FEATURES AND CHAINS GENESIS ANALYSIS OF EARTHQUAKE GEO-HAZARDS IN YUZI STREAM OF WENCHAN COUNTY

Under the influence of 5.12 earthquake, the geo-hazards are extreme serious in the reach of the Yuzi Stream form Gengda to Yingxiu. The geo-hazard chains are also very prominent. Through field investigation and remote sensing interpretation, this paper tries to reveal the characteristics and genetic process and conditions of geologic hazards in the stream. Based on their characteristics, the geo-hazards are divided into three types. They are (1) instability and crash of the highly weathered rock-mass and soil on the upper slopes, (2) collapse of massive rock-mass slope, and (3) local downpour types of debris flows. The geo-hazards are controlled by seismic fault and the density is higher on the north shore. They show significant deviations between the two sides of Maowen fault zone. 51 collapses and 17 debris flows are analyzed. The geo-hazards mainly took place on the slopes steeper than 40°. The instability positions are mainly upon the parts with slope gradients more than 0.4 and failure zones distribute on the upside of the slopes. Debris flows mainly occur in the valley of the large slope and manhood. The geo-hazards can be transformed into small-scale single collapses and debris flows induced by rainfall. There are mainly two types of geo-hazard chains: (1) geo-hazard chains due to inner dynamic-"earthquake → collapses → stream squeezed, road destroyed and the barrier lakes". The genetic process includes three stages: high-speed start, glide motion, stoppage; (2) geo-hazards chains due to the inner and outer dynamic coupling effect-"earthquake → collapses and shattered mountains → rainstorm →debris flows → stream squeezed, road destroyed and the barrier lakes". The genetic process can be divided into four parts: preparation, dam-breaking after the valley blocked, bed-making and stoppage. The genetic conditions of geo-hazard chains can be concluded as follows: vulnerable geology environment, high steep slope (>50m, >40°) collapsed and shattered by the strong ground motion, the heavy rainfall and suitable conditions for blocking the river.