本研究蒐集嘉南地區自1996年以來歷年的 GPS 觀測資料，使用瑞士伯恩大學天文研究所 (AIUB) 發展的 BERNESE GPS Version 4.2 與美國加州噴射推進實驗室 (JPL) 董大南博士開發的 QOCA Version 1.21 這兩套科學計算軟體進行處理分析，以估算此區在 1996-2000 年間之地殼變形，所得結果分為同震變形、平均速度場、主軸應變率、剪應變率、旋轉率、及面膨縮率六個部分討論。
GPS資料處理分為兩個階段，首先利用 BERNESE 軟體將 GPS 觀測原始資料加以處理得到寬鬆約制的“擬觀測量”，第二階段利用 QOCA 軟體的將擬觀測量加以處理分析而得到速度場、同震變形、各小網主軸應變率、剪應變率、旋轉速率、及面膨縮率。合併擬觀測量所使用的數學模式為卡曼濾波 (Kalman filter) 。且所有應變分析皆在均勻應變率模型下進行，即假設速度張量 Ｌ 不隨時間與空間改變，為等梯度速度場 (constant-gradient velocity field) 。
研究成果顯示： (1) 1998瑞里地震同震變形各站的同震位移為3-26mm，方向2°-342°；1999集集地震與嘉義地震造成的同震位移量為8-1349 mm，方向71°-337°，且位於觸口斷層的上下盤之測站同震位移方向大致呈現東西相對的形態； (2) 各測站平均運動速率為5-44 mm/yr，方向71°-345°； (3) 此區域19個小網最小主軸（壓縮）應變率為-0.2 ~ -1.3 mustrain/yr，方向75°-141°；最大主軸（伸張）應變率為 0.01-0.8 mustrain/yr；(4) 各小網最大剪應變率為 0.3-2.0 mustrain/yr，方向 30°-95°； (5) 各小網旋轉率為 0.02°-0.59°/ m.y.，且大部分皆為順時鐘旋轉；及 (6) 各小網面膨縮率為 —1.5 ~ 0.5 mustrain/yr。
本研究由 GPS 觀測結果獲得此區速度場的兩個特徵: (1) 速度量值之趨勢為由西向東及由北往南逐漸增大，速度向量之方向則由北向南逆時鐘旋轉；及 (2) 觸口斷層上盤之速度均較下盤為大，且跨越觸口斷層有 10-28 mm/yr之速度不連續。此結果與前人所得相當一致。
本研究所得此區平均壓縮方向為127° ± 6°，與前人由不同地質或地物方法所得古應力方向 (100°-131°) 相吻合，顯示此區現今應力方向與第四紀古應力方向一致。且比較最大剪應變率方向與主要斷層之走向，發現最大剪應變率方向較斷層走向偏東，其原因可能為受西側北港基盤高區屹立之影響。

Using Global Positioning System (GPS) we have investigated the crustal deformation of the Chianan area in southwestern Taiwan based on the observations from 1996 to 2000. The GPS network includes 40 GPS sites spaced approximately 70 km apart. GPS data were processed with BERNESE GPS software version 4.2 (developed by AIUB) and QOCA software version 1.21 (developed by Dong, Danna at JPL). The results include six aspects: coseismic deformation, average velocity field, principal strain rates, engineering shear rates, rotation rates, and area dilatation rates.
During the observation period there were three major earthquakes (ML > 6) affecting the GPS measurements: JuiLi, ChiChi and ChiaYi. We calculated the coseismic deformation caused by these earthquakes. Our results indicate that the maximum coseismic displacement vector for the Juili and ChiChi (including ChiaYi ) earthquake are 26 mm/ 62° and 1349 mm/ 304°, respectively. For both earthquakes, the coseismic displacements across the Chukou Fault (CKF) show opposite direction. The amount of displacement in the hanging wall is relatively larger than that in the footwall, and sites move toward NW (or W, SW) in the hanging wall as opposed to NE (or E, SE) in the footwall.
The pattern of average horizontal velocity between 1996 and 2000 follows those described first by Yu and Chen (1994). The velocity vectors of stations change directions from the south gradually to the north. A major discontinuity of about 10-28 mm/yr is detected across the CKF. Moderate to remarkable contraction rates of 0.7-1.3 mustrain/yr in 75°-141° are observed in the vicinity of the CKF and dies out both eastward and westward away from the CKF. The maximum engineering shear rate is 0.3-2.0 mustrain/yr and oriented 30°-95°. The rates of rotation for 19 subnets are 0.02°-0.59°/m.y. and mostly in CW rotation. The area dilatation rates of subnets are -1.5 ~ 0.5 mustrain/yr. Moreover, the distribution of engineering shear parameters and for 19 subnets showed that the dominant direction of compressive stress in the study area is SE-NW.
The average compressive direction is 127° ± 6°, which is quite consistent with the directions of present and Quaternary tectonic stress. Comparing the direction of maximum engineering shear rate and the strike of major thrusts in this area, we found that the direction of maximum engineering shear rate is more east than the strike of major thrust faults. This maybe caused by the presence of the Peikang High to the west.

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