Earthquake And Local Rainfall Triggered Giant Landslides

Essay add: 22-10-2015, 20:34   /   Views: 138

In almost all worldwide mountains with huge altitude difference and intense rainfall, landslides and debris flows frequently take place and are the major geological hazards, such as in the surrounding areas of the Tibetan Plateau. The Upper Yellow River, that locates at the north part of the Tibetan Plateau and consists of four basins areas (i.e., Guide, Jianzha, Xunhua and Guanting) from Longyangxia Gorge to Shigouxia Gorge, is such a geological disaster-prone area (Figure 1). In the region, the altitude difference of many mountain slopes is larger than 1000m. A large amount of landslides and debris and mud flows develop widely in the region.

At least more than 450 landslides have been identified from remotely sensed images.Figure 12. The enormous scale of landslide in the region is rare in the world.A lot of giant landslides developed along two sides of the Yellow river. The Xiazangtan landslide with the ~5.6km~3.8km range and the 15.05108m3 volume that located on the south bank of the Yellow river and directly impacts four villages, was reported as the largest one in the world (Yin et al., 2010; Li et al.,2011) (Figure 2). In its front edge, there are two sub-scale landslides that occurred later. On the south of the landslide bodys top, there was an ancient lake that had disappeared before the Holocene (Yin et al., 2010) because a deep ditch cut throughout the landslide body from south to north at the west section of its trailing edge.

In recent years, irrigation water accelerated the development of the ditch (Figure 3).Figure 2.Figure 3.However, another giant landslide with a ~5.6km~8.5km scale was found on the south bank of the Gongbaoxia Gorge from remotely sensed images according to its surrounding topography. A south-north wide valley suddenly narrows due to the landslide-like mountain bodys impact, showing an abnormal terrain features. A low ridge at the southeast side of the landslide-like mountain was remained between the south-north wide valley and an east-west gully with the slip surface-like slope that locates at the south of the landslide-like mountain, implying that there should be a big mountain at the position of the east-west gully.

We named the landslide-like mountain as the Xiuricun landslide. It may be truly the biggest landslide in the world if it can be confirmed by field work (Figure 4).Figure 4.In some narrow gorges, landslides body may block the Yellow river to form dammed lake in the upstream. Such as in the Jishixia gorge, a giant landslide that was called as the Gelongbu landslide formed a dammed lake along the river in the Xunhua basin (Zhou et al., 2009) (Figure 5). The ?14C age of organic matter from the top and bottom samples of the dammed lake sediment is 910040yrBP (cal) and 1059050yrBP (cal) respectively (measured by Beta Laboratory, USA), implying that the dammed lake was formed in a short time and had outburst in the early Holocene.Figure 5.3.

The unconsolidated sediments of Tertiary and Quaternary and Cretaceous sandstone are major materials of landslides.Quaternary sediment consists of two types: fluviolacustrine deposits and eolian loess deposits. The former mainly distributes in valleys and the latter usually distributes on the slope of mountains. In some areas, loess older than the last glacial period (LG) occurred on the fluvial terraces. However, loess that covers on most areas is the eolian deposit from LG to Holocene.

Tertiary red clay and sand sediments are semi-consolidated depositions that distributed widely in the upper Yellow river and constituted most of mountains (Figure 6). Because of the unconsolidated property of Tertiary and Quaternary sediments, landslides easily developed in mountains with the sediments. The Xiazangtang landslide body consists of Tertiary red clay and sand sediments.Figure 6.Cretaceous sediments mainly consist of purple red sandstones that distributed in several gorges of the Yellow river. Gelongbu landslide developed body is Cretaceous sediments. The Quaternary loess, the Tertiary semi-consolidated red clay and sandy sediments, and Cretaceous purple red sandstones consist of most landslide bodies.

The number of landslide developed in Cretaceous sediment regions is obviously less than in Tertiary and Quaternary sediment regions. Although there are Triassic and Carboniferous sediments in south of the region, more collapses not landslides developed here.It is significant to analyze and summarize landslide patterns on different sediments for revealing dynamic mechanisms of landslides.4. The upstream region of the Yellow river is a museum of landslide patternsThere are a lot of different landslide patterns in the region. Some landslides have the standard landslide morphology, such as Xiazangtan landslide.

Some landslides shapes are triangular or inverted triangular, or rectangular, etc. In some cases, several landslides developed on a slope like as several steps (Figure 7a). In other cases, several landslides developed as a row on a slope.

Some giant landslides that consist of unconsolidated sediments and developed on a mild slope may slip only a short distance to leave a scarp behind the landslide body and the surface of the landslide body likes as the fold dough, such as the Xiaqiongsi landslide (Figure 7b). Therefore, it is very significant to summarize all landslide patterns in the region.Figure 7.In addition, another geological process, mud flow, is an important disaster to seriously affect road traffic. In the Guide basin, mud flows formed a special diluvial fan on which rugged surface without any vegetation a lot of deep pits distributed (Figure 8). In fact, it is a kind of debris flow.

Because its debris materials come from Tertiary semi-consolidated fine sediments which grain size is less than 10?m and indicates the sedimentary environment of the lake Centre, these fine grain-built debris were easily broken as mud. The ?14C age of organic matter in a sample from the diluvial fan is 1621080yrBP (cal), indicating that the mud flow took place due to an rapid increase of melt water caused by warming in the last deglacial period. In the period, relative low temperature limited the growth of vegetation. However, it is still unknown how these deep pits developed on the mud-built fans surface.Figure 8.5.

Larger population was impacted by landslides in the regionBecause the average annual rainfall of this region is only about 300mm, semi-dry climate causes poor vegetation coverage, particularly on mountains of Tertiary sediments due to the poor water-holding capacity of unconsolidated sediments. Therefore, most people live in valleys and basins, and parts of people live on loess-covered slopes. Some villages directly locate on a landslide body and some villages are near the leading edge of landslide bodies. Thus, landslides seriously threaten these people-lived areas.

It is very important to investigate in detail the landslide distribution of the region in order to reduce the possible damage.6. Earthquake and thrust fault are important affecting factors of landslide distributionIn the region, there are two active deep fault belts: the Xiqinling-Jishishan fault belt and the Lajishan fault belt, that are the important thrust fault belts in north of Tibetan plateau and extend hundreds of kilometers (Li et al., 2011). Modern earthquake records shows that there were some earthquakes along the Lajiashan fault.In the region, earthquakes even severely damaged people-lived villages.

An example is the ~4000aBP Lajia ruin that located at the Guanting basin and the east side of Lajishan fault (I A, CASS, 2002; Yang et al., 2003). The oldest millet-noodle in the world was discovered in the ruin (Lu et al., 2005). The ancient loess cave-village was destroyed by a sudden earthquake and left many remains (Figure 9). The strike of the seismic fracture in the ruin is about NW345o, close to the Lajishan fault. Because remains in the ruin were covered by a layer of red clay, it was assumed that the flood caused by the Jishixia dammed lake (Figure 5) outburst submerged the Lajia ruin and deposited the red clay layer after the ancient village was destroyed by an earthquake (Wu et al.,2009).

However, the recent studies show that the outburst age of the dammed lake was earlier than the destroy age of the ruin and materials of the red clay layer came from mountains in north of the ruin not the upper Yellow river. However, it was still unknown where the epicenter of the earthquake was and which faults activity caused the earthquake.Figure 9.In most mountain ridges, many white belts were identified on the perspective view of remotely sensed images. Field investigation in Jiachangcun village of Baizhuang town, Xunhua demonstrates that the image features are some extensional normal fractures with ~20m width.

These fractures caused the development of deep ditches (Figure 10). Difference between these fracture-led ditches and water-eroded ditches on remotely sensed images is that the fracture-led ditches may not extend along the hillside tendency and lack of vegetation coverage due to poor water-holding capacity. It was noticed that the profile shape of these fractures is not a simple arc but a complicatedly-changed spade, strongly suggesting that any simulation of landslide must consider these complicated feature of slip-surface.Figure 10.6.1 Distribution of extensional fracturesIn south of the Yellow river, most of extensional fractures mainly distributed along the north side of the Xiqinling-Jishishan thrust fault belt and a part of extensional fractures distributed along the Jishishan fault belt.

Thus, these fractures development should be related to two trust fault belts activity.In north of the Yellow river, fractures mainly distributed in Yanglongcun and Xiangjialingcun areas where are two centers with intense fractures. Many ditches developed along fractures not slope tendency due to fractures extensional property and some of them directly became landslides that slipped southward.

The huge scale of gravity gliding tectonics in north of the Yellow river suggests that some of them can not be named as landslide again (Figure 11).6.2 Activity property of extensional fracturesAn obvious feature of these fracture-controlled landslides is that their landslide body slipped toward the Yellow river where the altitude is the lowest in the whole region, i.e., landslides in south of the Yellow river slipped northward and landslides in north of the Yellow river slipped southward (Figure 11).Figure 11.Because the fractures in south of the Yellow river may be related to two trust fault belts activity and the scale of gravity glide faults in north of the Yellow river was so big, the earthquake activity along these two thrust faults may be the major triggering force of these extensional fractures. It means that these fracture-led landslides are gravity gliding tectonics that was the indirect not direct generation of regional tectonic stress.

However, where each paleo-earthquakes center was, when each paleo-earthquake took place, which landslides and fractures were generated by each earthquake are still unsolved issues.In fact, any thrust fault-caused features was not interpreted from remotely sensed images. It is because it is difficult to identify thrust faults on remotely sensed images due to their extrusion property that restrained the development of ditches. Thus, more filed investigations are necessary to study these earthquake-generated fractures and landslides.7.

Water is important for some landslides developmentWater comes from local rainfall and melt-water of the Tibetan Plateau. Although the average annual rainfall of this region is only about 300mm, landslides easily developed on unconsolidated sediment-built slopes, such as the Xiazangtang and Xiaqiongshi landslides.However, the development of deep valley of the Yellow river that caused the development of large-scale gravity gliding tectonics should be due to the strong erosion of a large amount of melt-water from the Tibetan Plateau because the eolian deposition on slopes implied that rainfall in the region was lower at least since the last glacial period. The generation of melt-water is related to the increase of temperature or rainfall in the Tibetan Plateau.

Thus, it means that the past climatic change in the Tibetan Plateau and the region may be a key to reveal the development role of landslides in the region.Former studies shows that landslides in the region usually developed in the wet periods over the last 130000years, such as the Holocene Optimum, Marine oxygen isotopic 3th stage, etc (Yin et al.,2010). However, because changes of rainfall and temperature may generate different effects to the development of landslides in the region, it will be important to reveal the sequences of climate change, particularly rainfall and temperature, in the region.On the other hand, rainfall-driven and earthquake-led landslides should be distinguished although it is difficult for remote sensing.

Due to the lubricating effect of water, rainfall-caused landslides usually had smoothed slip planes and fold dough-like ground surfaces.Summarizing above analysis, dynamic mechanism of giant landslide development in the region is (1) the uplift of the Tibetan Plateau and the increase of melt-water led the strengthening of river erosion that in turn caused the increase of altitude difference along two banks of the Yellow river. (2) Earthquakes caused by thrust faults activity and the increase of local rainfall triggered landslides development. Its clear that more evidences are necessary to verify the mechanism and add more details.In the analysis, the powerful perspective view ability provided by Google Earth is a very useful tool for landslides identification and analysis. However, Google Earth cannot help us to real-time monitor the development of geological disasters, especially landslides and debris flows. It also cannot tell us the newest change of terrain.

Thus, if the stereo pair of a region can be real-time gotten from a double-satellite pair with same bands and different flight positions, geological disasters early warning and forecasting would be done better. We wish the satellite technique can be developed as soon as possible.




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