MANY of Manila's newly built middle-class suburbs are standing on a geological fault line which could activate a major earthquake at any moment. The warning follows a study by the United States Geological Survey and the Philippine Institute of Volcanology and Seismology. The West Mariki Valley fault is active, and an earthquake could occur within the next few years, said Dr Raymundo Punongbayan, director of the institute, and Dr Allen Nelson, spokesman for the geological survey. The fault line runs through 23 kilometres of Manila's outer suburbs, including many sub-divisions built by real estate companies in the past 10 years. The two scientists told the University of the Philippines Geological Society that the Government and the private sector should make contingency plans for an earthquake with the magnitude of 6.5 or seven on the Richter scale. According to the study, four big earthquakes between 6.5 and seven have occurred along the fault line in the past 1,400 years, the last one somewhere between 1599 and 1863. Dr Nelson said the recurrence of earthquakes was not uniform and that the average recurrence interval between big earthquakes was about 200 to 400 years. The estimates were based on charcoal fragments collected in a small fault trench near Amityville, a sub-division south of Manila. The institute warned of the danger posed by the fault in 1961, prompting real estate developers to complain that it had dampened the land sale business in areas like Amityville.