Yosemite rockfall - seismic signal

Improvements in the quality of seismic monitoring systems, in addition to increases in the numbers of instruments and better processing techniques have meant that in the last few years it has become increasingly common to detect large rockfall events from their seismic signature. A nice example of this occurred this week. On 28th March a large rockfall occurred from Ahwiyah Point near Half Dome in Yosemite. This is a pretty large lump of granite:

(Image from http://wikimapia.org/1881070/Ahwiyah-Point)

There is a very nice image of the rockfall scar and track available on Flicker, but it is not possible to import the file. Take a look. This is a less good image from the National Park Service - you do get a pretty good idea of what was involved:


The National Park Service description of this event is as follows:
"[the rocks] fell roughly [600 m] to the floor of Tenaya Canyon, striking ledges along the way. Debris extended well out into the Canyon, knocking down hundreds of trees and burying the southern portion of the Mirror Lake loop trail." They are also reporting that there was a small air blast, but no-one was killed.

Of course, seismic monitoring in the part of the USA is a bit of a fine art, so it is unsurprising that the event was picked up on multiple instruments. There is a nice image on Seismoblog of the recorded seismic events from a range of instruments located an increasing distance from the site:


Based on the sesmic data the USGS were quickly able to locate the fall - pretty impressive!

Terrific rockfall resource

The Colorado Geological Survey publishes a terrific quarterly newsletter on "all aspects of geology throughout the state of Colorado. The latest edition, which is available online for free from here, focuses on rockfalls. It is a terrific read, with some very interesting articles on how and why rockfalls happen; how they can be detected; mitigation approaches; and a number of case studies. It is illustrated with some great photos as well. My favourite two, showing the range of scales of hazardous rockfalls:


And this "spot the difference" is rather nice too. The images were taken about 120 years apart:

Images from the Sichuan earthquake part 5 - Xingyiu

This is the fifth and last part of my photographic description of damage caused by the Sichuan earthquake. The other parts are as follows:

Part 1: Beichuan town
Part 2: The Tangjiashan landslide
Part 3: Hanwang town
Part 4: The Mianyuanhe area
Part 5 (this part): The Xingyiu area

This final section shows images from a visit to the Xingyiu area, which is quite close to the main epicentral region. In this area the main fault rupture appears to sit just out on the plain, being picked out by the line of a river. This is of course bad news for any bridges in the local area. This one has been impacted twice. First, the displacement on the fault has cut through one section of the bridge itself, uplifting one half compared to the other:


The fault runs through the abutment in the foreground, which has been lifted up about 1.5 metres to form this scarp. In addition, because the fault was so close to the fault the bridge was also intensely shaken. As a result, a few of the decks collapsed completely:

Parts of the bridge do show rather nicely the advantages of having rebar in the concrete (these are steel reinforcing rods). Here the concrete has failed completely but the bridge has been prevented from collapsing by the rebar:


Fortunately, at this site rebuilding is well under way – a new bridge is already half constructed (it is visible in the background of the image below). The rush presumably is to get the structure open before the summer rains, when the river bed crossings will become very difficult:


The local authorities have turned the bridge into a memorial and have erected a monument to the event. Note the rockslope failure in the background.



Of course because the fault is in the plain, the mountain front was on the hanging wall block, which means that it was very seriously shaken in the earthquake. This is reflected in the presence of multiple landslides, which I have photographed through the numbers of the monument. Note the damaged town at the bottom of the image, and the temporary structures just in front of them:


In fact the following picture gives a rather more dramatic overview of the full range of landslide impacts:


Or perhaps most usefully, this picture shows the effects of the earthquake rather better. The fault is at the bridge, which has collapsed, severing communications. The town in the middle distance has been badly affected by the earthquake shaking – the damaged buildings and temporary structures are visible – and finally, in the background, lie the earthquake induced landslides:


Another section of the fault runs through one of the small towns in the area:


The tractor is on the trace of the fault (note the ramp and the cracks in the road. The buildings on the fault have collapsed completely, those near to it are intact but so badly damaged that they have had to be abandoned. In the mountains the picture is even worse. Here, whole communities have been devastated and have had to be abandoned. For example, in this settlement every building is very seriously damaged. There is clearly no-one living there now, and rebuilding has yet to start here:


In this area, which is quite beautiful actually, despite the devastation (mind you the weather helped as the pictures show), there was one very large landslide that created a barrier lake. This is a high speed rather complex earthflow, which has a steep rockslope source. The picture below provides quite a reasonable overview of it. The source is a steep rock wall. The main flow has come down a moderately steep gully to enter the main channel, which was duly blocked.


The complexity here is in the volumes, as you can probably see that although the rockslope is large, the amount of material that has detached from it is nothing like enough to have created this enormous deposit at the toe. My interpretation is as follows:
1. The landslide started with a substantial rockfall (or several rockfalls perhaps) from the back wall of the landslide. This is shown quite well in the following image:
 

2. At the toe of the slope was located a large body of old landslide and rockfall material. This was probably holding quite a large amount of water below the surface. The sudden fall destabilised this material, which started to flow. Note that the rock debris that can be seen in the picture above is mostly weathered (there are a few fresh fragments too, but not much), reflecting the fact that most of this material had been sitting on the hillside for a long period. This steep upper track is shown in the image below. Note also the very small volume of material at the toe of the rockslope, even though the face has clearly undergone wide scale rockfalls. This indicates that most of the rockfall material fell onto the landslide below before it moved, and was then carried downhill, supporting the notion that the trigger for the main slide was the rockfalls from the back wall.


3. As the slide moved down the slope it loaded and these entrained material along its track. In the image below you can clearly see how the landslide has eroded and entrained material along its track:


The landslide buried several houses towards the toe of the slope, killing the occupants. Several other houses very narrowly escaped being buried. The pile of debris behind the house is the landslide:


4. Finally, the landslide stopped in the valley allowing the formation of a barrier lake, the last vestiges of which can still be seen:



Note the landslide on the upper left side of the image, and the signs of the previously higher water level left on the rockwalls beside the lake. Of course, this was not the only landslide in this mountainous area. There are many other examples:



Finally, one of the great threats here in the aftermath of the earthquake is wholescale environmental damage. This is shown below, where local people are harvesting trees to be used in rebuilding. I mean no criticism by this (it is a very understandable thing to do), but the impacts could be severe:


That completes my photographic description of damage caused by the Sichuan earthquake. The other parts are as follows:

Part 1: Beichuan town
Part 2: The Tangjiashan landslide
Part 3: Hanwang town
Part 4: The Mianyuanhe area
Part 5 (thia part): The Xingyiu area

Of course there is a lot more that I could included - I will return to specific issues over the next few months. There are still some surprises up my sleeve...

Comments, corrections and requests are very welcome. Finally, thanks to my friends at the State Key Laboratory for Geohazards at the Chengdu University of Technology for arranging my visit and looking after me so well.

Images of landslides and other damage from the Sichuan earthquake part 4 - the Mianyuanhe area

This is the fourth part of my photographic description of damage caused by the Sichuan earthquake. The other parts are as follows:

Part 1: Beichuan town
Part 2: The Tangjiashan landslide
Part 3: Hanwang town
Part 4 (this part): The Mianyuanhe area
Part 5: The Xingyiu area

First a location map. The Mianyuanhe area is a fairly large river valley that cuts through the Longmen mountain chain, with its mouth as Hanwang as shown in the Google Earth image (which predates the earthquake) below:

As you may know, it has been shown that the earthquake was associated with movement on two different thrust faults (and an additional strike-slip fault that sits between the two). The Mianyuanhe shows the surface expression of the two thrust faults. I have annotated a perspective Google Earth view of the Mianyuanhe area to show the approximate position of the two faults below:

This image also gives an idea of the terrain in this area. The surface features of these two faults is pretty clear. This is the Jiangyou-Guanxian fault (JGF) - the car has tilted as it crosses the crest of the fault scarp:

The displacement on the Yingyou-Beichuan fault (YBF) is much larger:

The presence of these two faults has meant that this is an area that has been pretty badly affected by the earthquake, with a large amount of building damage and many, many landslides. It is also an area with some active mines (coal mines I think) - for example, this mine, located just on the hanging wall side of the JGF, has been pretty badly damaged by the earthquake:

Coal mining in China has a dreadful environmental and safety record, so one wonders what happened to the miners when the earthquake struck. In fact, the impact that the mine is having on the environment is pretty clear - the red colours here are acid mine drainage, associated with acidic water coming from the underground workings. Note the extensive earthquake triggered shallow rockslides in the background too:

There has also been a huge amount of damage to buildings, such as this school:

Unfortunately, destroyed buildings create a huge volume of waste material that is very hard to dispose of without causing real environmental damage:


The electricity infrastructure was seriously damaged, as this pylon shows:

And of course the roads are in a very sorry state in places:

However, there is a surprising and very impressive amount of rebuilding going on, some of which is now quite advanced:

In this valley the earthquake triggered a vast number of highly destructive landslides, some of which are quite large. This photo, taken just on the hanging wall side of the YBF, shows gives an idea of the number of slides, as well as the level of damage to buildings:

The second largest mass movement triggered by the earthquake, the Wenjiagou landslide, occurred in this area. It is located between the two faults. This is an overview of this large and complex slide:


The source is a dip-slope failure high up on the left side of the image. The picture below shows the source area more clearly:

Note that on the steep scarp on the centre-left side of the image dust being blown up from ongoing rockfalls is clearly visible. The landslide slide down the dip-slope, turned to the left and travelled down the valley, and then turned to the right before running out. The movement rate must have been very high as the landslide super-elevated (banked up) as it went around the second turn, as this image clearly shows:

About 45 people were killed by the landslide. There is now a problem here with ongoing debris flow activity - more on this below.

There were also some very impressive valley blocking landslides. The most visually dramatic is this one, which has created a large barrier lake:

The slide mass is evident in the gap between the cliffs on the upper centre right of the image. These barrier lakes are causing considerable difficulties (although they are not dangerous now). For example, they are causing flooding of properties and roads:

Even the natural vegetation is being killed:

There are several of these very large valley blocking landslides, such as this one (which is different from the one above):

Note the multiple landslides in the background.

One of the most interesting slides lies on a tributary valley to the main one. This is a c.1 km long rockslide that has created a very tall but restricted debris pile:

This slide appears to have originated as a wedge failure controlled mainly by a bedding plane that has then changed direction and slid into the valley. In the foreground is a school - it is lucky that the slide did not spread. The bedding plane is pretty clear on this image:

The landslide blocked the valley to a height of over 50 m. A lake built up behind the blockage, but in this case drainage has not been necessary as water flowed through the rock dam. The muddy sediments laid down in the lake show this very clearly:

And there are natural markings and pieces of debris showing the old lake levels on the valley sides:

Finally one of the major problems that lies ahead is that of ongoing landslide and debris flow activity, and the resultant impact on the river systems. There are already plenty of signs that these problems are very serious indeed:


Your comments, thoughts and corrections, and indeed you general impressions, are very welcome. Please feel free to use the pictures in lectures and seminars, but please do acknowledge me. I retain copyright on the images.

Finally, just a reminder that this is the fourth part of my photographic description of damage caused by the Sichuan earthquake. The other parts are as follows:

Part 1: Beichuan town
Part 2: The Tangjiashan landslide
Part 3: Hanwang town
Part 4 (this part): The Mianyuanhe area
Part 5: The Xingyiu area