Precursory Characteristics of Meta-Instability of Cross-Fault Deformation Before the Lushan Ms 7.0 Earthquake

  Objectives  The Lushan Ms 7.0 earthquake on April 20, 2013, occurred in the southern segment of the Longmenshan fault, which is the boundary zone of collision and compression between the Bayan Har block and the South China block. Within a few months before the earthquake, the Baoxing mobile cross-fault short-leveling site in the seismogenic fault displayed thrust accelerating, 25 km from the epicenter; the quasi-synchronous compressive turning was observed in the adjacent Xianshuihe fault, Zhuwo, Xuxu, Goupu, Longdengba, Laoqianning mobile cross-fault comprehensive sites for baseline and leveling, and the Xuxu and Longdengba sites also showed right-lateral turning, 147-295 km from the epicenter. We attempt to explore the mechanism of these abnormal phenomena and its relationship with the Lushan earthquake.

  Methods  Based on analysis of the spatio temporal evolution of mobile cross-fault deformation within the radius of 300 km around the epicenter where above sites are located, combined with continuous GPS, satellite remote sensing, well water level, seismic activities and other auxiliary data, the precursory characteristics of meta-instability of cross-fault deformation before the Lushan Ms 7.0 earthquake is discussed.

  Results  Over six months before the earthquake, the "inherited" fault activity which was conducive to stress accumulation slowed down obviously at first, deviating from linear variation, later changed to reverse inheritance state. Three months before the earthquake it was observed that, the "reverse" anomalies (contrary to the tectonic setting) of the Xianshuihe fault adjacent to the earthquake area increased and strengthened, the compressive turning obviously accelerated, the temporal variation of tension-compression component of grey correlation index was similar to the stress-time process obtained from the meta-instability experiment; the "reverse" change sites of the Longmenshan fault also increased significantly (at the same time, there existed Baoxing anomaly that may be related to pre-slip of the seismogenic fault). Moreover, the auxiliary data containing continuous GPS, satellite remote sensing, well water level and seismic activities also had supporting evidences.

  Conclusions  All of above showed characteristics of fault activities synergism and acceleration. It was very likely to be the precursor performance of meta-instability before the Lushan earthquake.