A reconnaissance ground magnetic survey was conducted in the southern Zambales area to verify the presence of a fault that separates the San Antonio Massif and the Cabangau Massif Magnetic readings were recorded at 182 different stations located randomly along accessible roads within the area of interest.

Preliminary results from the magnetic survey define a nearly E-W trending zone of magnetic lows and northwest and north-south trending lineaments delineated based on alternating highs and lows. The nearly E-W trending belt of magnetic lows is interpreted to correspond to the graben structure that has been recognized previously in this part of the Zambales area. Interpretation of the northwest and north-south trending structural features indicate this to be probably the Subic Bay Fault Zone (SBFZ). Field evidences support this geophysically-defined geologic fault.