Liquefaction effects of the 16 July 1990 earthquake were particularly pronounced in the provinces of Tarlac, Pangasinan and La Union. These provinces are situated in recent sedimentary environments that exhibited characteristic vulnerability to liquefaction: (1) backswamps in Gerona, Pura, Panique and Ramos, Tarlac; (2) river channels and points-bars in Dagupan city, San Carlos and Malasique, Pangasinan; (3) sandspits in Agoo and Sto. Tomas, La Union; (4) deltas in Aringay, La Union and Binmaley and Lingayen, Pangasinan. The liquefaction-affected areas are similarly underlain by relatively unconsolidated water-saturated, moderately-sorted, fine-grained sand deposits. The intense and prolonged ground-shaking during earthquake caused these sediments to lose strength and, subsequently, induce sand fountaining and lateral flow of the ground. However, the degree by which an area is affected by liquefaction was dominantly controlled by the intensity an duration of ground-shaking, the character of the geologic sub-settings and the level of infrastructural development in the area.
The experience learned from the 1990 earthquake could be applied to other areas situated in the same types of vulnerable environment. A shallow earthquake of magnitude 7.5 or higher will most likely bring about the same liquefaction-related damages in areas within 100 kms from its epicenter. Disasters like those that happened to Dagupan cannot be prevented but can be minimized with adequate understanding of the local geology and proper consideration of the geological factor in the land-use planning and policy-making. A first generation map of liquefactions-prone areas has been prepared to mitigate disasters arising from liquefaction hazards by stimulating preparedness planning. This map focuses on the potential vulnerabilities of all areas in the Philippines during an earthquake of magnitude comparable with the 16 July earthquake. The assessment takes into consideration the geological character of the lithology (recent alluvium), proximity to the earthquake generators and the slope (0-8) of the ground surface. (Author)