菲律賓馬里其納谷斷層活動性研究－－GPS連續觀測

Continuous GPS Monitoring Along the Markina Valley Fault , the Philippines

評估高度都市化地區鄰近斷層活動性及其地震潛能的重要性，於近來2010 Mw 7.0海地地震與2010 Mw 8.8智利地震所造成的災害中再度被凸顯出來。為了闡明位於菲律賓馬尼拉地區中，由West Marikina Valley Fault（WMVF）與East Marikina Valley Fault（EMVF）所組成的斷層系統Marikina Valley Fault System（MVFS）探討現今的地殼變形模式及地震潛能，本研究於2008年4月在MVFS地區架設6台雙頻與4台單頻GPS連續監測站來監測MVFS之活動性。各站之座標與速度場採用GAMIT/GLOBK軟體求解。GPS速度剖面指出WMVF為一帶有壓縮分量（2.1 mm/yr）的左移斷層（2.7 mm/yr）；EMVF呈現出微量右移型態，移動速率約2.1 mm/yr。由此區域靠近Laguna De Bay之GPS座標時間序列分析顯示，在乾季2008/12–2009/05與2009/12–2010/03時出現累積約19.1與17.5 mm的沉陷量，在濕季時則停止沈陷，甚至在2009年9月26日Ketsana颱風帶來單日250 mm豪雨後，發生20天累積抬升約14 mm的情形，此時間序列的週期變化推測與季節性地下水補注量的變化有關。此外，WMVF南段兩側測站東西方向基線長度在濕季2008/06–2008/11與2009/06–2009/11時產生5.9與11.8 mm/yr的伸長，推測可能因為降雨或地下水位變化導致地層間的孔隙水壓力增加，使得有效應力降低而促使WMVF南段發生潛移。 Recently, 2010 Mw 7.0 Haïti earthquake and Mw 8.8 Chile earthquake re–emphasize theimportance to evaluate earthquake potential and the fault activity near highly urbanized areas. Inorder to understand the recent deformation mode and earthquake potential of the Marikina ValleyFault System (MVFS) which includes West Marikina Valley Fault (WMVF) and East MarikinaValley Fault (EMVF), we set up 6 dual–frequency and 4 single–frequency continuous GPS stations inApril, 2008 to monitor the activity of MVFS. In terms of GPS analysis, we used GAMIT/GLOBK10.35 software to obtain the coordinate and velocity of the stations. GPS velocity profile showsWMVF is a left–lateral strike–slip fault (2.7 mm/yr) with a component of compression (2.1 mm/yr)and EMVF is right–lateral with a slight rate of 2.1 mm/yr. GPS coordinate time series indicates thatthe GPS stations in the region near Laguna De Bay accumulated 19.1 and17.5 mm subsidence duringdry season: 2008/12–2009/05 and 2009/12–2010/03 but stop subsidence in wet season. Even after the250 mm torrential rain which was brought by the Ketsana typhoon on September 26, 2009, the areauplifted 14 mm in the next 20 days. We speculated that the GPS time series cycles have something todo with the change of seasonal groundwater recharge. Furthermore, the baseline time series amongthe GPS stations located nearby the southern part of WMVF had 5.9 and 11.8 mm/yr extension in wetseason: 2008/06–2008/11 and 2009/06–2009/11, we infer that is because rainfall or groundwaterlevel change increases the stratum’s pore water pressure and decreases the effective stress then totrigger creeping in the southern part of WMVF.

Recently, 2010 Mw 7.0 Haïti earthquake and Mw 8.8 Chile earthquake re–emphasize theimportance to evaluate earthquake potential and the fault activity near highly urbanized areas. Inorder to understand the recent deformation mode and earthquake potential of the Marikina ValleyFault System (MVFS) which includes West Marikina Valley Fault (WMVF) and East MarikinaValley Fault (EMVF), we set up 6 dual–frequency and 4 single–frequency continuous GPS stations inApril, 2008 to monitor the activity of MVFS. In terms of GPS analysis, we used GAMIT/GLOBK10.35 software to obtain the coordinate and velocity of the stations. GPS velocity profile showsWMVF is a left–lateral strike–slip fault (2.7 mm/yr) with a component of compression (2.1 mm/yr)and EMVF is right–lateral with a slight rate of 2.1 mm/yr. GPS coordinate time series indicates thatthe GPS stations in the region near Laguna De Bay accumulated 19.1 and17.5 mm subsidence duringdry season: 2008/12–2009/05 and 2009/12–2010/03 but stop subsidence in wet season. Even after the250 mm torrential rain which was brought by the Ketsana typhoon on September 26, 2009, the areauplifted 14 mm in the next 20 days. We speculated that the GPS time series cycles have something todo with the change of seasonal groundwater recharge. Furthermore, the baseline time series amongthe GPS stations located nearby the southern part of WMVF had 5.9 and 11.8 mm/yr extension in wetseason: 2008/06–2008/11 and 2009/06–2009/11, we infer that is because rainfall or groundwaterlevel change increases the stratum’s pore water pressure and decreases the effective stress then totrigger creeping in the southern part of WMVF.