey-Juin Rau
• Format: Others
• Language: zh-TW
• Published: 2016
• Online Access: http://ndltd.ncl.edu.tw/handle/w472nm

碩士 === 國立成功大學 === 地球科學系碩士在職專班 === 104 === SUMMARY The Fengshan fault is proposed as an active creeping fault accompany with locked behavior in this study. We adopted the geodetic data, including GPS, precise leveling, and ALOS SAR images to analyze the fault surface movements. The GPS and precise leveling measurements provide discrete 3D surface velocities and the PSInSAR has dense Line-of-sight (LOS) velocities across the Fengshan fault. From the analysis of velocity profiles, the Fengshan fault is a left-lateral strike-slip fault in about 15 mm/yr and lengthening of about 3 mm/yr. This fault is creeping in the middle and southern segments. The northern segment of the fault is probably locked in about 1.5 km width. The locations of mud volcanos in the Niaosong, Kaoshung and the Wandan, Pingtung, are consistent with the fault trace of the Fengshan fault well and are proposed as the geological evidence of this fault. Key words: Fengshan fault, GPS, Leveling, PSInSAR, Creeping fault, Locked fault INTRODUCTION Previous studies based on GPS observations have found that the Fengshan fault may be a major active structure with surface creeping in southwestern Taiwan. The Long-term GPS measurements have shown that velocities at the western side of the Fengshan fault are faster than those at the eastern side. However there were no historic earthquakes along this fault and no geologic evidence besides geomorphic features to confirm whether the Fengshan fault is exist. Thus, the geometry and activity of the Fengshan fault are important but remain unclear. Once geologists in 19s had proposed Fengshan transfer fault zone (FTFZ) by the high resolution DEM and also geomorphological interpretation. And 2007 and 2013 Ching and Lee had used GPS velocities to prove the Fengshan fault may be creeping and exercising as a left-lateral, strike-slip fault. The SW Taiwan mainly covered with sediment bedding, which makes it hard to analyze the feature, property or behavior. On the other hand, there are also few uncertain faults makes this area even more complicate. But how it acts will make great impacts on the national constructions and public properties whether earthquakes or slow surface deformations. Thus, it’s necessary to evaluate the potential activities of Fengshan fault. METHODOLOGY ＆ DATA It should be considered about that there are several N20°- 40°E faults at SW Taiwan in the study area and also the Fengshan fault southern part overlapping the Pingtung plain southern part where undergoes strong land subsidence due to groundwater pumping. On the aims of proper investigation, there are 56 continuous GPS stations (mainly from CWB), 167 campaign mode GPS (from CGS and NLSC) stations used in SW Taiwan in study area and 483 leveling points (from CGS and MOI) for the horizontal and vertical displacements detecting. Therefore, precise leveling data can provide good constraints associated with the land subsidence. The velocities can be derived from the position measurements. The Fengshan fault can be analyzed by separated in three segments from north to south and derive the fault-parallel and fault-perpendicular velocities from horizontal velocity field and leveling for vertical velocities. To provide better spatial coverage of the data distribution, we apply the Synthetic Aperture Radar (SAR) image of ALOS ascending data to analysis the Fengshan fault. The vertical impact can be removed from the leveling data by integrating the data sets. Then the variety velocities show the fault properties and giving us more insights into the behavior analyzing of the Fangshan fault. RESULTS ＆ DISCUSSION On the northern, middle and southern segment, the fault parallel component has about 14.3 mm/yr, 12.6 mm/yr and 17.4 mm/yr difference across the fault, and the fault normal component has 3.5 mm/yr, 2.6 mm/yr and 3.6 mm/yr extension across the each segments. PSInSAR is helpful within the density of ground measurements. But it should still conscious about the patterns which might effects the final results of MLV (Mean Line-Of-Sight velocity). On the MLV field, one can see that the Chishan fault and the Lungchuan fault had played the role about 40 mm/yr to the LOS on the effect and so does the subsidence phenomenon on southern Pingtung plain. The two locations of mud-volcanos in Niaosong, Kaoshung and Wandan, Pingtung, lay around on the fault line which may consider as the geological evidence because the fault gap provides better structure for the volcanos releasing pressure. CONCLUSION The Fengshan fault is a strike-slip fault with left-lateral and extension component which can be proved by the long-term GPS surface deformation monitoring. The Fengshan fault is about 41 km from Nanzi, Kaohsiung to Chiatung, Pintung and creeping from the middle segment to southern segment, locked in the northern segment. The mud-volcanos at Niaosong and Wandan are almost corresponding to the creeping segment at the middle profile about 10 km distance between.