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5 December 2019

The Blackhawk rock avalanche – insights from imagery

The Blackhawk rock avalanche – insights from imagery

A couple of weeks ago I posted about the remarkable Blackhawk rock avalanche in the USA.  Since then my regular and highly valued correspondent who goes by the pseudonym funkenbeachin has been putting together some additional information to aid the interpretation of this very large landslide.  I would like to highlight two really interesting and important things that he has done.  The first is a really impressive and useful video, posted to Youtube, that combined the geological map of the site with multispectral satellite data.  You should be able to view this below:-

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Note that this uses Sentinel data.  The annotation on the Youtube video page provides information about the sources of information and the band combinations used.

This is really interesting for a number of reasons.  First, it illustrates really well the ways in which multispectral imagery can be used to provide insight into the geological processes in a complex location.  The different geological materials provide different spectral responses when the satellite band combinations are changed.  This means that it becomes possible to discriminate between different deposits that look similar under normal conditions, providing real insight into the evolution of the landslide.  This band combination is a really good example:-

Blackhawk rock avalanche

A multispectral image of the area around the Blackhawk landslide. Still from a Youtube video by funkenbeachin. Data from ESA Sentinel, collected on 28 June 2019.

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There is fascinating geological complexity in this image, in the bedrock source area, the sediments on the plains below and the landslide deposit itself.

The second is the insight that this analysis provides into the evolution of the landslide itself.  The very linear lateral eastern margin of the Blackhawk rock avalanche deposits for example is really interesting.  I would be really interested to receive comments about your interpretation of the imagery.

Funkenbeachin has also annotated and enhanced the digitased Shreeve geological map of the Blackhawk rock avalanche to make it easy to understand:-

Blackhawk rock avalanche

The digitised Shreeve geological map of the Blackhawk rock avalanche. Image created by funkenbeachin.

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He has very kindly provided a PDF of the map with far higher resolution than the image above – if you would like a copy then please drop me a line via [email protected]

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4 December 2019

Les Mees: a spectacular and unusual rockfall in France

Les Mees: a spectacular and unusual rockfall in France

On 2 December 2019 a spectacular rockfall occurred above the village of Les Mees in Alpes de Haute Provence, France.  The failure, which was triggered by the exceptional rainfall that has occurred through the autumn, is reported to have developed on a steep rock bluff known as Les Pénitents (although judging by the imagery I suspect it actually occurred on an adjacent bluff rather than Les Pénitents themselves).  The blocks, which must weigh over a thousand tonnes, destroyed a house:-

Les Mees

The aftermath of the 2 December 2019 rockfall at Les Mees. Image via L’Obs / AFP.

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Note the very large block at the toe of the slope and the smaller, but still very substantial block 50 m or so further down the slope.

The landslide occurred at 4:15 pm local time.  Fortunately the occupants of the destroyed property were at work, so no fatalities occurred.  The Twitter account @ALERTES_INFO tweeted this image of the aftermath of the rockfall, taken from the road.  This is the smaller of the two blocks, with the larger one being visible in the background:-

Les Mees

Devastation caused by the 2 December 2019 rockfall at Les Mees in France. Image tweeted by @ALERTES_INFO.

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Les Pénitents are a set of geological columns that sit above the ancient village of Les Mees, named for their resemblance to a group of monks wearing pointed hoods.  The rocks are eroded columns formed from the Valensole Formation, a Miocene and Pliocene conglomerate formed from subalpine debris.  The images suggest that the rock mass has few discontinuities such as joints, which both allow it to form these large pillars and enable the detachment of large, coherent blocks when failures occur.  In this case, large rockfalls are likely to be comparatively infrequent but to have very high potential cost.

This rockfall is likely to present an interesting hazard management challenge for the village.  Determining the likelihood of a failure of this type will not be straightforward, but as the images above show the consequences could be substantial.  Clearly catching or retaining such a boulder is difficult, and modifying the rock mass is likely to be unacceptable.

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3 December 2019

Planet Labs images of the huge new landslide at Kumtor Mine in Kyrgyzstan

Planet Labs images of the huge new landslide at Kumtor Mine in Kyrgyzstan

On 1 December 2019 a very large mining-related landslide occurred in a waste pile at the Kumtor Mine in KyrgyzstanConcerns have been raised previously about the large landslides occurring at this site, and indeed I wrote a blog post in September about these risks.

The new landslide at Kumtor occurred at 5:43 am on 1 December.  Sadly, two workers appear to have been killed, although search parties have yet to find them.  Early reports suggest that the landslide was about 12 million cubic metres, although this remains uncertain.

Planet Labs have captured high quality before and after images of the site, which is located at 41.90, 78.18.  This is the site on 29 November, prior to the failure:-

Kumtor mine

Planet Labs image of the site of the Kumtor mine landslide of 1 December 2019. Planet Labs PlanetScope image, collected 29 November 2019. Copyright Planet Labs, used with permission.

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The aftermath of the landslide is captured on an image from 1 December 2019, just a few hours after the failure:-

Kumtor mine

Planet Labs image of the aftermath of the Kumtor mine landslide of 1 December 2019. Planet Labs PlanetScope image, collected 1 December 2019. Copyright Planet Labs, used with permission.

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The images show that the landslide was very large – the length appears to be about 2 km and the width is about 500 m, possibly more in the lower parts.  The failure appears to have started in recently tipped materials close to the bottom of the images.  The 29 November 2019 satellite image shows vehicles in this area.  The rear scarp of the landslide is in partial shadow, but can be seen in the image of 1 December.

A hint of what might have happened at Kumtor Mine is shown in this sequence of Planet Labs images from the last few weeks, taken on 8, 20 and 29 November 2019:-

Kumtor mine

A sequence of Planet Labs images of the upper parts of the Kumtor mine landslide of 1 December 2019, prior to failure. Collection dates as per image. Planet Labs PlanetScope images, collected November 2019. Copyright Planet Labs, used with permission.

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The images appear to show a new area of active tipping in early November, which expanded rapidly through the month.  The image of 29 November appears to show the development of a large failure in the area that developed into the full landslide ultimately.  The debris appears to be loading the next bench down.

It is not possible to say whether this is the start of the events that ended so tragically on 1 December, but I strongly suspect that this is the case.  If so, I wonder if this was detected at the time, and how the risk was being managed?

Reference

Planet Team (2019). Planet Application Program Interface: In Space for Life on Earth. San Francisco, CA. https://www.planet.com/

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2 December 2019

Losses from mining-related landslides

Losses from mining-related landslides

Next Tuesday I will deliver a lecture in Hong Kong that focuses on mining-related landslides.  In the latest of my sequence of papers on global losses from landslides, published last year with my then post-doctoral researcher Melanie Froude, we highlighted the increasing levels of global loss from mining and quarrying related landslides.  In preparing for the lecture in Hong Kong I have been revisiting the data.

The graph below shows the cumulative total number of mining-related landslides from the start of 2004 to the end of 2018, as recorded in my dataset:-

mining

The cumulative total number of mining-related landslides from 2003 to 2018 inclusive.

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The data clear shows an increasing trend through time.  This is a complex pattern, and we need to be careful in interpreting the data.  For example, it does appear that the number of mining-related landslides that I recorded in the first year might have been anomalously low – perhaps through 2004 I was still learning how to collect the data.  But since then the graph has steepened with time, and by the end of 2018 I had recorded 408 mining-related landslides, causing almost 2,800 deaths.

It is really interesting to try to understand what is driving this change.  One way to get at this is to look at the data in terms of location.  The sad reality is that the vast majority of fatal mining-related landslides occur outside of Europe, North America and Australia/New Zealand.  This reflects the dramatic effects of mining regulation – in areas where mining is managed properly landslides are less common, and are far less likely to kill large numbers of people.

The graph below breaks the data for mining-related landslides into four geographical areas:-

mining-related landslides

The cumulative total number of mining-related landslides from 2003 to 2018 inclusive, broken down according to four main geographical areas.

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The trends are really interesting.  In SE. Asia large numbers of mining-related landslides occur, but the rate is essentially constant with time.  East Asia suffers rather less (although I am increasingly concerned as to whether we are really capturing the true extent of these accidents in China).  Thus, t significant global increases are driven by changes in Africa and in particular in South Asia, both of which have seen a dramatic rise since about 2010.

I can only speculate on what has driven this change.  Of course it is possible that I have simply become better at capturing these events, although that seems unlikely.  It is more likely that the accelerating demand for resources in these poorer parts of the world over the last decade has driven an increase in unsafe mining activities, resulting in higher numbers of landslides.  In Africa and South Asia in particular, mining can be very poorly regulated.

The vast majority of these losses are avoidable.  We know how to manage slopes around mines to minimise risk.

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28 November 2019

West Pokot: a landslide disaster in Kenya

West Pokot: a landslide disaster in Kenya

Extremely heavy rainfall affected parts of Kenya on 23 and 24 November 2019, triggering extensive landslides and floods.  Worst affected was the county of West Pokot in the Rift Valley, where landslide events has caused at least 53 fatalities.  It has been hard to track down exactly what happened at this site, although the reports are consistent with a channelised debris flow.  An earlier report in the Independent for example indicates loss of life over multiple locations:-

Interior cabinet secretary Fred Matiangi claims 17 people died in a mudslide in the village of Takmal in the Pokot Central district, while 12 others lost their lives in mudslides in the villages of Parua and Tapach in Pokot South.

I’m wary of posting images from news reports in this case as it is clear that many of the pictures circulating show landslides from events elsewhere in the world.  However, the weather has now cleared and satellite images are becoming available.  Planet Labs captured an image of West Pokot on 27 November 2019.  The part of the image below shows the area around Takmal, which was reported to be a site of extensive landsliding:-

West Pokot landslides

Planet Labs Planetscope image of some of the West Pokot landslides. Copyright Planet Labs, used with permission.

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This image appears to show multiple channelised debris flows, starting as small slips high in the catchment and then combining in the channel.  This is particularly apparent on the west side of the image.  However, note that this is also the case for Takmal itself (the marker shows the location).  Whilst there is some cloud at that point, the landsliding appears to be particularly acute there.

Meanwhile, there are also reports of a similar event in Kinshasa in the Democratic Republic of Congo on 26 November 2019.  Some reports suggest at least 42 fatalities.  Once again there is a lack of clarity about exactly what has happened.

Reference

Planet Team (2019). Planet Application Program Interface: In Space for Life on Earth. San Francisco, CA. https://www.planet.com/

 

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25 November 2019

Savona: a landslide-induced bridge collapse in Italy

Savona: a landslide-induced bridge collapse in Italy

Over the weekend extremely heavy rainfall affected a wide swathe of Europe, most notably in France and Italy.  The Italian region of Liguria has suffered from multiple landslides, the most dramatic of which occurred on an elevated section of the A6 motorway on the road between Savona and Turin.  There are severe rainfall alerts in Emilia Romagna (red); in Abruzzo, Calabria, Piedmont, Veneto, Marche, Lombardy and Puglia (orange); and in Val d’Aosta, Trentino, Alto Adige, Campania, Molise, Basilicata, Umbria, Sicily and Sardinia (yellow).

The event near to Savona was serious – it was fortunate that there was no loss of life given the impact:-

Savona landslide

The aftermath of the landslide on the A6 near to Savona in Italy. Image via Il Fatto Quotidiano.

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A video on Youtube, posted by La Repubblica, provides an overview of the full extent of the landslide:-

Savona landslide.

The aftermath of the landslide on the A6 near to Savona in Italy. Image via Youtube.

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On first inspection this appears to be a slip in soil and regolith in a steep topographic depression or bowl.  The released debris appears to have transitioned to become a channelised debris flow that has exceeded the capacity of the channel below the road.

The Savona landslide occurred close to Madonna del Monte at 44.295, 8.434. The topographic depression from which the landslide originated can be clearly seen:-

Savona landslide

The location of the landslide on the A6 near to Savona in Italy, as shown on Google Earth

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In essence this event will be down to inappropriate design of the channel below the road.  It highlights the difficulty of designing structures that are able to handle the very short lived, high discharge events that occur when chanellised debris flows evolve from slope failures.  As usual, I highlight the video from the 2008 Lantau events in Hong Kong to illustrate the dramatic and devastating effects of these landslides.

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22 November 2019

Deep-seated gravitational slope deformations: Piz Dora

Deep-seated gravitational slope deformations: Piz Dora

Deep-seated gravitational slope deformation (DGSD) is one of the most poorly recognised, but most dramatic, types of landslide. Mauro Soldati defined them as follows:-

A deep-seated gravitational slope deformation (DGSD) is a gravity-induced process affecting large portions of slopes evolving over very long periods of time. A DGSD may displace rock volumes of up to hundreds of millions of cubic meters, with thicknesses of up to a few hundred meters…Deep-seated gravitational slope deformations (DGSDs) are not considered hazardous phenomena because they evolve very slowly. However, they must not be neglected when defining slope instability in a territory and the related hazard implications. Despite their slow deformation rates, DGSDs may cause damage to surface and underground (e.g., tunnels) structures. In addition, they may evolve into faster mass movements or favor collateral landslide processes.

A really nice example is described in an article in Engineering Geology (Agliardi et al. 2019) at Piz Dora in Switzerland. This landslide, which is located at 46.601, 10.350, is shown very nicely in the Google Earth image below:-

Piz Dora

Google Earth image of the Piz Dora deep-seated gravitational slope deformation in Switzerland. The view is towards the west.

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There is an excellent interpretation of the geomorphology and structure of the Piz Dora DSGSD is provided by Agliardi et al. (2019):-

Piz Dora

A geomorphologic and morpho-structural map of the Piz Dora deep-seated gravitational slope deformation. Map from Agliardi et al. (2019).

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As the map shows, the entire mountain side is moving on a deep-seated shear surface (or on multiple shear surface), creating a set of scarps and counter scarps high on the hillside. One of the largest ones is seen below:-

Piz Dora

A counter scarp on the Piz Dora deep-seated gravitational slope deformation. Image from Google Earth.

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Downslope from the scarp is a large displaced mass.  Smaller scarps extend across the slope – at the main peak (Piz Dora itself, there is a pin to mark it in the first image) the scarps extend right the way through the crest of the slope:-

Piz Dora

Google Earth image of the rear scarps of the Piz Dora deep-seated gravitational slope deformation.

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The lower part of the mobile slope consists of a series of smaller, but from insignificant, rockslides, mostly covered in forest.

Agliardi et al. (2019) have measured the rate of movement of the Piz Dora deep-seated gravitational slope deformation. They found that, as is usual for this type of landslide, the mass creeps continuously at low rates.  Movement rates vary across the landslide, with a maximum velocity in the order of 30 – 100 mm per year.

These types of landslides are very common in high mountain areas.  Because they do not generate significant levels of hazard, they are poorly investigated and are rarely reported.  However, they are one of the most fascinating types of landslide, and they deserve more attention.

Reference

Federico Agliardi, Federico Riva, Marta Barbarano, Stefano Zanchetta, Riccardo Scotti and Andrea Zanchi. 2019.  Effects of tectonic structures and long-term seismicity on paraglacial giant slope deformations: Piz Dora (Switzerland) Engineering Geology, 263, 105353.  https://doi.org/10.1016/j.enggeo.2019.105353.

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19 November 2019

Landslides triggered by the Mw=7.8 Haida Gwaii earthquake in Canada

Landslides triggered by the Mw=7.8 Haida Gwaii earthquake in Canada

On 27 October 2012 the Mw=7.8 Haida Gwaii earthquake struck off the coast in British Columbia in Canada.  This was a shallow earthquake with a dominantly thrust-type mechanism, which would be expected to generate substantial numbers of landslides if it occurred directly under hilly terrain.  In this case, a comparatively small area of Haida Gwaii (formerly the Queen Charlotte Islands) was affected by significant shaking.  The impacts in terms of landslides are documented in a new paper (Barth et al. 2019) published in the journal Landslides.  This is a particularly interesting case as the study area, which extends over about 1350 km2, consists of undisturbed, forested hillsides.  This is important for two reasons:

  1. It removes the effects of human activity on landslide susceptibility, giving a dataset that is less complex to interpret than has been the case elsewhere;
  2. The contrast between native forest and the bear rock / soil of a landslide makes mapping comparatively straightforward using satellite imagery.

Thus, this is a key event for understanding the landslide impacts of large earthquakes.

In total Barth et al. (2019) have mapped 244 landslides triggered by the earthquake.  The authors provide an example of these landslides in the area of Gowaia Bay in Haida Gwaii:-

Landslides from the Mw=7.8 Haida Gwaii earthquake

Landslides from the Mw=7.8 Haida Gwaii earthquake. Image from Barth et al. (2019).

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Barth et al. (2019) have looked at multiple epochs of satellite imagery, and examined the rates at which landslides occur.  They find that:

  • Prior to the earthquake, the average rate was 1.44 new landslides per year per 100 km2 in the study area;
  • The 244 landslides triggered in the earthquake dramatically increases this to 18.1 new landslides per year per 100 km2
  • In the three years after the earthquake the rate of new landslides decreased to 1.93 per year per 100 km2
  • And finally, in the period from 2016 to 2018 this reduced to 1.0 new landslides per year per 100 km2.

So, it is clear that the earthquake caused a very dramatic spike in the landslide rate and, as has been established elsewhere, an above average rate continued in the post-seismic period, before settling back down to the background rate.  This is a dataset that should allow a more detailed understanding of the control on landslides from different physical parameters, and thus it is an important new dataset in the growing catalogue of coseismic landslide inventories.

Reference

Barth, S., Geertsema, M., Bevington, A.R. et al. 2019. Landslide response to the 27 October 2012 earthquake (MW 7.8), southern Haida Gwaii, British Columbia, CanadaLandslides. https://doi.org/10.1007/s10346-019-01292-7

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18 November 2019

90 years after the 1929 Grand Banks earthquake: the hazards of submarine landslides on the western North Atlantic passive margin

90 years after the 1929 Grand Banks earthquake: the hazards of submarine landslides on the western North Atlantic passive margin

On 18 November 1929 (90 years ago today) the Mw=7.2 Grand Banks earthquake triggered a submarine landslide off the coast of Newfoundland, which in turn generated a significant tsunami.  Whilst the shaking damage was limited, the tsunami struck the Burin Peninsula in Newfoundland, Canada, killing 28 people:-

Grand Banks earthquake

The aftermath of the 1929 Grand Banks Earthquake. This image shows The home of Steven Henry Isaacs of Port au Bras, which was towed back to shore after being swept out to sea by the tsunami and anchored to the fishing schooner Marian Belle Wolfe. Image via Natural Resources Canada.

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Considerable work has been undertaken on that particular event since the disaster.  It is thought that the earthquake triggered a submarine landslide with a volume of about 200 cubic kilometres, which in turn triggered the tsunami.  However, the frequency of such events has been unclear until now.

Over the last few years there has been a dramatic improvement in the understanding of the hazards associated with submarine landslides, and the tsunamis that they can generate, on the margins of ocean basins, driven by deep ocean exploration.  Indeed, even this weekend The Daily Express carried an article featuring the work (inevitably in a sensational style) of David Tappin from the British Geological Survey, explaining that tsunamis generated by submarine landslides may be more common on the coasts of the UK than had been understood previously.

In a new, very timely, paper published in the journal Geology (Normandeau et al. 2019), and available Open Access, report on the results of mapping with multibeam bathymetry, supported with the analysis of cores, off the coast of eastern Canada.  This map shows the location of this work:-

Grand Banks earthquake

The location of the landslide deposit, and the epicentre of the 1929 Grand Banks earthquake. Image from Natural Resources Canada, via CBC.

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The researcher found extensive turbidite deposits, which they interpret as having been generated by submarine landslides similar to the 1929 Grand Banks earthquake event, more than 200 km from the coast and at about 4000 metres water depth.  Analysis of these deposits suggests that they were formed in a series of landslides, representing four events over the last 4,000 years, with a total volume in the range of 300-400 cubic kilometres.

This study indicates that large submarine landslides may be more common on this section of the ocean basin than had been previously considered.  The implications of this are not entirely clear without further research, as Normandeau et al. 2019) highlight:

“Although these landslide events occurred far from the coast, there are considerations for impacts to seabed infrastructure. Three submarine cables cross the Laurentian Fan region, all located on the large Laurentian Fan levee landslide described here. Additionally, the Canadian Atlantic margin is an area of active oil and gas exploration, with recent exploration wells drilled in water depths >2000 m and a potential for deepwater oil production. The tsunamigenic potential of these newly identified landslides is unknown, but the potential threat to coastal communities of eastern North America should not be discounted. A reevaluation of submarine-landslide risk across the western North Atlantic margin is recommended, and would require more systematic seafloor mapping, analysis of the distal record of large events, targeted slope stability analysis, and numerical modeling of landslide tsunamigenic potential.”

This is an excellent study, casting further light on these huge, potentially hazardous landslides.  It is clear that they need to be investigated in more detail.

Reference

Alexandre Normandeau, D. Calvin Campbell, David J.W. Piper and Kimberley A. Jenner. 2019. Are submarine landslides an underestimated hazard on the western North Atlantic passive margin?. Geology 47 (9): 848–852. doi: https://doi.org/10.1130/G46201.1

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15 November 2019

The Blackhawk landslide

The Blackhawk landslide

The Blackhawk landslide is without doubt one of the most impressive rock avalanches on Earth.  It is located at 34.393, -116.773 in the Lucerne Valley on the escarpment that divides the San Bernardino Mountains to the south from the Mojave Desert to the north, in California.  This is a Google Earth image of the landslide source and deposit:-

The Blackhawk landslide

A Google Earth image of the Blackhawk landslide in the Lucerne Valley of California.

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As the image shows, this is a landslide on a very grand scale.  With an estimated volume of about 300 million cubic metres, the landslide extends over a distance of about 9 km from the crown to the toe, with a deposit that is up to 2 km wide and up to 30 m thick.  The fall height is estimated to be about 1,200 m in total, making this a highly mobile landslide.

The event is not recent.  Dating of the deposit suggests that it may have occurred about 18,000 years ago, although there is huge uncertainty in that date.  However, in the dry desert environment of inland California the landslide is exceptionally well-preserved, and of course the landslide mass contrasts with the valley floor, rendering the mass highly visible.

This is a landslide that is surprisingly poorly investigated.  There is a very nice PhD thesis from 1959 by Ronald Shreeve that is online, which describes the geology and mechanics of the landslide.  This was in the days in which a PhD thesis could be just 84 pages long (there are lessons to learn from that!).   Shreeve describes the geology, and tries to explain the exceptional mobility of the landslide, hypothesizing that it may have moved on a lubricating basal air layer.  There is also a nice blog article about it on the excellent Looking for Detachment blog.  Finally, there is a book chapter from 1978 about the landslide by Brann Johnson, but even the University of Sheffield subscription to Science Direct does not provide access to that one.

This is landslide that would really benefit from a revisit using up to date techniques.  There is exquisite hummocky morphology in the landslide deposit, captured well in the this 2006 Google Earth image:-

The Blackhawk landslide

Google Earth image of the Blackhawk landslide in California

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And to me, the now dissected landslide source area suggests that this might have been quite a complex event too.  This is a Google Earth image of the landslide scar area; I have annotated the approximate boundaries of the upper part of the deposit:-

The Blackhawk landslide source

Google Earth image of the source area of the Blackhawk landslide.

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The implication appears to be a highly unusual morphology of the landslide scar.  It is not clear to me how the mechanics of this part of the landslide motion would have operated – is there a large volume of stalled material still within the scar area?

Acknowledgement and footnote

Many thanks to my friend funkenbeachin for pointing out this landslide, and for the discussions about it.  He has hypothesised that there may be other events in this area as well – take a look at the image below.    It is not hard to believe that there is more than one landslide deposit on the valley floor:-

The area around the Blackhawk landslide

Wide angle Google Earth view of the valley floor around the Blackhawk landslide

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