Characteristics of land subsidence, earth fissures and related disaster chain effects with respect to urban hazards in Xi'an, China

The ancient capital of Xi'an is a typical city in China with a shortage of water resources. With an urban population of 7 million, its domestic and industrial use of water depends almost entirely on groundwater. Large-scale long-term extraction of groundwater has caused severe land subsidence and triggered the reactivation of active normal faults in the Quaternary under-consolidated sediments underlying the city. This has further led to ground ruptures that have gradually evolved into earth fissure zones. During this process, there has been a close spatio-temporal relationship between land subsidence and the formation of earth fissures. From field monitoring data, the subsidence bowls are typically located in loess depressions between earth fissure zones within structural blocks of sediment cut by active Quaternary faults and preexisting fault planes. The subsidence bowls are generally elliptical in shape, with the long axes consistent with the preferred orientation of the earth fissures. The underlying Quaternary active faults and preexisting fault planes predispose the intervening sediment blocks to formation of earth fissures, and intense groundwater extraction has led to accelerated subsidence and earth fissuring during the past 50 years. Mining groundwater, land subsidence and earth fissures occur in a sequence and constitute a disaster chain with respect to urban hazards in Xi'an. Thus, it is important to understand the relationships between these events. This paper summarizes the characteristics of land subsidence and earth fissures in Xi'an and discusses the chain of relationships that connects them.