First news of the Qinghai Earthquake in China

This morning's Mw=6.9 earthquake in China is now reported to have killed at least 300 people, with the toll likely to rise over the next few hours.  Although the earthquake occurred in an area with a low population total and density, vulnerability of both buildings and the landscape to shaking is likely to be high, especially given the reported shallow nature (depth = 10 km) of the event.

The Google Earth imagery of the area shows a landscape that is sparsely populated with substantial mountains such that landslides may well be a serious issue.

The landscape appears to vary, with rolling hills in the south (image from here):

To the north the landscape is undoubtedly steeper and more rugged, and thus landslide-prone: (image from here)

The town in the foreground above is Jiegu (sometimes spelt Gyegu). Early reports suggest that up to 80% of the buildings in the town have been destroyed.  Images of the town before the earthquake suggest that it is likely to have been very vulnerable both to ground shaking and to landslides:

Given the remoteness of this area, and the likelihood of landslides on the roads, the delivery of aid is going to be a major challenge.  My earlier comments on the reporting of large earthquakes in mountain areas applies here once again (insert China for Haiti):

1. Everything stops at night. At the time of writing it is still night time on Haiti. In the aftermath of an earthquake electricity and power supplies are wiped out, so for the night time period it appears that the disaster is not as bad as is feared. As the sun comes up so the reports on the true picture start to emerge, and the fatality statistics start to increase rapidly. This increase will continue for several days at least, but may ultimately exceed the final toll;

2. The initial focus is often wrong. In the immediate aftermath of a disaster the initial focus of the media reports is often on the biggest city. This is rarely where the biggest impact has occurred, but it is most accessible so will be the focus of the reports.

3. No news is very, very bad news. The biggest impacts are often in rural areas with the highest levels of shaking. These areas had poor communications to start with, but when an earthquake strikes the roads become blocked, power is lost and there is no telephone service. Therefore, no news comes out for some time after the quake. The picture is actually the opposite of the obvious. If news starts to emerge quickly from those areas with the highest shaking then the picture is not as bad as we feared - at least some communications are open - although it may still be quite grim. If there is almost no news at all from the rural areas for a day or two, then the picture is probably very bad indeed, with almost all of the communications wiped out.

4. The media focus will quickly change to the foreign rescue teams. However, although these efforts are valuable, their overall impact is very small. The real work is actually done by local people - most rescues are made by untrained people in the first 24 hours - this should really be the focus.

The Pareechu landslide and flood, 2005

In July 2003 a landslide occurred on the Pareechu stream at 32.322N, 78.735E, a remote tributary of the Satluj River in Tibet (see Fig. 1). The landslide was quite large, blocking the river and allowing a lake to form that eventually breached. In this post I will try to use the range of satellite imagery and reports that are now available to describe the sequence of events.

Fig. 1: Google Earth location map of the Pareechu landslide (click on the image for a better view in a new window)

NASA produced an ASTER image of the site that was collected in October 2003, before the landslide occurred (Fig. 2). This shows the rather strange landform at this site - i.e. a broadening of the river in which there has clearly been extensive sedimentation.

Fig. 2: NASA ASTER image of the Pareechu landslide site before failure

It is not clear what triggered the landslide, but another ASTER image collected in July 2004 very clearly shows the immediate aftermath of the slide (Fig. 3).

Fig. 3: NASA ASTER image of the Pareechu landslide site after failure

Note that the basin is now being filled by a lake. The landslide itself is not particularly clear, although the extensive dust kicked up by the slide is fairly obvious as a whitish colour mantling the slopes just to the east of the lake. It is certainly clear that the valley is blocked. An enlargement of this area helps a little (Fig. 4).

Fig. 4: Enlargement of the NASA ASTER image of the Pareechu landslide site after failure

There is an image available in the following presentation (slide 7) that provides a view of the site in August 2004 (Fig 5):
http://www.managingclimaterisk.org/document/SAARC.pdf

Fig. 5: Presentation image of the Pareechu landslide site after failure

The landslide is clearer here, but note also that there is water flow clearly evident, so presumably at this point the landslide had overtopped the barrier. Note also though that the lake is still present, so presumably the flow was not large enough to induce significant erosion.

The next available image is from 20th February 2005 (Fig. 6). Of course this is the middle of winter, so unsurprisingly everything is frozen up. Nonetheless the lake is clearly still present.

Fig. 6: Google Earth image of the Pareechu landslide site in February 2005. The site is of course mantled with snow.

It does appear that there is some flow in the river, but not over the dam, presumably indicating that some seepage was occurring. This high resolution image also gives the first opportunity to get a good look at the slide, especially when the Google Earth terrain model perspective view is used (Fig. 7).

Fig. 7: Perspective Google Earth image of the Pareechu landslide site in February 2005.

It is clear that the landslide here was actually a comparatively small failure of a much larger rock slope failure. However, in turn this active section is part of a much larger rock slope failure that is delineated by a large fracture running across the hillside. I have highlighted this on Fig. 8.

Fig. 8: Annotated Google Earth image of the Pareechu landslide site in February 2005.

At its maximum the lake was 2,100 m long, 1,100 m wide and about 40 m deep storing about 64 million cubic metres of water, according to Gupta and Sah (2008). The dam appears to have failed at about 11 am on 26th June 2005, releasing a flood wave down the river that had a maximum measured discharge of about 2000 cubic metres per second. Fortunately, according to Gupta and Soh (2008) no lives were lost, although press reports at the time indicated that a small number of bodies were washed into India from Tibet. In India the flood did wash away 8 bridges, 15 km of road and caused damage estimated at US$177 million. It is not clear how much damage was caused in Tibet.

The final image, also from Google Earth and dated 4th September 2006, shows the site after the dam breach (Fig. 9).

Fig. 9: The most recent (Sept 2006) Google Earth image of the Pareechu landslide site

Note that the dam is still holding back a little water, but that this volume is now quite low. However, the slope above still looks highly unstable, so a repeat looks inevitable in due course.

Reference
Gupta, V. and Sah, M.P. 2008. Impact of the Trans-Himalayan Landslide Lake Outburst Flood (LLOF) in the Satluj catchment, Himachal Pradesh, India. Natural Hazards, 45, 379-90.

The Yigong Rock Avalanche, Tibet

Occasionally I like to take a look back at a significant landslide. Today I thought I'd write about the amazing Yigong rock avalanche in Tibet. The location of the landslide is shown on Figure 1: it's location is 30°14' N, 94°59' E, located high on the Tibetan Plateau.

Figure 1: Google Earth image showing the location of the Yigong landslide. Click on the image for a better view.

The landslide itself occurred at midday (UT) on 9th April 2000, when a wedge failure at about 5,500 m elevation (3,300 m above the valley floor) detached a volume of rock of over 100 million cubic metres.

the landslide occurred in Zhamu Creek. It took only 10 min to travel down a horizontal distance of 8 km through a vertical elevation difference of 3330 m from its source at 5520 m a.s.l. to its sediment fan at 2190 m a.s.l.(Fig. 3). The trigger for the failure is not clear, but no obvious rainfall or seismic event was recorded. The debris travelled down the valley (Fig. 2) at velocities of up to about 14 m/sec (Shang et al. 2003). Over the next 10 minutes the landslide travelled about 10 km, entraining debris, snow and ice as it flowed to generate a deposit with a volume of about 300 million cubic metres. The landslide came to rest on the valley floor at an elevation of 2190 m, blocking the Yigong River to a height of 60 m over a distance of about 1000 m (Fig. 3).

Figure 2: The track of the Yigong landslide. Click on the image for a better view. Image from Yueping, Y. 2008. Landslides in China: selected case studies. China Land Press, Beijing.

Figure 3: The deposit the Yigong landslide. Click on the image for a better view. Image from Yueping, Y. 2008. Landslides in China: selected case studies. China Land Press, Beijing.

The landslide itself is rather well shown in Google Earth, although unfortunately the source area has a slightly lower resolution than does the track (Figure 4). It is clear that the debris travelled down a comparatively linear tributary valley until the main valley was reached. In the latter stages of the movement the landslide was technically a distal debris flow, essentially meaning that it entrained a large volume of water (from snow and ice). This would have been immensely destructive in an inhabited area, but fortunately the area is sparsely populated.

Figure 4: The track the Yigong landslide as shown on Google Earth. Click on the image for a better view.

Of course the landslide did create a major problem in that it blocked the valley. A lake rapidly started to fill, and over the next two months a volume of about 3 billion cubic metres of water accumulated. The authorities sought to mitigate the problem by relocating the 4,000 living in the path of the flood whilst simultaneously constructing a drainage channel across the landslide deposit. This lowered the peak height of the dam to 44 m above the river level. The water broke through the channel on 10th June and the lake drained over a period of two days. 17 km downstream a peak discharge of 120,000 cumecs (cubic metres per second) was recorded, with the river level rising to 32 m above the deck of the bridge across the river. The trimline created by the flood is clearly visible in Fig. 5. The area that had been inundated by the lake is clear on Figure 6.

Figure 5: The trimline created by the flood resulting from the overtopping of the Yigong landslide dam. Click on the image for a better view. Image from Yueping, Y. 2008. Landslides in China: selected case studies. China Land Press, Beijing.

Figure 6: The area inundated by the landslide dammed lake at Yigong shown on Google Earth. Click on the image for a better view.

The emergency measures were effective in preventing loss of life in Tibet, although there was considerable damage to infrastructure downstream. However, the flood did cause an international incident as the Chinese authorities failed to warn their counterparts in India that the flood was coming. As a result, the people of Arunachal Pradesh were not prepared for the event, leading to about 130 fatalities and 50,000 being rendered homeless (Yin and Wang 2005).

Finally, as an aside, the team that successfully built the drainage channel in this case was kept intact after the event and were thus ready to respond to the landslide dams created by the Wenchuan (Sichuan) earthquake this year. Their experience at Yingong was undoubtedly critical in their success in mitigating the major landslide dammed lakes that were created. It appears that their skills continue to be needed.

References:
Shang, Y. et al. 2003. A super-large landslide in Tibet in 2000: background, occurrence, disaster, and origin. Geomorphology. 54 (3-4), 225-243.
Yin, Y. and Wang, S. 2005. Landslide hazard and reduction strategy in China. In: Hungr et al. Landslide Risk Management. Tayor and Francis, 423-6.
Yueping, Y. 2008. Landslides in China: selected case studies. China Land Press, Beijing.