4 February 2016
Landslides near Muzaffarabad from the 2005 Kashmir earthquake
The M=7.6 2005 Kashmir earthquake in northern Pakistan is thought to have killed over 80,000 people (and most likely over 100,000), and left 3.5 million people homeless. A key factor in the impact of this earthquake was the devastating effect of many thousands of landslides. One of the most seriously affected areas was Muzaffarabad – the city itself was very seriously damaged by the shaking, and there was also a huge impact of landslides in the local area. Google Earth now has very high quality imagery of the area before and after the earthquake, and a further set collected more recently (in 2014), almost a decade after the event. This is an image of one of the most seriously affected areas before the earthquake, taken in 2002:
The river that runs north – south across the image is the Neelum. On the southern edge of the image the main part of the city of Muzaffarabad is clearly visible. There are landslides in the image, most notably along the Neelum and on the walls of some of the smaller channels. Contrast this with an image of the area after the October 2005 earthquake – this image was collected on 31st June 2006. Note that the green area on the right is the pre-earthquake image:-
The most dramatic area of landsliding is the huge cliff collapse events that occurred on the east bank of the river to the north of the city (and note how much the city had expended in the few years prior to the earthquake), but there is also dramatic landsliding along the Neelum river, to the east of Muzaffarabad and along the river to the northwest. I took photographs of some of the landslides at the time. This is the area of massive rock cliff collapse to the east of the city:-
Whilst this some of the landslides on the Neelum River. This area was extremely badly affected my shallow rockslope failures:
Landslides are one of the enduring legacies of an earthquake in a mountain chain. This Google Earth image was collected in April 2014. It is heartening to see reconstruction in the city, but note just how obvious the landslides are, and many are very obviously fresh failures. A decade on it is clear that earthquake induced landslide remain a major issue in the Muzaffarabad area:
3 February 2016
Another debris flow on Aconcagua
Last week I blogged about a debris flow video on Aconcagua in Argentina. It turns out there is another video of an earlier debris flow, collected on 26th December 2015:
The text associated with the video, which was shot by Kyle Peterson, is as follows:
I shot this video Dec 26, 2015 and our group nearly got caught in a rock/mudslide hiking out of Horcones Valley after summitting Aconcagua the previous day.
This video beautifully illustrates that dry nature of these types of flow, even though they look wet. Look at the amount of dust the flow is generating as the dry rocks scrape along the walls of the channel:
I’d be very interested to know the source of these debris flows.
A fascinating landslide at Muothatal in Switzerland
Meanwhile, this amazing landslide was apparently triggered by a combination of rain and snow at Muothatal in Switzerland:
Drone footage of coastal erosion at Pacifica
Repeated El Nino storms have driven more rapid erosion on the bluffs at Pacifica in central North California. The resultant erosion appears to be condemning a number of buildings, as very impressive drone footage shows:
2 February 2016
Landslides in Chile Part 5: Water waves triggered by landslides and the Mentirosa Island Landslide complex
Landslide in Chile 5: the Mentirosa Island landslide complex
Cymophobia, the fear of waves and wave-like motions, is capitalised on by the 2015 Norwegian film, The Wave (originally titled Bølgen). Opening sequences of the trailer remind us of the devastating Norwegian Tafjord slide of 1934. Here an estimated 3 million m3 of rock slid into the fjord, generating a <60 m displacement wave of water that travelled 4.5 km up-fjord; inundating shoreline communities with c.15 vertical metres of water (Baathen et al., 2004). Distinction is made between the term displacement wave and tsunami, the former generated by subaerial landslides impacting into water bodies, and the latter caused by the movement of submarine landslides (Hermanns et al., 2014). With all the merits of a blockbuster disaster movie, The Wave presents a worst case scenario, fully CGI-realised, depiction of the catastrophic failure of the Åknes rockslide (also in Norway) and subsequent displacement wave. It is no surprise that this landslide has gained attention from filmmakers, having a colossal estimated volume of 30-40 million m3, and the potential to deliver a 40 m wall of water to the villages of Hellesylt and Geiranger (Oppikofer et al., 2009). The Åknes rockslide is one of the most extensively monitored landslides in the world, and alarm thresholds with public alert levels have been established based on displacement, velocity and acceleration of the slope surface and subsurface (via boreholes).
Water waves triggered by landslides, either originating from an exposed slope (e.g. valley wall) or occurring under water (submarine), pose a danger to communities living around confined water bodies such as fjords, coastal bays, rivers, lakes and reservoirs. Large masses of rock are effective at generating waves when impacting water at high speeds. Waves are then guided by local narrow topography, which may amplify wave height and reflect waves sending them in an alternative direction; leading wave activity to last for hours and extensive damage to shorelines from run-up. Several landslides that triggered waves of different magnitudes have been reported in this blog: (1) Porcupine Bay, Washington State, (2) Nord-Statland, Norway, (3) Vaiont, Italy, (4) Daning River, China, (5) Sokjosen, Norway and (6) Lituya Bay, Alaska. The largest wave occurred in Lituya Bay and had a maximum run-up height (vertically up the valley wall) of 530 metres. Sonny and Howard Ulrich provide an eyewitness account of the event for the BBC Mega Tsunami documentary, available here.
Differences in source landslide trigger, size, material and mechanism, as well as local variations in topography, challenge scientists who use numerical computer models to forecast the formation and propagation of water waves to inform hazard assessment. There is much research published in this field for sites in the fjords of Norway, which is unsurprising as Oppikofer et al. (2009) point out, historical and geological evidence indicates one catastrophic rockslide event has occurred approximately every 100 years in the Storfjord in Western Norway alone. Shifting hemispheres, and focusing on the fjords of Chilean Patagonia, I was surprised that water waves triggered by landslides were not more commonly reported given the extensive network of fjords containing steep slopes of often fractured bedrock and volcanic surficial deposits. Landslides may be occurring out of sight of coastal population centres, historic events may not be documented due to the relatively recent mass settlement of the region (following the 1902 Cordillera of the Andes Boundary Case), or the physical conditions for occurrence of a landslide of sufficient magnitude and mechanism to locally generate a displacement wave or tsunami, may not be often realised. With the exception of the landslides triggered by the Aysén (Aisén) earthquake on 21 April 2007, and a local testimony from Puerto Aysén regarding landslides and waves generated by an earthquake in November 1927 (Naranjo et al., 2009), very little is published on landslide-induced tsunamis/displacement waves in Chilean Patagonia. Of course, there is also the possibility that events and historical studies reported in Spanish have not been identified, and I welcome contributions from readers highlighting these.
The Mentirosa Island Landslide complex, Chilean Patagonia
My last blog post discussed the Punta Cola rock avalanche, providing some detail on Aysén (Aisén) fjord and the 2007 earthquake on the Liquiñe-Ofqui Fault zone (LOFZ). The Mentirosa Island landslide complex is composed of four soil-rock slides from the north-eastern slopes of Aysén fjord, with a total estimated volume of 8 million m3 (Sepúlveda & Serey, 2009). It is located less than 5 km from the earthquake epicentre. Research on this landslide is ongoing by Sergio Sepúlveda and others at the University of Chile; observations here are preliminary from fieldwork in January 2016.
All four soil-rock slides failed from near the top of the hillside, suggesting the topographic amplification of seismic waves. Scar A appears to have derived from two smaller rock wedge failures, which have coalesced to form a debris fan at the slope base. Inspection of scar of B indicates the rock mass failed as a simple joint-controlled wedge, sliding downslope and disintegrating rapidly. The scar contains significant amounts of loose debris, stored on lower gradient rock ledges, showing the persistence of rockfall activity. Scar C has a slightly more complex geometry suggesting failure derived from two main wedges of rock; the translational movement is believed to be similar to B. The steeper geometry of the scar means debris is transported rapidly from rockslope to debris fan downslope. Large masses of highly fractured rock are exposed within the scar, highlighting the potential of future block failure. Landslide D is likely to have failed as a series of blocks; mineral veins are evident within the scar and the rock is highly fractured.
After detachment and disintegration it is possible that material from each soil-rock slide travelled downslope as a rock avalanche/ debris flow (depending on slope saturation) before impacting the fjord water surface. Fan deposits at the base of slide B show some inverse grading and larger boulders (~0.5 m) are located close to the surface. Material is predominantly angular gravels and boulders, and there is evidence for post-2007 rockfall and sediment-water flows (incision and cross-cutting channels) on the surface of the debris fan.
The impact of two of the landslides into the fjord were photographed by F. Olivera from a research vessel on the water at the time. These excellent photographs are published in Narangio et al.’s (2009) paper, which is free to view. Interpretation of the photographs suggests one of the landslides to the east of the Mentirosa Island complex impacted the water first (likely landslide B). The impact generated a displacement wave, which ran up the eastern coast of the Mentirosa Island. A second displacement wave generated by slide C impacted the island within 20 seconds of the first. The waves removed soil and vegetation from the lower <65 m of the eastern slopes of Mentirosa Island, and propagated into the fjord, increasing in amplitude in shallower waters, resulting in runouts of several hundreds of meters on coastal areas around Puerto Aysén. Landslides which triggered displacement waves and tsunamis were within 5 km of the epicentre of the 2007 Aysén earthquake. Seismic activity associated with the LOFZ poses a significant hazard to coastal communities in the Aysén region of Chilean Patagonia. The landslide and tsunami hazard for other towns such as Hornopirén, is being assessed using landslide inventories from the 2007 event and laboratory testing of slope materials, by members of the Newton Fund/NERC project on earthquake-induced landslides.
Baathan, A., Blikra, L.H., Berg, S.S. & Karlsen, F. (2004) Rock-slope failures in Norway; type, geometry, deformation mechanisms and stability, Norwegian Journal of Geology, 84 (1), pp.67-88
Hermanns, R.L., Oppikofer, T. Roberts, N.J. & Sandøy, G. (2014) Catalogue of Historical Displacement Waves and Landslide-Triggered Tsunamis in Norway (Chapter 13), in: Lollino, G., Manconi, A., Locat, J., Huang, Y., Artigas, M.C. (eds.) Engineering geology for Society and Territory- Volume 4: Marine and Coastal Processes, Springer, 235 pages
Naranjo, J.A., Arenas, M., Clavero, J., Muñoz, O. (2009) Mass movement-induced tsunamis: main effects during the Patagonian Fjordland seismic crisis in Aisén (45°25’S), Chile, Andean Geology, 36 (1), pp. 137-145
Oppikofer, T., Jaboyedoff, M., Blikra, L., Derron, M.-H., Metzger, R. (2009) Characterization and monitoring of the Åknes rockslide using terrestrial laser scanning, Natural Hazards and Earth System Sciences, 9, pp.1003-1019
Sepúlveda, S.A. and Serey, A. (2009) Tsunami-genic, earthquake-triggered rock slope failures during the April 21, 2007, Aisén earthquake, southern Chile (45.5° S). Andean Geology 36: 131-136. Doi: 10.4067/S0718-71062009000100010.
1 February 2016
Images of the Tbilisi Zoo landslide and flood disaster
Last week I posted Google Earth images of the Tbilisi Zoo landslide and flood disaster last year, and suggested that contrary to reports at the time, the losses were not caused by a valley blocking landslide, but rather a more conventional landslide that transitioned into a debris flow and combined with many other smaller events. Since then I have been contacted by Dr Sergey Chernomorets, who is Senior Scientist of the Laboratory of Snow Avalanches and Debris Flows in the Faculty of Geography at Lomonosov Moscow State University in Russia. He visited the site in September 2015 with Prof. Igor Bondyrev from Tbilisi State University and Elena Savernyuk, also from Lomonosov Moscow State University. Sergey confirms that the major cause of the disaster was the landslide at Akhaldaba village, and that there is no sign of valley blockage.
He also points out that the disaster at Tbilisi Zoo was a new road junction at Gmirta Moedani (Heroes Square), which dammed the flow.
Sergey has very kindly provided some photographs of the disaster, which I reproduce here with his permission. These two images show the landslide at Akhaldaba that initiated the Tbilisi zoo landslide and flood disaster:
This is the road junction at Gmirta Moedani (Heroes Square) that dammed the flow and caused the terrible flooding that caused the loss of life the Tbilisi landslide and flood disaster:.
This is the site of the zoo that was so profoundly damaged:
And this image is an example of the damage to the buildings in this area of Tbilisi. Note the height of the flood water and sediment on the walls:
28 January 2016
Google Earth imagery of the Tbilisi landslide and flood
On 14th June 2015 a major flood swept through parts of Tbilisi in Georgia, causing the loss of 19 lives. The greatest damage associated with the Tbilisi landslide occurred at Tbilisi zoo – I blogged about this a few days later because the reported cause of the flood was a landslide that had blocked the valley, allowing a lake and then a dam break flood to develop. I noted at the time that there was a lack of clarity about exactly where the landslide that caused the disaster had occurred. Google Earth has now posted imagery of the area collected on 24th June 2015, about ten days after the disaster. This is an overview of the affected area from the south, the outskirts of Tbilisi are on the bottom left of the image:
The most obvious feature here is a major landslide in the upper right portion of the image, with many smaller landslides in the vicinity. The path of the flood along the river is also clear. This is the same location in March 2014:
The major feature is the large landslide that reportedly caused the disaster. This is a substantial feature that appears to be a comparatively shallow but extensive translational rock and soil slide. The main source appears to be in the vicinity of the road that crosses the upper part of the slope:
This landslide is about 1 km long and 300 metres wide at its maximum extend
The pre-landslide image of the same location gives little indication of the potential problems at this site, and there is no obvious sign that the road played a part in the failure:
However, it is not obvious to me from the image that there was a major landslide dam event in this area, at least not associated with this landslide. Downstream there appears to be extensive evidence of shallow landslides and extensive flood scour on the river, but no obvious evidence of a dam and lake.:
Thus, to me on first inspection it appears that the disaster might have been caused by a large debris flow and flood that originated in the landslide shown above, with contributions from many other smaller landslides and channels, rather than a valley blocking event.
27 January 2016
The Punta Cola rock avalanche in Aysén Fjord, Chile
Aysén (Aisén) fjord is located at 45.26° latitude in Zona Austral, Chile (Chilean Patagonia). In common with other mountainous regions in Chile, the landscape is tectonically controlled, however in contrast to valleys such as the Maipo valley in the Central Andes, the coastline of south Chile is dominated by fjords. The fjords were formed during deglaciation, when rising sea levels inundated glacier-carved valleys as the continental ice cap retreated towards the end of the Pleistocene (about 15,000 years ago). Erosion by the ice has left deep fjords (Aysén fjord has a depth greater than 350 m), steep bedrock slopes and ice covered mountain peaks, with lower level hummocky terrain attributed to glacial moraines and landslides. Topography is overlain with shallow volcanic soils and temperate dense virgin forest, which thrives in the wet climate; Puerto Aysén has an average annual rainfall of 2,600 mm although this may reach 4,000 mm in the fjord.
Nestled within a steep drainage on the north side of Aysén fjord is the Punta Cola rock avalanche. Triggered by the Mw 6.2 Aysén earthquake on 21 April 2007 on the Liquiñe-Ofqui Fault zone (LOFZ), this was the largest of a total 538 mapped earthquake-triggered landslides ( Sepúlveda and Serey 2009; Sepúlveda et al., 2010), with an estimated total volume of 22.4 M m³ (Oppikofer et al., 2012).
Without a site visit, the magnitude of this landslide is difficult to comprehend in real terms; the panorama and insert below, illustrate quite how small ‘we’ are and present an intimidating reminder of the extent of damage caused when large landslides occur in densely populated regions.
The Punta Cola rock avalanche deposit is composed of a very large event and seven smaller secondary landslides. The main rockslide was composed of a <135 m thick block of North Patagonian Batholith, which broke into small fragments, sliding into the valley and running up the opposite slope by up to 180 m (Hermanns et al., 2014). The slide transformed into a highly mobile rock avalanche travelling down valley for 1.5 km into Aysén fjord, generating a displacement wave (akin to a tsunami). The rock avalanche accelerated as it descended into the fjord causing a large section of the pre-existing shoreline sediment ramp to collapse. Bathymetric data obtained in March 2013 (see Hermanns et al., 2014) show deformation across the full 3.5 km fjord width, interpreted as the impact of the avalanche as it slammed into sediments on the fjord floor, generating up to 10 m of vertical erosion and deformation structures, including concentric compressional ridges and large (< 70 m) sediment fragments.
Oppikofer et al. (2012) used terrestrial LiDAR to create a high resolution topographic model (DEM) of the landslide scar and deposit, from which an estimate of landslide volume was obtained, and geomorphological units were mapped. They hypothesised that failure was initiated at compartment A (see below), triggered by the 2007 earthquake, which led to the sequential failure of compartments B and C shortly after the main event. Material from compartments A and B was thought to have contributed to the rock avalanche, while compartment C was likely to have deposited at the base of the slope, possibly temporarily damming the river valley.
The evolution of the Punta Cola landslide scar and deposit are being monitored by a team from NGU, headed by Reginald Hermanns, in collaboration with the University of Chile. The team repeated the terrestrial LiDAR survey earlier this month, and will be comparing the new data with their previous survey (from 2010) to identify recent landslide activity and measure changes in the morphology of the debris fan.
Large sections of slope eroded by the 2007 rock avalanche run-out are now revegetated, primarily by Gunnera Tinctoria (Chilean rhubarb), which seems to be the main colonizing plant on disturbed areas around the fjord. Debris slides and rockfall persist in the valley, primarily from unvegetated sections of loose boulders near the valley floor and from within the main and secondary bedrock scars, which are composed of highly fractured rock. Comparison to photographs in the Oppikofer et al. (2012) paper show that over 10 m of rock avalanche deposit has been remobilised, highlighting the transport capacity of water-sediment flows over just five years, in sediment-charged mountainous environments.
The displacement wave from this landslide and a second wave from the Metirosa landslide (see Sepúlveda et al., 2010), caused significant damage to salmon farms in the fjord and to infrastructure on the shoreline (with run-ups of several tens of metres). Ten people were killed by these waves and debris flows associated with the earthquake. Operations in the fjord including fishing and tourism are of great economic value to the local community. Critically, the April 21 2007 Aysén earthquake is the first seismic event to be directly attributed to the LOFZ. Ongoing research is focused on the seismic and landslide hazard posed by future earthquakes on the LOFZ.
Hermmans, R.L., et al., (2014) Ch 14: Earthquake-triggered subaerial landslides that caused large scale fjord sediment deformation: combined subaerial and submarine studies of the 2007 Aysén fjord event, Chile. In: Lollino, G. et al. (eds.) Engineering Geology for Society and Territory – Volume 4. Doi: 10.1007/978-3-310-08660-6_14, Springer International Publishing, Switzerland
Oppikofer, T., Hermanns, R.L., Redfield, T., Sepúlveda, S.A., Duhart, P., Bascuñan, I. (2012) Morphologic description of the Punta Cola rock avalanche and associated minor rockslides caused by the 21 April 2007 Aysén earthquake (Patagonia, southern Chile). Revista Asociación Geológica Argentina 69:339–353.
Sepúlveda, S.A. and Serey, A. (2009) Tsunami-genic, earthquake-triggered rock slope failures during the April 21, 2007, Aisén earthquake, southern Chile (45.5° S). Andean Geology 36: 131-136. Doi: 10.4067/S0718-71062009000100010.
Sepúlveda, S.A., Serey, A., Lara, M., Pavez, A. and Rebolledo, S. (2010) Landslides induced by the April 2007 Aysén fjord earthquake, Chilean Patagonia. Landslides 7: 483-492. Doi: 10.1007/s10346-010-0203-2.
24 January 2016
The greatest ever debris flow video? Aconcagua
This one, on the flanks of Aconcagua in Argentina, is quite spectacular. It starts slowly, but hang in there!
The original text accompanying the video is in Spanish, a tidied up Google Translation version is as follows:
“Avalanche between Horcones (park entrance) and Confluence (first base camp of the Aconcagua field). Julian Insarralde [who posted the video], Nico Aguero and Naco Choulet were working for INOUT ADVENTURE. During a trek lasting three days. We are going to customers to avoid them being splashed with mud as it is an area of avalanches at that time of year. The warning was a sound similar to an airplane sound, which is why Julian Insarralde is looking back and is able to warn that an avalanche is coming. That’s why we ran and we did not abandon people so that we were in the safe zone. They are things that can happen when we work in real natural environments”.
This is the moment that the debris flow arrives:-
This is a classic debris flow – the front end is almost entirely dry (note the dust in the image above) and mostly large boulders. The tail of the debris flow has more water and finer material. Note that the debris flow goes through a series of surges.
The weather appears to be dry and sunny. The very small debris flow that the hikers are crossing at the start is also quite intriguing. Even this appears to be debris rich. I wonder if this is the tail of an earlier surge? The walls of the gully appear to be wet?
Its a pretty good job that the trekkers heard the debris flow coming, and respect to everyone for getting out of the way, despite a couple of slips and trips.
22 January 2016
The Punta Tre Amici rockslide
On 16th December 2015 the Punta Tre Amici rockslide occurred on a rock slope on the flanks of Monte Rosa in northern Italy. The rockslide, which had a volume of about 200,000 cubic metres, was detected on a network of three seismometers, indicating that the main collapse occurred at about 8:25 pm local time. There is a very detailed description (in Italian), and lots of images, of this landslide on the Nimbus Web Glaciologia website. Google Translate does a fine job on the text, which is rendered quite clear. This is a post-event image of the Punta Tre Amici rockslide:-
The rockslide left the cliff in a quasi-stable state, and it has subsequently been subjected to repeated subsequent rockfall events (and avalanches, as the image above shows). This means that the rockfall scar and deposit has continued to evolve with time. Nimbus Web Glaciogia has this image of the landslide, taken on 8th January:
The Nimbus Web Glaciologia article indicates that this is an area that has been subject to considerable slope instability in recent years. It also indicates that there might be an interesting temperature-related aspect to this rockslide. They provide this graph of temperatures in the days leading up to the rockfall event:
The Nimbus Web Glaciologia (translated by Google and then corrected by me) notes that:
The late fall 2015 in the Alps has been marked by persistent subtropical anticyclones, therefore exceptional mildness and lack of snowfall. In particular, a period of extreme warmth was observed in the days leading up to 10th November 10, with extremes never previously recorded at that time of year, with temperatures of about 7 ° C at about 3000 m in the free atmosphere above the Northern Italy and Monte Rosa, therefore with [ground temperature] well above 0° C and the likely presence of liquid water still in circulation to the portion of the detachment of the landslide (at about 3400 m). In late November, there was a temporary cold period (-15 ° C at 3000 m)…followed by milder weeks in December, but without reaching the extremes of the previous month, while there was a continued extraordinary lack of snow, limited to a few centimeters on the shady slopes.
It is well established that global warming is driving an increase in high elevation temperatures, and a dramatic increase in permafrost degradation and thus an increase in rockfalls, in mountain chains. This may well be yet another example. We will see many more in the years ahead.
21 January 2016
The Meson Alto rock avalanche
A few kilometres upstream from the Las Cortaderas rock avalanche lies one of the truly great landslides, the Meson Alto rock avalanche. This landslide is so large – 4.5 cubic kilometres, which is more than 10 billion tonnes of rock – that it can only really be shown properly on a satellite image:
I have annotated the Google Earth image below to show the main features of the landslide. Note that the boundaries are approximate, particularly on the downstream part of the deposit, where there may be a combination of rock avalanche deposits and redeposited landslide material from the outburst flood:
The landslide scar is very clear, as is the huge landslide deposit. Note the way that it has spread downstream. The rock avalanche is described in detail in a great paper, available online, by Deckart et al. (2014). They point out that this is a rock avalanche that has been deposited on top of glacial moraine. The landslide is believed to have occurred about 4,500 years ago. This is the landslide from downstream:
Note the fluvial (river) terraces between the camera and the deposit in the background. These may be a combination of the lake deposits from the Las Cortederas landslide and the flood terraces from the breach of the Meson Alto rock avalanche. This is the artificial dam that has been built to create a lake at the site of a previous natural lake impounded by the landslide. At some point the natural lake must have drained through a substantial breach event:
The landslide deposit is the huge pile of debris in the background, Note how large the dam is in this image – compare that with the Google Earth image above. This gives an idea of the scale of this landslide. Capturing the scale of the landslide is extremely difficult. This is an attempt at a composite image to take in all of the landslide features, taken from the banks of the lake:
Part of the landslide scar is on the left of the image, and the landslide extends across the image to the rock slope on the right side. The dam blocks the space between the mound of debris and the rock wall on the left side of the image.
Deckart, K., Pinochet, K., Sepulveda, S., Pinto, L., and Moreiras, S. 2014. New insights on the origin of the Meson Alto deposit, Yeso Valley, central Chile: A composite deposit of glacial and landslide processes? Andean Geology 41 (1): 248-258/
20 January 2016
Landslides in Chile Part 2: Las Cortaderas Landslide
Located not far from Santiago, in the upper reaches of the Maipo River catchment, is a very large mass movement, known as the Las Cortaderas Landslide. This is a fascinating landslide, not least because there are two distinct phases. The main landslide is vast – an ancient rock avalanche that entered the valley at some point in the past (of which more below). The frontal portion of this landslide deposit then failed again in the 1958 Las Melosas earthquake, blocking the Yeso River. This makes the later landslide sound like a small event – it is certainly not. Sepulveda et al. (2008) estimated that it had a volume of 15 to 20 million cubic metres, which is a large landslide by any standard. The river was blocked for a few years before breaching naturally. This is a view of the landslide from upstream:
To illustrate the features of the lower part of the landslide, I have annotated the photograph below:
This image shows the landslide scar with the deposit on the right hand side (the scar is just to the left of the power cables):
But, as mentioned above, the 1958 La Cortaderas landslide is a comparatively small failure of the deposit of a much larger landslide body. This is the Google Earth image of the site – I have annotated the 1958 landslide and the larger rock avalanche deposit from which it originated:-
Taking a wider view though, the scale of the ancient rock avalanche becomes clear:
This is a truly huge landslide that would have blocked the river before breaching. The current river cuts through the landslide deposit, leaving a portion of the deposit on the other side of the valley. This is clearly exposed by the river. Note the chaotic landslide deposit, with the predominance of the largest boulders occurring in the upper portions:-
But if you think this is large, you should see what lies upstream – of which more tomorrow.