Characteristics and mechanism of the Longyao ground fissure on North China Plain, China

• The characteristics of the Longyao ground fissure were obtained by the methods of field investigation and measurement.
• The profile characteristics of the Longyao ground fissure were mainly revealed by trenching.
• The relationship between the Longyao ground fissure and Nei-jv fault was revealed by drilling and seismic exploration.
• The mechanism of the Longyao ground fissure was analyzed.

Among more than 1000 ground fissures developed over the past 30 years having induced severe geohazards in the North China Plain, the Longyao ground fissure in Hebei Province is famous for its longest length and causing the worst damage and widest influence in this region. In order to reveal the origin of its formation, the geological background, developmental characteristics, and genetic mode of the Longyao ground fissure were investigated in detail by geological surveys, measurements, trenching, drilling and seismic exploration. This fissure has a total length of 39 km and is characterized by normal fault extension, left-lateral slip, and dominant along-polyline distribution. These characteristics are well consistent with its underlying fault — the Nei-jv fault. The fissure causes the damage to buildings (structures) along a polyline and in the form of a horizontal tensile crack, subsidence, and shear failure. According to the trenching, drilling and shallow seismic profiling, this fissure possesses the characteristics of synsedimentary faults reflecting the historical stages of fissure activities. Regarding its genesis, this fissure was generated under the coupling action of multiple factors, which took shape after the earthquake, and was enlarged under the action of groundwater pumping. The regional extension first caused a fracture system in the surface strata, and then the action of seismic activities started to control the fracture, and finally the pumping action of excessive groundwater and other influential factors promoted the formation of the current ground fissure.