Submarine landslides and formation of the East African Rift System

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The risk of landslides in Africa

The most widespread risk from natural hazards is, with little doubt, that posed by ground instability; landslides and landslips; mudflows; rock avalanches and a range of other categories in which large volumes of surface material are set in motion. They can be triggered by earthquakes, volcanism or heavy rainfall that changes the physical properties of rock and soil. Not only steep slopes pose a risk, for some affect ground with quite gentle topography, as witness the terrible scenes from Sulawesi triggered by the 28 September 2018 magnitude 7.5 earthquake beneath the Minhasa Peninsula. This set in motion mudflows on gently sloping ground when the seismic waves caused liquefaction of unconsolidated sediments that not only shattered dwellings by the lateral motion, but whole communities sank into the slurry with little trace. The rapid events left a death toll confirmed at 2010 people with about 5000 missing, feared to have been swallowed by the earth. In the last 9 months mass movement has resulted in fatalities in many places, the most publicised being in Uganda, Japan, Philippines, Sulawesi, Ethiopia, Sumatra, South India, Bangladesh, California, Nepal, and the list grows as it does every year.

Types of mass movement (Credit: US Geological Survey

As well as purely natural causes, human activities, such as deforestation, excavations and dumping of materials, greatly exacerbate risks. The South Wales former coal-mining communities commemorate every year the collapse of a mine spoil heap on a steep hillside on 21 October 1966 that engulfed a primary school at Aberfan, killing 116 small children and 28 adults. Wherever they occur, there seems to be little chance of escape for those in their path. Slowly it has become possible for geoscientists to outline areas that are potentially at risk from catastrophic mass wastage, sometimes from the distribution of scars of previous events on remotely sensed images, but increasingly by multivariate analysis of landscapes in terms of the factors that may contribute to future ground failures. The principal ones are: topographic slope and relief; annual rainfall, especially the likely precipitation in a single day; vegetation cover, particularly by trees; strength of surface rock and soils, including degrees of consolidation, interbedding and water content; geological structure, such as the trajectory of faults, degree of  jointing and the dip of strata. Modelling risk has to grapple with the global scale of the problem, which cannot be addressed in the least developed regions by piecemeal local studies, although those are urgent, of course, in areas with recorded instances of catastrophic ground failure. Regional studies can screen vast areas of probably low risk so that meagre resources can focus on those that appear to be most dangerous to populated places.

Degree of risk from landslides of all types in the northern part of the East African Rift System (Credit: Broeckx et al. 2018; Fig. 6)

Belgian engineering geologists and GIS specialists have assembled a monumental risk assessment of Africa, together with a bibliography of all published work on mass movement across the continent (Broeckx, J. et al. 2018. A data-based landslide susceptibility map of Africa. Earth-Science Reviews, v. 185, p. 102-121; DOI: 10.1016/j.earscirev.2018.05.002). They point out that Google Earth’s 3-D viewing potential at fine spatial resolution provides a free and rapid means of mapping scars of previous earth movements in considerable detail over areas that data analysis suggests to be susceptible. Their paper provides continent-scale maps of the parameters that they used as well as maps showing several versions of their risk analysis. The supplementary data to the paper include downloadable, full-resolution maps of landslide susceptibility.

Earthquakes in Nepal

The magnitude 7.8 Gorkha earthquake hit much of the Himalayan state of Nepal on 25 April 2015, to be followed by one of magnitude 7.3 150 km to the east 18 days later. As would have happened in any high-relief area both events triggered a huge number of landslides as well as toppling buildings, killing almost 9000 people and leaving 22 000 injured in the capital Kathmandu and about 30 rural administrative districts. Relief and reconstruction remain hindered 9 months on in many of the smaller villages because they are accessible only by footpaths. Nepal had remained free of devastating earthquakes for almost 6 centuries, highlighting the perils of long quiescence in active plate-boundary areas.

Damage in Kathmandu, Nepal, after the Gorkha earthquake in May 2015 (Credit: CNN)

The International Charter: Space and Major Disasters consortium of many national space agencies was activated, resulting in one of the largest ever volumes of satellite images ranging from 30 to 1 m resolution to be captured and made freely available for relief direction, analysis and documentation. This allowed more than 7500 volunteers to engage in ‘crowd mapping’ coordinated by the Humanitarian OpenStreetMap Team (HOT) to provide logistic support to the Nepal government, UN Agencies and other international organizations who were swiftly responding with humanitarian relief. Most important was the location of damaged areas using ‘before-after’ analysis and assessing possible routes to remote areas. The US NASA and British Geological Survey with Durham University coordinated a multinational effort by geoscientists to document the geological, geophysical and geomorphological factors behind the mass movement of debris in landslides etc that was triggered by the earthquakes, results from which have just appeared (Kargel, J.S. and 63 others 2016. Geomorphic and geological controls of geohazards induced by Nepal’s 2015 Gorkha earthquake. Science, v. 351, p. 140 – full text purchase).

The large team mapped 4312 new landslides and inspected almost 500 glacial lakes for damage, only 9 had visible damage but none of them showing signs of outbursts. As any civil engineer might have predicted, landslides were concentrated in areas with slopes exceeding 30° coincided with high ground acceleration due to the shaking effect of earthquakes. Ground acceleration can only be assessed from the actual seismogram records of the earthquakes, though slope angle is easily mapped using existing digital elevation data (e.g. SRTM). It should be possible to model landslide susceptibility to some extent over large areas by simulation of ground shaking based on various combinations of seismic magnitude and epicenter depth modulated by maps of bedrock and colluvium on valley sides as well as from after-the-event surveys. The main control over distribution of landslides seems to have been the actual fault mechanism involved in the earthquake, assessed from satellite radar interferometry, with the greatest number and density being on the downthrow side (up to 0.82 m surface drop): the uplifted area (up to 1.13 m) had barely any debris movements. Damage lies above deep zones where brittle deformation probably takes place leading to sudden discrete faults, but is less widespread above deep zones of plastic deformation.

The geoscientific information gleaned from the Gorkha earthquake’s effects will no doubt help in assessing risky areas elsewhere in the Himalayan region. Yet so too will steady lithological and structural mapping of this still poorly understood and largely remote area. As regards the number of lives saved, one has to bear in mind that few people buried by landslides and collapsed buildings survive longer than a few days. It seems that rapid response by geospatial data analysts to the logistics of relief and escape has more chance of positive humanitarian outcomes.

In the same issue of Science appears another article on Nepalese seismicity, but events of the 12^th to 14^th centuries CE (Schwanghart, W. and 10 others 2016. Repeated catastrophic valley infill following medieval earthquakes in the Nepal Himalaya. Science, v. 351, p. 147-150). As the title suggests, this relates to recent geology beneath a valley floor in which Nepal’s second city Pokhara is located. It lies immediately to the south of the 8000 m Annapurna massif, about 50 km west of the Gorkha epicentre. Sections through the upper valley sediments reveal successive debris accumulations on scales that dwarf those moved in the 2015 landslides. Dating (¹⁴C) of interlayered organic materials match three recorded earthquakes in 1100, 1255 and 1344 CE, each estimated to have been of magnitude 8 or above. The debris is dominated by carbonate rocks that probably came from the Annapurna massif some 60 km distant. They contain evidence of extreme pulverisation and occur in a series of interbeds some fine others dominated by clasts. The likelihood is that these are evidence of mass movement of a more extreme category than landslides and rockfalls: catastrophic debris flows or rock-ice avalanches involving, in total, 4 to 5 km³ of material.

Related articles

• Bigger Earthquakes May Be Coming to Nepal
• Fewer landslides than expected after 2015 Nepal earthquake