Trigger Effect of Earthquake Geohazards around Baoxing County Induced by Lushan 4·20 Strong Earthquake

Even though Lushan 4·20 strong earthquake has characteristics of high seismic magnitude and strong destructiveness, its secondary geological disasters are dominated by rockfalls rather than landslides. Compared with secondary geological disasters induced by 5·12 Wenchuan earthquake, their size and number are both mild. However, Baoxing County, which is in the earthquake-affected zone, is an exception. Rockfalls and landslides develop well around the county because of steep mountains. Geological field investigations of the geohazards around Baoxing County before and after the earthquake show that after earthquake the number of rockfall, landslide and debris flood changes into 21, 1, and 3 from 3, 1, and 3, respectively; geohazards development density and modulus change into 2.02 km-1 and 5.59×104 m3/km from 0.38 km-1 and 3.4×103 m3/km, respectively, which are 5.3 and 16.4 times that before the earthquake; and the effect of slope failure induced by the earthquake is significant. Based on the investigation, the following recognitions are obtained by segmental analysis: Seismic geological disasters occur mainly at thin ridges and steep slopes, rockfalls occur mostly at rock slopes, and landslides occur at loose deposited slopes, but large-scale landslides are rare. The overall revival of large-scale loose deposits is not caused by the earthquake, and only some collapse or vibration cracks take place in some parts of steep-gentle junctions. Thus, the loose deposited ancient landslides are stable. This suggests that strong earthquake may not induce overall revival of ancient landslides.