At 4 :20 AM on August 2, 1968, a major earthquake, the most powerful of a series to shock the country in at least two decades, jarred Northern Luzon and battered some sectors of Manila. The sudden sharp strain caused the six-storey Ruby Tower Apartments in Sta. Cruz District to collapse resulting in the death of more than 300 persons. Several other structures sustained damages which amounted to millions of pesos.

A few days after that major tremor, there were reported in the metropolitan newspapers the existence of two faultlines, each approximately 15 kms. long, traversing the Manila area in a northwesterly direction. One fault, as reported, extends from Manila Bay cutting through the intersection of Herran Street and Taft Avenue and continues to the Marikina Valley passing through Guadalupe in Makati (Fig. 1). The other fault, reportedly passing directly under the
Ruby Tower Apartments possibly causing its collapse, runs from Manila Bay through Tutuban Station at Claro M. Recto Street and, likewise, extends to Marikina Valley passing through the northern approach of the Guadalupe Bridge in Makati. Further, as reported, these faults bound a postulated graben.

As a form of public service and for geological information, the Board of Directors of the Geological Society of the Philippines formed an AD HOC Committeei to make a review of the geology of the Greater Manila area to verify such reports.