潛移斷層可透過持續潛移來釋放斷層面上能量，且潛移斷層之通過將造成鄰近建物與財產之持續破壞，因此我們需要對潛移斷層活動性進行分析，做為往後都市規畫之參考。過去的研究工作指出，位於台灣西南部平原區北起台南南邊至高屏交界處有一西北–東南走向的鳳山斷層，具有約15 mm/yr之左移分量，但由於野外調查工作並沒有發現確切之地質證據來確實鳳山斷層之存在與活動，故於地調所2010年的活動斷層版本中被移除。為了了解鳳山斷層之存在與其活動性，本研究使用GPS點狀資料與PSInSAR之面狀觀測資料進行分析，推估鳳山斷層更可靠之地表位置與影響區域及其影響量。本研究分析2002年至2015年之GPS連續站與移動站量測資料共223站，以Bernese v.5.0解算得到相對於穩定大陸測站金門站(KMNM)每日座標解，並以最小二乘法求得各站速度；本研究也採用內政部一等二等衛星控制點、三等加密控制點於1997年公告之TWD97座標與2010年公告之TWD97[2010]座標之成果共63點，求其座標差平均速度。在PSInSAR方面，本研究採用ALOS PALSAR於2007 至 2011年間總共17張雷達影像，求解衛星視線(LOS)平均速度；同時將藉由2006 - 2011年屏東平原南側147個水準點之垂直速度，評估地層下陷對該區LOS速度的影響量。從GPS所得之水平速度場將鳳山斷層北中南分成三段剖面，在有最明顯的速度落差之中段區，跨斷層速度變化在平行斷層分量上約為12.6 mm/yr，而垂直斷層走向分量的速度落差約為2.6 mm/yr，垂直速度跨斷層的落差約為4.1 mm/yr；而斷層北段之平行斷層分量約為14.3 mm/yr，垂直斷層走向分量的速度落差約為3.5 mm/yr；南段則在平行斷層分量上約為17.4 mm/yr，垂直斷層走向分量的速度落差約為3.6 mm/yr；然而相較於中段在鳳山斷層北段與南段因為GPS連續站點位分布不足，無法藉由GPS明確判斷斷層之運動行為，然可從平行斷層之速度分布確認鳳山斷層為左移之走向滑移斷層，略帶伸張分量；其中，北段更由於點位不足無法得跨斷層的垂直速度落差。而從PSInSAR所求得之LOS速度場觀測結果顯示，在鳳山區至屏東崁頂有較穩定明顯的速度落差。本研究將利用精密水準資料移除研究區域的高程影響，以得到水平項對於LOS方向的速度影響，成果顯示斷層北段接近鎖定狀態，中段與南段則為斷層潛移。近期位於高雄鳥松與屏東萬丹的兩處泥火山噴發，其座落位置與噴發裂隙與鳳山斷層地表位置走向相互呼應，沿線也正為斷層中段潛移最顯確之處，使泥貫入體以壓力較小之裂隙作為噴發路徑等地質特性，與鳳山斷層為走向滑移略帶伸張分量之特性相為呼應。整體而言，鳳山斷層自楠梓至佳冬全長約41 km，為左移潛移斷層，左移分量約15 mm/yr等斷層特性與前人研究相符，北段近地表有鎖定情形；未來針對國土開發與其上線型國家建設維護如台88等應納入位移變型考量。也期待未來能使PSInSAR的監測資料更確切模擬出鳳山斷層的斷層型態及能量累積、釋放等狀況，對斷層活動監測可以有更進一步的掌握。

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.

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