Figure 1. Tectonic setting of the Marikina Valley fault system (MVFS) in central Luzon, The Philippines. In A, subduction zone trenches are shown by white barbed lines and other faults with high rates of Quaternary activity by heavy black lines. White dots show locations of recent earthquakes on the Philippine fault in Luzon (magnitude 7.8; 1990) and the Aglubang River fault in Mindoro (magnitude 7.1; 1994). Inset B shows how the Marikina Valley pull-apart basin (MVPB) may have been formed through extension caused by clockwise rotation (dashed circle) and shearing of this part of central Luzon, which is caught between two active left-lateral strike-slip faults--the Philippine fault and the Lubang fault. A zone of extension and young volcanism has also influenced the structural development of the valley.