Behavior of Dams During the 1990 Philippines Earthquake

On July 16, 1990, a devastating earthquake struck Luzon Island, Republic of the Philippines. The earthquake, measuring ML = 7.7, caused more than 2,000 deaths and extensive damage in an 8,000 square mile area extending from Manila northwest through the central Plains of Luzon and the mountains of the Cordillera Central. The earthquake occurred as a result of movement along the Philippine Fault and one of its main branches, the Digdig Fault. As compiled by the USGS National Earthquake Information Center from reports by numerous stations around the world, the epicenter was located about 25 miles northeast of the town of Cabanatuan, at a depth of 16 miles. The Philippine Institute of Volcanology and Seismology reported that a second earthquake occurred about 2 minutes after the initial one; its epicenter located about 10 miles northeast of Santa Fe. Ground rupture was mapped for a distance of 70 miles extending from the east coast of the island to Santa Fe. The fault movement was mainly horizontal (left-lateral strike slip) with the land on the northeastern side slipping to the north a maximum of about 20 feet. Five dams owned and operated by the country's National Irrigation Administration (NIA) were among the major structures severely affected by this event. These earthfill and rockfill dams are situated at distances between 1-1/2 to 12-1/2 miles from the surface trace of the ruptured Digdig Fault. All of the dams underwent settlement, deformation, and cracking. In one case, major slumping occurred; however, none of the embankments failed or were damaged enough to require being taken out of service. The NIA Dam Safety Group inspected the dams immediately after the earthquake; the authors visited the sites three weeks after the event. The main findings from these examinations are summarized in the remainder of this paper. Affected dams: Canili, Diayo, Pantabangan, Aya, Masiway.