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Mapping of
Areas Affected by Liquefaction during the 16 July 1990 Earthquake
INTRODUCTION At 4:26 PM of 16 July 1990, Central Luzon was rocked by the strongest quake to hit northern Philippines this century. The earthquake registered a magnitude 7.8 on the Richter Scale with epicenter at 15.6N and 121.0E near the town of Rizal, Nueva Ecija. A 125 km long ground rupture was consequently formed along the Gabaldon (Nueva Ecija)-Kayapa (Nueva Vizcaya) segment of the Philippine Fault-Digdig Fault system (Fig. 1). The 1990 Luzon Earthquake was felt in many places at Intensity VIII (based on a modified version of the Rossi-Forel Intensity Scale of I to IX currently used in the Philippines). The partial isoseismal map shown in Fig. 1 gives the limit of felt intensities during the event. However, some modifications may be necessary because there were isolated areas, like Dagupan City, Baguio City and the town of Rizal in Nueva Ecija which seemed to have experienced the earthquake at Intensity IX based on observed wave-like motion of the ground surface (Punongbayan and Torres, 1990) and documented occurrence of thrown-up boulders (Umeda, et al., 1990). The severe and unusually long groundshaking caused widespread destructions in the form of collapsed manmade structures, liquefaction and associated lateral spreading, and slope failures in places near and far from the ground rupture and epicentral area. Dagupan City and the rest of low lying areas in Central Luzon, which experienced the earthquake at Intensity VIII, suffered largely from liquefaction-related processes. Purpose of the Study This study aims to achieve the following:
Our study focuses on Dagupan City for a number of reasons. Firstly, Dagupan City is a well-developed population center. Secondly, the phenomenon is of interest not only to geologists, but for engineers as well. The presence of buildings and other infrastructures provides valuable information regarding structural responses to liquefaction. In effect, Dagupan City virtually became a laboratory where theoretical scenarios for liquefaction occurrences were tested against actual geological conditions. Finally, the availability of the record of the city's physical development made it easier to relate damages sustained to the rate of its physical development. Definition of Terms Liquefaction is a process that transforms the behavior of cohesionless soil from a solid to a liquid (Seed, 1979; Leeder, 1982; Bates and Johnson, 1987). This phenomenon is also referred to as "quick" condition (Leeder,1982). During strong groundshaking, a liquefying body of sediment loses its strength or stiffness and undergoes compaction. The attendant decrease in the volume available for interstitial fluids causes an increase in pore water pressure. Liquefaction occurs when the pore water pressure equals the weight of the overburden. Liquefaction affects loosely-packed and water-saturated sediments. Its occurrence is influenced by the magnitude of earthquake, ground acceleration, proximity to earthquake generator, duration of shaking, relative density, seismic history and age of the deposit (Seed, 1979; Committee on Earthquake Engineering, 1985; Bennet, 1990). Liquefaction has been observed to be induced by earthquakes with magnitude of at least 5.5 and in areas hundred of kilometers away from the epicenter (Leeder, 1982). A deposit exhibiting liquefaction undergoes unlimited deformations yielding readily to overlying man-made structures. Severe tilting and subsidence and extensive sand boil occurrences are commonly observed, most noticeably at places with multi-storey concrete structures. Since shear waves are unable to effectively propagate through the liquefied layer, effects of further groundshaking are not very pronounced. Historical Accounts of Liquefaction in Northern and Central Luzon A review of the history of major disasters in the Philippines (Selga,1926; Repetti, 1946; SEASEE,1985) shows that central and northern Luzon had been repeatedly visited by very destructive earthquakes. In vulnerable areas, such high magnitude earthquakes resulted in liquefaction-related damages. Fig. 2 summarizes the documented occurrences of liquefaction in the Philippines during strong earthquakes. Reference to "sinking" grounds, cracks and outpouring of water and black sand were observed in Dagupan during the 1892 earthquake and are probably analogous to ground subsidence, tension cracks due to lateral spreading and sand boil during the 16 July liquefaction phenomenon. >> Methodology
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