| The 135 km-long Marikina Valley fault system (MVFS) is an active dextral strike-slip fault that transects the eastern part of metro Manila, Philippines. We infer kinematics from geomorphic analysis to understand the role of the MVFS in accommodating the deformation resulting from plate interactions. We deduce the rupture segmentation pattern of the MVFS from offset data and recent scarp morphology. We also employ paleoseismic analysis to examine the nature, control, and trigger of fault creep along one segment. These studies are critical in estimating seismic hazard and risk for Metro Manila. The current sense of displacement of the MVFS is explained by pure shearing and by lateral advection of the block between the MVFS and the Philippine fault zone (PFZ) resulting from the westward drift of the Philippine sea plate and northern Luzon. During block extrusion, the east Marikina Valley fault (EMVF) and the west Marikina Valley (WMVF) move at lower rates (∼5–7 mm/yr) than the PFZ. Long-term offset data and single-event scarps indicate that the EMVF has had a history of surface rupture involving multiple segments. Based on length and amount of offsets of the most recent scarps, the EMVF is capable of generating earthquakes of magnitude M > 7. Our modeling also indicates that the northern and southern segments of the WMVF ruptured separately and at earlier times than the 1863 event along the EMVF that produced major shaking and damage in Manila. Vegetation is probably a limiting factor in the accelerated degradation of recent scarps in the MVFS region, where diffusivity is ∼4 m2/kyr. The kinematics, pattern, and location of the creeping traces of the WMVF's Sucat-Biñan segment and occurrence of some of this creep on pre-existing (tectonic) fault traces point to tectonic control of fault creep. However, groundwater withdrawal is strongly indicated as a short-term trigger by the high creep rates (to 10 cm/yr), the spatial correlation between the creeping faults and drop in water table at a number of wells, and the reactivation of a pre-creep strike-slip fault zone by vertical creep. |
