The Setback Distance Concept and 1999 Chi-Chi (Taiwan) Earthquake

The October, 1999 Chi-Chi earthquake in Taiwan, while devastating in the damage to property and life, has become a prototype for faults and earthquake study in Taiwan. The extensive on-site observations taken after the event provide useful and pertinent data in developing measures to mitigate property damage and loss of life in future earthquakes. According to the Taiwan National Center for Research on Earthquake Engineering (NCREE) statistics, at least 30% of the building damages in the Chi-Chi earthquake resulted from surface faulting and ground movement. The State of California, USA enacted in 1973 the Alquist-Priolo Earthquake Fault Zoning Act in an aim to establish setback distances from an active fault to minimize fault movement induced damage. Such a concept was also incorporated into the Taiwan Building Code recently as a result of Chi-Chi earthquake. The principal reason for establishing a Setback distance is to define a “reasonable” distance from a fault within which most buildings for human occupancy are prohibited so as to minimize damages to buildings and human life in the event of a fault rupture. This paper utilizes the data gained from the Chi-Chi earthquake within and without setback areas to study and evaluate the correlation between damages and distance from fault. First, ground distortion was analyzed based on the elevation data recorded before and after the earthquake in Wu-Fong Township. These data are then used as a basis to evaluate the correlation between observed ground distortion, &stances from fault rupture line and damage to buildings in three specific sites. The purpose of the study and analysis is to evaluate the established setback codes as relates to observed building failures along the Chi-Chi earthquake fault lines.