為了解呂宋地區之地殼變形，中央研究院地球科學研究所和菲律賓火山及地震研究所合作從1996年至2006年期間，在呂宋地區重複實施七次GPS衛星測量。整合這些長達十年的呂宋GPS重測與部分台灣GPS連續觀測站資料，並加入亞太地區14個IGS追蹤站做為約制，一併利用GAMIT/GLOBK軟體解算獲得各測站之坐標及GPS時間序列資料，再據此以QOCA軟體估計各測站之間震期速度場。呂宋地區相對於歐亞板塊速度場大致上是由南到北逐漸增加，菲律賓斷層西側測站之速率為44到59 mm/yr，東側為59到79 mm/yr；速度向量方位角大致為285o至314o之間，顯示呂宋島弧內部有顯著變形。
根據GPS觀測估計之1996-2006年呂宋地區現今地殼運度速度場，並藉Matsu’ura（1986）提出之間震期地殼變形模式，分別以二維及三維的錯位模型推求菲律賓斷層幾何參數及滑移速率。在二維錯位模型中，在呂宋地區切南北兩條與斷層垂直的剖面（方位角55o），透過地質資料獲得斷層傾角（89o），並利用網格搜尋及bootstrap統計方法求得斷層最佳參數解及其信心區間。在北段剖面，塊體運動速率是37 (95%信心區間，35.3 ~ 40.9) mm/yr，斷層滑移速率為36 (30.9 ~ 44.3) mm/yr，斷層寬度是22 (8.5 ~ 28) km；在南段剖面，斷層滑移速率是25 (15.7 ~ 41.2) mm/yr，塊體運動速率是32 (29.7 ~ 37.2) mm/yr，斷層寬度是35 (27.2 ~ 58.9) km。在三維錯位模型中，模型斷層之地表位置與菲律賓斷層系統之斷層線大致相同。因已知菲律賓斷層為左移斷層，故在模型計算中約制為只有走向滑移分量，利用逆推及bootstrap統計方法求得斷層最佳參數解及信心區間。斷層平均滑移速率南段為20.6 mm/yr，北段為32 mm/yr，塊體運動速率是31 (23 ~ 35) mm/yr，塊體運動方位角是332 o (323 o ~ 341o)，斷層寬度被設定為30 km；本研究根據GPS速度場以二維或三維錯位模式推求之間震期斷層滑移速率與前人研究結果大致符合。粗估地震重現週期北段為128 ~ 183年，南段為83 ~ 217年。由於測站密度及廣度不夠，無法完整偵測整個斷層系統及分支斷層之變形，斷層寬度不能由GPS觀測資料透過模型得到最佳解，僅能根據地質及地震資料加以估計。

For the purpose of understanding the crustal deformation in the Luzon region, the Institute of Earth Sciences, Academia Sinica cooperated with the Philippine Institute of Volcanology and Seismology to conduct 7 GPS surveys from 1996 to 2006. Summing up the 10-year survey-mode GPS data in the Luzon region and continuous GPS data in Taiwan, along with other 14 IGS sites in the Asian Pacific region, we use the GAMIT/GLOBK software to calculate the coordinates of each site and obtain the GPS time series. Then the intersesmic velocity field in the Luzon region is derived by utilizing GPS time series data with QOCA software. The velocity field in the Luzon region gradually increases from south to north with respect to the Eurasia plate. The velocity vectors to the west of the Philippine fault range from 44 to 59 mm/yr; while that to the east of the fault are from 59 to 79 mm/yr; The azimuths of velocity vectors range from 285o to 314 o. This indicates that there’s significant intend deformation in the Luzon Arc.
Based on the GPS observed 1996 ~ 2006 velocity field in the Luzon region and using the intersesmeic crustal deformation model suggested by Matsu’ura et al.（1986）, 2D and 3D dislocation models are utilized to invert for the fault geometry parameters and fault slip rates of the Philippine fault. In the two dimensional dislocation model, two profiles(azimuth 55o) perpendicular to the Philippines fault are studied. The fault dip (89o) is determined from the geological data. By utilizing grid search and bootstrap methods, optimal values of fault parameters and their confidence intervals are estimated. In the northern profile, the block motion is 37 (95% confidence interval, 35.3 ~ 40.9) mm/yr; fault width is 22 (8.5 ~ 28) km; and slip rate of the fault is 36 (30.9 ~ 44.3) mm/yr. In the southern profile, the block motion is 32 (29.7 ~ 37.2) mm/yr; fault width is 35 (27.2 ~ 58.9) km; and slip rate of the fault is 25 (15.7 ~ 41.2) mm/yr. In the three dimensional model, the surface location of the model fault is more or less consistent with the fault trace of the Philippine fault. Since the Philippians fault is a left-lateral strike slip fault, we constrain the fault slip with only strike-slip component in the model calculation. The fault width is fixed to be 30 km. The invented results indicate that the average slip rate of southern segment is 20.6 mm/yr, and northern segment is 32 mm/yr; the block motion is 31 (23 ~ 35) mm/yr; the azimuth of the block motion is 332 o (323 o ~ 341o). Our estimates of the fault slip rates from either two dimensional or three dimensional dislocation models are generally consistent with the previous studies. Recurrence interval of major earthquakes is roughly estimated to be from 128 to 183 years in the northern segment, and from 83 to 217 years in the southern segment. Because the density and aperture of the current GPS array are insufficient, it is not able to successfully detect the deformation of the entire fault system and the branch faults. The fault width can not be well determined from GPS observations through modeling, it can only be estimated for geological and earthquake data.

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