Identification of active fault traces of the Abra River fault system, northwestern Luzon, Philippines, from tectonic geomorphic features

The Philippine fault system is characterized by primarily sinistral fault segments and traverses the entire Philippine archipelago. On the populous Luzon Island, the northern segment of this active fault system poses significant seismic hazards, as evidenced by the 1990 M_W 7.7 Luzon earthquake in central Luzon and the 2022 M_W 7.0 Abra earthquake in northwestern Luzon. However, the precise location and characteristics of the fault traces along some of the fault’s segments still remain poorly understood, such as the Abra River fault system (ARFS) in the Abra Province. Therefore, this study aims to identify and characterize the active fault traces of the ARFS on the basis of tectonic geomorphic features related to strike-slip faulting using a 5-m resolution DEM, augmented by field investigations.

Based on geomorphic manifestations and results from our field investigations, we identified at least three major sinistral fault traces of the ARFS along the Abra River valley. Although our mapping results are generally consistent with the published map by the Philippine Institute of Volcanology and Seismology (PHIVOLCS), the new mapping provides better constraints and information for several fault segments that were previously uncertain. Along the fault traces, numerous offset channels, offset alluvial fans, and offset bedrock ridges indicate that the ARFS exhibits primarily left-lateral motion. During field investigation, we found two fault zone outcrops aligned with offset geomorphic features with vertical fault plane and horizontal slickensides, consistent with strike-slip faulting of the ARFS. Flexural scarps and pressure ridges that deform Quaternary fluvial sediments show that these ARFS traces are active. The predominantly sinistral motion of the ARFS is not consistent with the focal mechanism of the 2022 Abra earthquake, which is characterized by reverse motion on a gently dipping fault plane. This suggests the ARFS is not the seismogenic fault of the 2022 event, and the accumulated strain along this structure may have not yet been fully released within the time period of written history. As a result, the ARFS poses a great seismic hazard for the area, and it is necessary to further understand its earthquake behavior and paleoseismic characteristics.