Subsurface Characteristics of the Lubao Fault in Pampanga, Philippines Defined by Two Dimensional High-Resolution Seismic Reflection Surveys

In the absence of surface ruptures and manifestations, fault mapping in zero-relief areas prove challenging and require other methods such as geophysics and remote sensing. One such fault is believed to exist in the riverine areas of Pampanga, Philippines where sprawling deltas and active sedimentation hide any trace of a possible fault line. Previous studies have identified the Lubao Fault in Pampanga from a prominent surface lineament trending southwestward across deltaic plains north of Manila Bay, separating dry alluvial plains northwest of it from its moist southeastern block, which long-term ground subsidence and rapid sedimentation have dropped 3.5 m over the past 1.5 Ma. Field mapping and remote sensing has shown that the fault extends southwestward through Mount Natib Volcano into the South China Sea, and presently may be undergoing 3.2 cm/y of right-lateral motion. This study augments these previous data by examining shallow subsurface of the fault of the alluvial plains to the northeast flank of Mount Natib with high-resolution seismic-reflection. The seismic profiles show normal components of faulting at a maximum depth of 250 m. Low (800-2000 ms^-1) P-wave velocities of layers that define the profiles are supported by lithologic data from coring; thus, demonstrating shallow faulting through Quaternary alluvium and pyroclastic rocks in Lubao and Hermosa. The high-resolution seismic reflection profiles across the Lubao Fault supplied a key structural information required for assessments of seismic hazards and risk.