This blog is a personal project that does not seek to represent Durham University.
Sunday, 31 August 2008
The terrain in which the earthquake struck is pretty mountainous as the following Google Earth image perspective shows. The location is the estimated epicentre, as derived by the USGS:
This is clearly the type of terrain in which landslides occur, and of course this region received a good shake in the Wenchuan event. However, of particular concern this time is the weather at the time of the earthquake. NASA TRMM data are very useful for looking at the locations of heavy rainfall. Their latest image shows that SW China has been having some pretty intense precipitation of late:
Given that material may have been loosened by the Wenchuan earthquake, the shallow depth of the earthquake and the rainfall at the time, there must be a reasonable chance that some landslides have occurred. It will be interesting to see if reports of landslides emerge.
UPDATE: The official news agencies are now reporting that there were 40 fatalities, but is not specifying how these occurred. However, Xinhua has released the following incredibly sad and poignant image showing soldiers recovering two bodies. There seems to be little doubt that these people were buried in a landslide.
UPDATE 4th SEPTEMBER: It does appear that landslides have proven to be a substantial problem. CCTV is now reporting that:
"Local residents say there are areas in the remote mountainous regions that suffered even more severe devastation than Huili. But quake-triggered landslides have damaged roads leading to those areas, hampering relief efforts. And it takes several hours at the very least...for relief supplies to get through."
Saturday, 30 August 2008
In particular, on 21st August at about 8:45 am (local time) a series of tidal oscillations were recorded in St Helena Bay (Fig. 2). These tidal oscillations were reportedly large enough to damage some factories, pull a car into the sea and to cause problems for boats navigating into the harbours. A couple of days later, a report emerged from the National Sea Rescue Institute (NRSI) suggesting that the "mini-tsunami" was caused by a seismically-induced submarine landslide. Fig. 2 shows that some anomalous tidal activity certainly did occur.
In the last couple of days a more considered analysis from NSRI has been reported, recognising that the cause is more likely to be meteorological than the effects of a landslide. The South African Weather Service have proposed that atmospheric gravity waves might have been the cause, although they are sitting on the fence a little.
Whilst these tidal oscillations are clearly strange, a landslide cause does not really seem to be very likely.
Wednesday, 27 August 2008
First, South Asia shows a very strong monsoon signal. Look in particular at the mean trend for the five years. The level of fatal landsliding in the winter months is very low, with most of the events occurring in June to September. It is also clear that the number of landslides increases rapidly at the start of the monsoon as the rain bands push northwards, but then decline slowly as the monsoon disperses. There is also considerable year-on-year variation; in particular the very strong and prolonged monsoon rainfall across parts of India is very clear for 2007.
East Asia (below) also shows a strong monsoon signal, of course slightly modified by the impact of typhoons too. Here though the pattern is quite different - again look at the graph of the mean of the other data. In this case the monsoon signal is far more symmetrical, again reaching a peak in July but rising and declining at about the same rate. Again there is considerable year-one-year variation both in terms of number of events and the variation through the year. The big spike in July 2006 is particularly clear.
SE. Asia present a completely different picture (below). As much of SE. Asia is tropical there is far less monthly variation. Unsurprisingly no monsoon spike is seen. The occurrence of landslides is slightly higher in the latter part of the year as the rainy season affects Indonesia for example. Of particular note are the variations in the distribution between years - this really does reflect the occurrence of extreme tropical cloud bursts.
So the impact of the really large scale climate systems on Asian landslides is absolutely clear. This is particularly interesting in the context of climate change - exactly how the Asian monsoon in particular will change will partially determine the landslide impact. Unfortunately the models are not resolving this issue too well at the moment, but most do suggest an increase in the occurrence of the most intense rainfall events. The implications for landslides of this are stark.
Monday, 25 August 2008
The talk is to the various branches of the New Zealand Geotechnical Society and the New Zealand Society of Earthquake Engineering.
I have uploaded the presentation to authorstream so that it can be downloaded (note that I have now given up using slideshare as I have found it too unreliable. The presentation can be found (and can be downloaded as a Powerpoint file from) here and should be viewable below:
Uploaded on authorSTREAM by Dr_Dave
Earthquake induced landslides – lessons from Taiwan and Pakistan
In the second part of the presentation, the occurrence of landslides in the years following the earthquakes is explored. It is shown that there is a notable increase in the occurrence of landslides in the aftermath of the seismic event, and that these landslides release large volumes of sediment into the river system. This increase in landslide activity can have a major impact on the rehabilitation of earthquake-affected areas for long periods of time. Thus, there is a need to improve preparation for earthquakes in landslide-prone areas and to recognise that the impacts of such events are prolonged, rendering the recovery phase rather difficult from an engineering perspective.
Saturday, 23 August 2008
The problem that arose was the occurrence on 12th July of a rockfall on the canyon wall above the river in an area upstream of that used by the jetboats. The Google Earth image of this area is quite good (Fig. 1), illustrating well the steep topography, narrow valley and the location of the jetboat station.
The rockfall was pretty intriguing - a large pillar of schist had detached from the rock mass and was creeping downwards (Fig 2). The mass was pretty large - about 30,000 tonnes and as the image shows it was quite unstable. Fortunately, the authorities in New Zealand are pretty well set up to deal with such things, so quickly swung into action. Perhaps unsurprisingly, the gorge had to be closed to rafters and other users, although the jetboats were essentially unaffected. A monitoring programme was put into place whilst a strategy was formulated.
Clearly the block needed to be brought down so that rafting etc could be restarted, so a fire hose was used to try to induce failure (this is sometimes called water jacking). Unfortunately, the weather over the last month has been very cold, which has meant that this approach has been slower than had been anticipated. Nonetheless the block continued to move. The main block finally detached at 6:30 am yesterday after temperatures rose and there was a few hours of heavy rainfall (Fig. 3). The block has broken up and has not blocked the river, meaning that the crisis is finally over. The local council hopes to reopen the river next week.
Wednesday, 20 August 2008
The town shot into the limelight on 4th March 1995 when a landslide slipped off the slope and buried or damaged seven houses. Fortunately no-one was killed, but it clearly caused considerable concern. The landslide is the subject of perhaps one of the most famous of all landslide pictures, taken by the USGS (Fig. 3). The landslide was large (120 m wide, 330 m long, and >30 m deep, with an estimated volume of 1.3 million cubic metres. However, the slide occurred as a coherent earthflow and was comparatively slow.
Unsurprisingly, in the aftermath of the landslide concerns were raised about the safety of the town. In particular, it is clear from Figure 2 that the slope is mantled with old landslide scars and deposits. There was consistency in the views of the experts that further landslides on the slope were likely. Sadly, this proved to be correct because on 10th January 2005, during a period of very heavy rainfall, a further landslide occurred. On this occasion a portion of the failed mass from 1995 remobilised and hit the upper part of the town.
Amazingly, the failure of the upper part of the landslide was caught by a TV crew (see here or below).
Saturday, 16 August 2008
Perseid Meteor Shower Triggers Mountain Landslide
The Perseid Meteor shower was so intense on the night of August 11th that it triggered a landslide along Interstate 74 in North Carolina.
Meteorites fell on a top of a mountain that overlooks the four lane interstate highway between Murphy and Andrews in western North Carolina. Rocks and boulders rained down on the road and authorities stated that traffic has been re-routed through the small town of Marble. The detour takes drivers along a scenic two lane road that runs parallel to I-74.
Full article here - not bad!
Friday, 15 August 2008
As a comparison, below are the statistics for July 2007 and July 2008, followed by the fatal landslides map for July 2008.
The number of fatal landslides is about the same, but the number of fatalities is very different. At first glance these two maps are also quite similar, but if you look carefully there are some key differences:
1. They are similar in that both have a clear cluster in Central China, although the location is subtly different;
2. In 2008 there were a number of landslides in Central America, whereas in 2007 there were none;
3. They both have a clear cluster along the Himalayas, but close examination shows that there are key differences between the two. In 2007 there were far more fatal landslides in Asia in 2007 than there were in July 2008. The southern edge of the Himalayas is particularly interesting - in 2007 there was a big cluster of landslides in northern Pakistan. This is not the case in 2008. In Nepal the 2007 distribution was concentrated in the west, whereas the 2008 focus was in the central part of the country. On the other hand, in 2007 there was a number of landslides in Bangladesh, but this was not the case in 2008, when only one occurred. Finally, in July 2007 there was a cluster of landslides in southern India, but only one landslide in this month is 2008.
So what drives the differences between these two years? The most obvious explanation is that of rainfall patterns between the two years. It is possible to look at this issue because TRMM provides monthly maps of rainfall anomaly - i.e. the difference between the recorded monthly rainfall and the monthly total averaged over several preceding years. Unfortunately, this map is not available for 2008 yet, but 2007 is online. Given that most of the landslides in July 2007 occurred in Asia, let's take a look there. First the landslide map:
Now the TRMM rainfall anomaly data. The units are mm / day difference from the long term July mean.
It should be fairly clear that there is a strong correlation between the distribution of fatal landslides and the rainfall anomaly in July 2007. In particular, the clusters along the southern edge of the Himalayan arc are focused almost exactly where the rainfall was unusually high. The apparent gap in central Nepal corresponds to an area of lower than average rainfall. Similarly the cluster in south-west India is also related to a high rainfall anomaly.
Finally, below is a NASA image of population density for Asia. Dark red colours represent high population density, orange is low. It is clear that in Asia the population is very unevenly distributed. Note the high population densities in places that were particularly badly affected by landslides in July 2007. Thus, it is clear that the distribution of rainfall probably determines the distribution of landslides, and the density of population then determines the fatality total. Of course this is a bit of an over-simplification, not least because landslides only appear on the database if they kill someone, which means that the population density plays a role here too, but I am sure that you get the general idea.
Finally, it should be noted that Dalia Bach, Yang Hong and the TRMM team have also been looking at this issue and have completed an analysis for landslides in general. See this web page for details:
Tuesday, 12 August 2008
Number of fatal landslides: 55
Number of fatalities: 2127
Once again, the number of recorded fatalities this month has been substantially below the average for 2002-2007, which is 504 deaths. This is consistent with the monthly totals for the whole of the year - indeed at the moment 2008 has the lowest number of fatalities that I have recorded for any year, excluding the Wenchuan (Sichuan) earthquake of course. I will in due course write a post about the likely causes of this very interesting pattern.
Figure 1 shows the distribution map for July. As ever, click on the map for a bigger version. Please acknowledge any use of it or Fig. 2.
The effect of the monsoon in South Asia is now being clearly felt, with a very marked line of landslides along the southern edge of the Himalayas. The usual cluster of landslides in Central China is also clear. Once again there is a higher incidence of landslides in Central and S. America than is normal.
Fig. 2 shows all of the recorded fatal landslides in 2008 to the end of July, excluding those associated with the Wenchuan (Sichuan) earthquake. The typical annual patterns are now becoming clear, with clusters in Central China, S. Asia along the southern edge of the Himalayas, the Philippines and Central / S. America. There are certainly more recorded fatal landslides in the latter area so far this year than has been the pattern over the last few years, but there are also rather fewer in the Himalayas. The western part of the Himalayan chain (e.g. Kashmir) in particular seems to have far fewer fatalities than has been the case for the last few years.
Please leave a comment about this map and dataset! I would really like to know what you think about the data, the maps and the general approach.
Of course, if you know of an event that I might have missed I would like to hear from you...
Thursday, 7 August 2008
Two sets of colleagues in Canada have very kindly provided follow ups to that post. First, Frank Baumann, who is a geotechnical engineer in Squamish has very kindly sent an image (Fig. 1) of the landslide for inclusion on the blog. This very nicely captures both the scale of the problem and the topographic setting of it.
Second, Steve Evans and Keith Delaney from the Landslide Research Programme at the University of Waterloo in Canada have very kindly provided the following explanation of the landslide, including this excellent comparison image:
Steve and Keith write:
"The site of the July 29th 2008 Sea to Sky rockfall/rockslide was examined in 1997. The main photograph in Figure 2 shows the site at that time. The photo at the lower right of Figure 1 was taken soon after the 2008 event by Canadian Press. The slope consists of resistant Coast Plutonic Complex granite but as seen on the 1997 photograph (and on the cover of the book “Rock Slope Engineering” by Hoek and Bray) the rock mass is characterized by more-or-less planar stress relief (sheeting) joints that dip west (downslope) towards Howe Sound (pole concentration is red dot on the stereonet in figure 2). Undercutting of the slope during construction of highway in 1958 resulted in the sheeting joints daylighting in the rock slope (pole concentration is red dot on stereonet). The location of the 2008 rockfall is known as Porteau Bluffs, they run for just under 1 km along Howe Sound (49 33 52.15N; 123 14 01.44W), and the rock slopes along this section of highway exhibit similar rock mass characteristics to those involved in the July 2008 event."
Frank Baumann has kindly also provided a couple of other images of another landslide at the north end on Highway 99 - this time an incipient failure (Figs 3 and 4). It clearly shows that there are other potential problems on this road - although it is also important to stress that, surprising though this might seem, cracks like this do not mean that collapse is imminent or even likely.
This event was first picked up at about 14:50 UT on 1st August at the seismometers at the Cascades Volcanic Observatory, who have kindly put the seismic record online (Fig. 2).
The seismologists quickly recognised that this event was not volcanic in origin, but was in fact associated with a mass movement. As a result, they contacted a local photographer, who went up there and confirmed that a rock and snow avalanches about 2 miles (3 km) has occurred. H.C Tupper has been able to collect a photograph of the event (Fig. 3).
Tuesday, 5 August 2008
The suggestion is that the landslide has opened cracks and fissures in the ground that have penetrated down to a pocket of hydrocarbon, which has now spontaneously started to combust. This probably sounds a little weird and surprising, but interestingly there are quite a few reports of such events from around the world. The area in which this occurs with which I am most familiar is Dorset on the south coast of England. Indeed, near to the town of Ringstead in Dorset there is a coastal landslide known as "Burning Cliff" (Fig. 1).
Perhaps the most famous example however is that of the "Lyme Volcano", which occurred on the Black Ven landslide (see this previous post). This well-recorded event occurred in 1908 when a mound of material on the landslide, probably representing a rock fall deposit, smoldered over several months. It became a sufficiently important tourist attraction that when the heat and smoke started to diminish the local sought to maintain it by throwing paraffin onto it.
So what is the cause of this rather strange phenomenon? In general these events always occur in deposits containing reasonable amounts of hydrocarbon - in the case of the Ventura County event above the landslide is located in a production onshore oil field. In the case of Black Ven Dorset, the Liassic rocks of Black Ven contain about 7% organic materials and represents the source rock for the Wessex Basin. In Ringstead, the Kimmeridge Clay is the cause - this is the main source rock for North Sea oil, containing as much as 20% organics. Whilst I am not sure what the source would be of the ignition for the Ventura County event, in Dorset this is provided by oxidation of the abundant iron pyrites in the shale. Thus, when the landslide occurs the pyrite is exposed. This oxidises, producing heat that then ignites the hydrocarbon in the cliff.
If you are interested in this strange phenomenon then can I recommend the following web page by Ian West of Southampton University, which provides a very details account of such events in Dorset:
West, I. undated. Burning Cliffs of Dorset. http://www.soton.ac.uk/~imw/kimfire.htm
Monday, 4 August 2008
The UK news today carries a very sad report of the death of a teenager on a beach in the UK:
"Paramedics tried to resuscitate the boy after he was pulled out of the sand at Cefn Sidan beach, near Burry Port in Carmarthenshire, on Sunday evening. The teenager was airlifted to Morriston Hospital in Swansea but doctors there pronounced him dead. The boy was from Wrexham in north Wales and had been on holiday with his family and friends when the incident happened. He had been playing with a group of up to 15 other children in the dunes behind the beach in Pembrey Country Park when he became trapped. Swansea Coastguard watch manager Dave Hughes said the alarm was raised at around 1910 BST and Burry Port coastguard team were on the scene within 15 minutes. He said: "By that time the child had been trapped in the sand for approximately 20 minutes."
Whilst this is not really a story about landslides, such a tragic event is closely related to slope failures. I have seen children digging deep holes in the sand on beaches several times, and they always deeply scare me.
A random web search shows up a couple of examples of people digging deep holes in beach sand (I have obscured the faces of the people involved):
It might surprise you to find that this was studied in an academic paper:
Maron et al. 2007. Sudden Death from Collapsing Sand Holes. New England Journal of Medicine 356:2655-2656.
They documented 52 examples of such collapses over a ten year period in the USA. The shallowest hole involved was just 60 cm deep. As a result, 31 people died. The mean age of the victims was 12 years old; the youngest was just three. They suggest that their report under-estimates the occurrence of such events.
So, can I just make a few observations:
- Burial in as little as 30 cm of sand, and sometimes less, can lead to suffocation as the victim cannot inflate their chest due to the mass of material compressing it. Children are particularly vulnerable to this.
- When collapse happens, the victim needs to be extracted in just a few minutes;
- It is often very difficult to find the victim once a collapse has occurred. Digging into sand quickly to get to a buried person is also a substantial challenge;
- If the victim is located it is essential to both clear their nose and mouth and also to get the weight of the sand off their chest. In many cases cardiopulmonary resuscitation is required.
- Never allow children to dig a hole more than a foot deep without supervision;
- Do not allow the child to dig a hole that is deeper than the waist of the smallest child;
- Do not allow them to put their head or chest into the hole;
- Do not allow them to bury each other in a hole.
Sunday, 3 August 2008
Updated (corrected re confusion between naina Devi and Nainital): The Naina Devi temple stampede in India
The Naina Devi temple has a long history of landslides, which may at least in part explain the rumour. The temple was destroyed in a landslide in 1880 that killed 151 people, a fact that may have been in the back of the mind of the pilgrims when the rumour spread, though this is of course just speculation.
Friday, 1 August 2008
Many readers will be familiar with the classic book "Rock Slope Engineering" by Evert Hoek, which still represents the bible for understanding the mechanisms of failure of hard rock slopes. The cover of the original version featured a photograph of a 1965 rock slope failure on the so-called "Sea to Sky" highway in British Columbia, western Canada:
This highway is the subject of some scrutiny at present as it represents the main transportation link between Vancouver and Whistler, which in 2010 will be hosting the Winter Olympics. The road, which has a history of being landslide prone, is currently undergoing a US$780 million upgrade. In Canada the national, and indeed now the international, press are quite exercised by a failure that occurred on this same road on the night of 29th July. Intriguingly, as Erik Eberhardt has pointed out, this rockslope failure occurred directly next to the 1965 failure featured on the cover of Evert Hoek's book:
Photograph, courtesy of Erik Eberhardt of the University of British Columbia, of the rockslide on the Sea to Sky highway. Used with permission.
This failure, which has a reported volume of about 16,000 cubic metres, has completely blocked the road and the railway. It appears to have occurred on a set of pre-existing and somewhat unfavourable joints:
The scale of the landslide can be judged from this Reuters image taken from a helicopter. Note the workers on the debris - some of the blocks are rather large:
The inevitable media jamboree appears to be focused on two elements:
1. At the time of the landslide there was a bus passing by that was fortunately not buried but was hit by some of the debris. This has given the media the opportunity to run the traditional story line of a narrow escape. This from the Toronto Star (who incidentally have a great image of the slide taken from a helicopter, plus a video):
2. The inevitable concerns regarding access to the Olympic Games should such an event occur at the wrong moment. So, for example, Vancouver 24 Hours have run this:
"Where is the contingency plan? What would they do in the event of a major accident or a landslide as we see here today?" said B.C. NDP Olympics critic Harry Bains. "When we are inviting the world, we need to assure them that they will be able to watch the Games, that they will not be stuck on some highway for hours."
In the meantime, the clearance crews are working hard to deal with the debris and the obviously unstable portions of the cliff. CBC News has posted this terrific sequence of images of an unstable block being blasted. This is accompanied by an impressively sensible analysis of the situation on the ground.Update: The Vancouver Sun has printed some rather interesting statistics about the hazards of this highway:
- "Prior to the improvement project the stretch of highway between Vancouver and Whistler averaged 405 rock falls, slides and debris torrents of varying size and severity each year"
- "Since 1906, at least 50 people have been killed in more than 13 debris torrents, 16 floods and two events of unknown cause that have been recorded on 13 of the 23 creeks along the route."
"Oct. 28, 1921: A heavy Lower Mainland rainstorm triggered a washout that killed 37, injured 15 and flattened 50 houses at Britannia Beach.
March, 1964: Six motorists and passengers missed death by seconds when tonnes of granite thundered on to the highway.
Aug. 25, 1976: A rockslide near Lions Bay knocked the engine of a BC Rail freight train off the tracks and buried 30 metres of road under seven metres of rock and mud
Oct. 28, 1981: Sixty years to the day since the Britannia Beach tragedy, nine people were killed as heavy rains brought down debris that knocked out a bridge at M Creek. Unsuspecting motorists drove over the ripped edge of the bridge into the raging creek below.
Dec. 4, 1981: One person was swept away and drowned when a debris torrent buried a concrete bridge at Strachan Creek.
Feb. 11, 1983: Two teenaged brothers were killed when Alberta Creek, flooded with rain, overturned a small trailer in which they slept and buried it under mud and debris at Lions Bay. A highway bridge and three houses were destroyed."