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Preliminary knowledge on landslides induced by the Lushan, Sichuan earthquake

2014/5/27 14:52:28

On April 21, 2013, the Lushan earthquake emergency scientific research team under the Institute of Geology of China Earthquake Administration consisting of 13 members carried out field investigations on the seismogenic structure and landslide hazards of the Lushan earthquake under the leadership of Deputy Director Wu Xiwei. Now we present the earthquake-induced landslides investigated today and some simple analysis as follows:

The investigated areas include Luyang Town, Longmen Township, Gaohe Town, Shuikou Town and Yuxi Township of Lushan County and other areas in the east and northeast of the epicenter. According to the preliminary field investigations, the earthquake-induced landslides developed in this region mainly include the following few types: rock collapse, superficial destruction-type slide, rock slide and soil collapse.

1.      Rock collapse

Rock collapse is the most common type of landslides induced by this earthquake,  mainly manifested by broken substances of landslide mass and high moving speed. The volume is small (a few cubic meters) ~ medium (tens of thousand cubic meters) in general.

The most common type of rock collapse triggered by this earthquake is rolling stone-type collapse. The blocks generated by this collapse mostly are of below 10 cubic meters. The collapse occurred on high-slope positions. Its mechanism is that under the condition of strong vibration, the mass loses stability at a high initial kinematic velocity. Plus the steep clinoform, the phenomenon of long-distance and fast stone rolling is generated. Some photos of collapse of rolling stones are shown in Fig. 1. Although the speed of this collapse is high, the earthquake region is mostly covered with dense vegetation and the densely distributed vegetation on the slope lowered the moving speed of the rolling stones to some extent. Fig. 1 indicates these rolling stones are mostly distributed on highways and destroyed highway guardrails to some extent, but they didn’t completely paralyze highway traffic. After simple handling, traffic can be resumed. Affected by the vegetation on the slope, the rolling stones were stopped at slope foot and didn’t move farther. Such collapse is ubiquitous in the Lushan earthquake region. In the dense areas, more than five rolling stone hazards can be found every 100m. Firstly, it is difficult to count them due to small volume and large quantity. Secondly, unless the source areas are very obvious, it is difficult to obtain the accurate positions of their source areas and retention areas from remote-sensing images, so the preparation of a detailed later-stage earthquake landslide chart does not consider these small-scale rolling stones.

It is noteworthy that the vegetation in the earthquake region is luxuriant and a great many of blocks are stopped on slopes by vegetation and may roll again in the future due to aftershocks, rainfall, even wind, so some preventive measures against rolling stones must be taken in the road sections vulnerable to rolling stones. Other types of collapse except rolling stone-type collapse are shown in Fig. 2. The relation between the attitude of rock on a slope and the surface of the slope in Fig. 2 is adverse slope. A medium rock collapse happened along a nearly perpendicular weak structural plane roughly consistent with the slope surface.

Fig. 1  Rolling stone-type collapse

Fig. 2  Photo of a typical rock collapse

2.      Superficial destruction-type slide

This landslide is also a common type of landslides induced by this earthquake and mainly developed in weathered crust on earth surface. Its thickness is less than 1m in general and its volume is small. The landslide substance is extremely fragmentary. See Fig. 3.

Fig. 3  Photos of two landslide masses with superficial destruction-type landslide

3.      Rock slide

This kind of landslides is characterized by large thickness, large volume, continuous and complete sliding surface and serious destruction of landslide mass. Fig. 4 shows a typical rock slide-type landslide. The landslide mass is seriously destroyed. From the sporadic vegetation on the landslide mass, we infer it is a slide type landslide. It has a continuous sliding surface. The residual loose substance on the sliding surface is very likely to create a new slide under the condition of aftershocks or rainfall.

Fig. 4  A long-shot photo of a rock slide

4.      Soil collapse

Soil here is not soil in a real sense. It means the structure of the slope mass is loose before a slide and is not like rock landslide of which structure is strong as a whole. The soil collapse in Fig. 5 is in a section of roadbed. The roadbed is made of loose soil and stone substance accumulated artificially. Under the condition of strong vibration, it loses stability. Such highway roadbed has great potential risk and causes great harm to highway. This hazard needs to be treated with much time and energy.

Fig. 5  Collapse of roadbed

Influence of aftershocks on landslides: according to the measurement of China Earthquake Network, an Ms 4.9 aftershock happened in Lushan County, Ya’an City at 11: 59 April 11. By coincidence, we witnessed the impact of this aftershock on landslide rickle on the slope. After the aftershock, yellow dust was lifted from the slope where a large number of earthquake-induced landslides had developed. Many broken stones rolled down from the slope. On the slope where a large number of landslides triggered by the main shock were generated, following the occurrence of this Ms 4.9 aftershock, many stone blocks rushed down along the slope and the slope was in yellow dust. At the moment, the water in the hydropower station about 1km from the landslide site was still clear (Fig. 6), but a few minutes later, the water gradually became turbid.

Fig. 6  Affected by the aftershock happening a few minutes ago, a large amount of soil and rocks were piled in the river and the water in the hydropower station became turbid

The rolling stones from the secondary landslide triggered by this aftershock injured a soldier of Chinese People’s Armed Police Force who was doing rush road repair work and put the work at a standstill. We planned to go to the possible seismogenic fault (Shuangshi–Dachuan fault) and investigate the disaster situation in the afternoon of that day, but our plan was suspended due to traffic interruption. In the end, we went to Luyang Town of Lushan County to investigate and evaluate the disaster situation. In the earthquake-induced landslide areas where aftershocks happen frequently, constant attention shall be paid to secondary slide of loose accumulation bodies on the mountain triggered by external disturbance. Such slides usually have an extreme high speed due to steep topography. Even small stone blocks are deadly. During rush repair of the road, in the road sections vulnerable to landslide, makeshift and effective measures against rolling stones of the slope shall be taken if possible before rush road repair. Otherwise, in the case of aftershocks, the small stone blocks rolling down not only may destroy the traffic but also may cause serious human injury.

In the chart for rough forecast and analysis on landslide development trend of the 2013 Lushan earthquake prepared 2h after the earthquake, we think this earthquake will trigger a large number of landslides and forecast the dense distribution areas of landslides are mainly in the north and northwest of the epicenter. Limited by objective conditions, we only have made preliminary investigation on the landslides developed on the heading side (we temporarily consider Shuangshi – Dachuan Fracture as the seismogenic fault of this earthquake). On the heading side, many landslides have been developed and caused serious damage. Consideration of the hanging side effect of the landslides in the Wenchuan earthquake, we may foresee denser and more harmful earthquake-induced landslides will happen on the hanging side (mainly in the mountainous areas in the north, northwest and west of the epicenter) of this Lushan earthquake with similar structure and topographic and geological conditions, compared with the heading side. These forecasts will be proved by subsequent field investigations of seismotectonics and landslides and future explanation of earthquake-induced landslides based on high-resolution remote-sensing images.

Due to limitation of field work conditions and lack of all-round remote-sensing images of the earthquake region, the above content is based on current day’s investigation and the preliminary knowledge on the eastern and northeastern areas of the epicenter. In the following investigation, we will acquire more comprehensive knowledge on seismogenic structure and coseismic landslides.

Institute of Geology, China Earthquake Administration

Lushan, Sichuan “4.20” Ms 7.0 Earthquake Emergency Scientific Research Team

Night of April 21, 2013