On early Friday morning, August 2, 1968, an earthquake with a Richter magnitude of 7.3 rocked the whole of Luzon and was felt as far south as Northern Samar. The quake left a swath of destruction in the City of Manila claiming more than 300 lives.

The tremor originated from the release of stress along an Island Arc structure located east of Northern Luzon. Its epicenter was plotted at 122.5“E and 16.0’N and its hypocenter was estimated to be 30 to 35 kilometers deep.

No crustal movement occurred in Manila and the surrounding areas. Intensities and distribution of damages were a function of the design, construction techniques and the stability of the foundation. Shear stresses produced the most extensive damages while their combination with torsion produced heavy structural damages. Shear stresses acting on vertical planes caused heavy damages to buildings constructed before World War II while tall modern buildings yielded to horizontal stresses and torsional movements.

The morphological evolution and composition of the Manila delta were studied and correlated to ths effects of the earthquake. The destructiveness of the quake was determined by the relative stabilities of parts of the Manila delta rather than by any existing fault structure. Damages have no distinct pattern of distribution but are sporadic or in clusters. Based on physiography and behavior of the various man-made structures, the Manila delta area is classified into immediately critical areas, critical areas and relatively stable areas.

Two phases of study are recommended: (1) an inter-agency long-range research on the occurrences of earthquakes and. at minimizing their ill-effects, emphasis on local phenomena, and (2) the study of the sub-surface configuration of the Manila delta.