32 Geomorphic Hazards

Daoud Firdausi and Dr. S. Fazal

    Objectives

To understand Geomorphic Hazards, Earthquakes, Landslides, Tsunami and Avalanche, Earthquake and Plate Tectonics, Risk Reduction Measures

Keywords

Endogenous and Exogenous, Hazards, Risk Reduction

Natural hazards are threatening events that can cause widespread damage to life and property. They have a long term consequence and their continuous impact can change or modify both physical and social space. In this section we discuss the hazards that are intrinsically related to geomorphology as they are integral elements of our dynamic earth. The endogenous geomorphic hazards like earthquakes and exogenous geomorphic hazards such as tsunamis, landslides and avalanches are discussed in detail.

Earthquakes

Earthquakes are one of the most devastating and destructive natural disasters experienced by an individual and the society. They occur without warning in different areas throughout the world.

Earthquakes can cause maximum damage and deaths in densely populated areas. Earthquakes are the result of vibrations generated by sudden movements and ruptures in the rocks that is being stressed beyond its elastic limits. These stresses can be natural or man-made. The intensity of an earthquake can vary from a faint tremor to wild shaking of the ground. The point at which earthquake is generated is called as the focus or hypocenter. The point located just above the focus is called as epicenter. Near the epicenter of the quake, the impact is direct, resulting in immediate damage such as collapse and destruction of buildings and other infrastructure, the area near epicenter then experience secondary or indirect impact such as fires and landslides. Usually the depth of the focus has been traced between 10-700 Km from the surface.

Fig. 1.1. Focus & Epicenter

Earthquakes occur in a sequence of shocks. The largest earthquake in a sequence is called as Mainshock. It is preceded by one or more Foreshocks and followed by Aftershocks. The smaller earthquake that occurs after the large one is designated as Aftershocks. They are the result of the adjustment of ‘fault plane’ formed by the displacement of the part of the earth crust after the occurrence of Main shock. Aftershocks are very dangerous because they are usually unpredictable can be of large intensity. It can raze buildings to ground already damaged by the Mainshock. Scientific study of earthquake is known as Seismology (Seismos in Greek language means earthquakes). Therefore, the waves generated by the earthquakes are also called as seismic waves. Mainly there are three kinds of seismic waves, Primary, Secondary and Surface waves.

    Causes of Earthquakes

Earthquakes are mainly caused due to the disturbance of equilibrium in any part of the earth crust. Such happenings may be the result of diastrophic forces or sudden movements. It may sometimes be caused by expansion and contraction of surface area due to hydrostatic pressure produced by human-made reservoirs and water bodies. Following are the major causes:

Plate Tectonics: The theory of plate-tectonics provides best possible explanation for the occurrences of earthquakes. Earth crust is made up of solid and moving plates. These plates can be oceanic or continental. There are seven major and twenty minor plates that are constantly movingunder the influence of thermal convective currents originating deep within the earth. It is the interaction at the margins or edges of earth’s tectonic plates that generates World’s most earthquakes and volcanoes. There are mainly three types of boundaries where the plates interact – convergent, divergent and transform fault.

Fig. 1.2: Different Plate Interactions

Convergent Boundaries: is the area where two tectonic plates collide with each other. In case of the collision between two oceanic plates, one of the plate which is older, larger and heavier is forced to be subducted under the lighter plate. The deepest trench, Mariana lies above the place where Pacific plate is being subducted under the Philippines plate. In case where one oceanic plate collides with continental plate, usually the denser plate is subducted under the lighter plate. Nazca plate is subducted under South American plate near Peru-Chile trench. When edges of colliding boundaries are continental crust, it results in the formation of huge Mountains such as Himalayas. All these plate interaction may generate shallow to deep focus earthquakes. Just beneath volcanic arcs and continental margins lies subduction zone also known as Wadati-Benioff zone. The Benioff zone is major zone of earthquake activity because of the thrust faulting of two plates over a wider area.

Divergent Boundaries: This margin is characterised by moderate earthquakes, where two plates are moving in opposite direction. It results into rupture of the crust and the formation of mid-oceanic ridges. Shallow focus earthquakes (0-70 km)are mostly produced near the constructive plate boundaries.

Transform Fault:

The area where plates slide past each other in opposite directions. Most famous transform fault is the San Andreas Fault situated in California, USA. Here, the southeast moving North American Plate meets the northwest moving Pacific plate.As the two plates brushes each other, stick together and slip along transform fault, earthquakes frequently occur in the surrounding regions.

Fig. 1.3:  San Andreas Fault, California, USA

Elastic Rebound Theory and Faulting: Sudden displacement and slippage of rocks take place due to tensile and compressive forces lead to the creation of faulting. It can also trigger tremors due to re-adjustment of rock blocks. According to H.F. Reid (1960) Earthquakes are linked to elastic rebound of previously stored elastic stress. If a rubber is stretched or broken, the elastic energy that was stored during the stretching process in released suddenly. Likewise, the earth crust can also store elastic stress that can be released during an earthquake.

Fig. 1.4: Elastic Rebound Theory

Vulcanicity:Volcanic activity and earthquakes are intimately related to each other. They have cause and effect relationship with each other. The eruption of Krakatoa volcano in Sumatra produced severe earthquake that was experienced 12000 km away.

Hydrostatic Causes and Anthropogenic Activities: Most prominent is Reservoir Induced Seismicity due to the disequilibrium in already isostatically adjusted rocks below the reservoir. For examplethe earthquake of Koyna (Maharashtra, India) 1967, Hoover Dam (USA) 1936 and Marathon Dam (Greece), 1931.

Magnitude and Measurement of Earthquakes

It is commonly measured on Richter scale. The scale is based on the energy release at the earthquake centre. Therefore, it measures the severity of particular earthquake. The intensity of the earthquake is measured at a scale of 1 to 10. On the logarithmic scale each whole number represents 32 times more energy. On the other hand Mercalli Scale is based on power of destruction and severity of earthquake remembered and felt by humans. It measures the damage caused to buildings, dams, bridges and other infrastructures.

Fig. 1.5. Richter scale                                        Fig. 1.6. Modified Mercalli Scale

Global Distribution of Earthquakes

The location of World’s earthquakes presents a striking pattern. Occurrences of earthquakes are concentrated in certain areas and stretches across the World. Area with high volcanic activity and earthquake is called as “Pacific Ring of Fire” or the Circum-Pacific Belt. It is the zone of plate collision, subduction and young fold mountains. It includes North America, South America, East Asia coastal areas and Island arcs. The 2004 Sumatra, 1964 Alaska and 1960 Chile earthquakes occurred in subduction zone connected to Pacific Ring of Fire.

Fig. 1.7

    Mid-Continental or Alpine-Himalayan belt is also major earthquake prone area of the World.

It is particularly characterised by collision and subduction of continental plates (or part of plates). It consists of Alpine Europe, Mediterranean Sea, North Africa, Himalaya and Burma.

    The belt contains weaker zones of folded mountains, where fault-generated earthquakes are very common. For exampleChamoli earthquake of 1991, massive earthquake of Pakistan in 2005 due to the convergence between Indian and Eurasian plate. Earthquakes also occur in areas of transform fault such as California, East African Rift Valley and Mid-oceanic ridges. The 1906 San Francisco earthquake has been found to be linked with 1820 km long San Andreas Fault.

Earthquake Hazard

Earthquakes are associated with variety of specific hazards. Some are characterised as primary hazards such as:

i. Ground Motion: when the seismic waves travels through populated area, ground motion is felt as shaking. The destruction linked to ground motion depends upon the design and construction of buildings.

ii. Ground Breaking: It includes wide opening in the ground due to earthquakes. These ground breaks may have vertical, horizontal or combined displacements.

iii. Mass Wasting: It may trigger the downhill movement of material lying on the slope. It can range from gradual creep to rolling of large blocks of rocks. Earthquake may induce landslides and avalanches on steep slopes.

iv. Liquefaction:It is a process where sudden and intense vibrations and shaking converts certain types of sands and muds into a slurry or a substance with a consistency of liquid.

v. Changes in Ground Level: Due to earthquakes sometimes blocks of earth shift relative to one another. It may lead to changes in ground level, base level and water table.

Other secondary and tertiary hazards associated with earthquakes are:

Tsunamis

Seiche waves

Fires and Explosion

Displacement of People

Loss of jobs and livelihood

Earthquake Prediction and Risk Reduction

It  is  very  difficult  to  predict  earthquakes,  no  definite  means  of  predicting  earthquakes  are available.  However,  it  can  be  predicted  indirectly  by  analysing  unusual  animal  behaviors, studying hydro-chemical pressures and increase in turbidity. Seismic in many parts of the World are monitored using sensors, global positioning systems and satellite technology. Earthquakes cannot be stopped but risk can be reduced through preventive measures. These measures include building earthquake-proof or resistant structures and safer homes. Spreading awareness, conducting earthquake preparedness drills and capacity building for emergency situations are some of the significant solutions.

Tsunami

Tsunami is a wave, or series of waves produced by sudden vertical displacement of the column of water. The displacement may be caused by seismic activity, volcanic eruption and a landslide above or below water. Tsunami waves are sometimes also referred to as tidal waves due to its long wavelength. However, it is not related to the attraction of sun and moon. Tsunami waves are generated in oceans, bays and other water bodies. The word Tsunami comes from Japanese Tsu (harbour) and Nami(waves) because it mainly affects coastal areas and harbour. In 1990s, around 14 Tsunami events occurred throughout the World, it did not caused much death and destruction but Tsunami of 26, December, 2004 perturbed the entire world. It struck due to the largest underwater earthquake ever recorded off-the coast of Northern Indonesia. It generated devastative tsunami that swept the northern Indian Ocean and killed thousands of people who never anticipated such event.

Causes of Tsunami

Earthquakes: It is the most common cause behind the origin of tsunami. Over 80 per cent of all tsunamis occurred in Pacific oceans were generated due to seismic activity. When the earth crust is displaced by several metres during under water earthquake, it covers thousands of square km area and induces tremendous potential energy to the water body. Tsunami can only be triggered by earthquakes that originating mainly in the upper 100 km of the oceanic crust. It has been found that earthquake-induced tsunami is associated with the MsMagnitude of 7.0 or greater on the Richter scale. Most tsunami forming earthquakes are shallow foci and occur at a depth of 0-70 km. The greater the vertical displacement, the greater the amplitude of tsunami, therefore thrust fault associated with subduction zones are preferred mechanism for the generation of tsunami.

Landslides: The topography along continental shelf and margins are often very steep, particularly near ocean trenches. Sediments lying over the continental shelf moves under gravity down the slope, generating marine landslides. It can form small to mega tsunamis, whose magnitude may even surpass than those generated by earthquakes. The most notable example is Grand Bank Tsunami (1929).

Volcanic Eruptions: The contribution of volcanic eruption in generation of tsunami is relatively lesser (4.6%) than seismic events and underwater landslides. Explosive volcanism with caldera formation can cause tsunami, it is mainly limited to few areas such as Japanese- Kuril Islands and Philippine and Indonesia archipelagos.

Comets and Asteroids: Any asteroid and comet entering into atmosphere at a shallow angle is more likely to reach the ocean without breaking, can create cavity that would be ten times greater than the diameter of the objects. It can generate waves in different direction that may result into tsunami.

Mechanism and Propagation of Tsunami

When an earthquake struck undersea, the vertical displacement of seafloor also displaces overlying water. All the water rushes towards the point of displacement to fill the depression created by downthrown region, as a result water recedes from the shore. Once the surface rocks are adjusted all water rapidly moves towards the shore forming tidal waves. If the seafloor is lifted up, then it will create a hill that would collapse eventually and sends waves travelling towards shore leading to tsunami.

    In open sea, the tsunami waves propagates fast, low and long wavelength, although they resemble similar to other waves of the sea. Tsunami, travel across the open sea a series of long waves with low crest (1-2 m high) (people on ship would not able to detect the deadly tsunami passing below them). Tsunami does not loose energy during its travel to the shore. When tsunami reaches close to the shore, it enters shallow waters, it slows down, portion of wave closest to the coast or beach slow down but its back maintains fast speed forming higher and steeper waves. The tsunami produced by Krakatoa explosion caused tsunami with a wave height of 98 feet (30 metres).

Tsunami Hazard

The regions were tsunami occurs frequently are Pacific ring of Fire, Mediterranean Sea, Caribbean Region and Indian Ocean. Following are the major hazards associated with tsunamis.

Inundation: tsunami waves are able to push lot of water onto the shore, leading to flood like situation.

Destruction and Damage to property: It destroys anything in its path such as boats, buildings, houses, telephone lines and other infrastructure.

Death and Fatalities: One of the worst impacts of tsunami is loss of human life. December, 2004 Indian Ocean Tsunami caused death to 30, 974 people in Sri Lanka, 122, 232 in Indonesia, 6400 in India and 5395 in Thailand.

Tsunami

Fig. 1.9 Indian Ocean Tsunami

Fires and Explosions: It causes damage to oil and natural gas storage and pipelines resulting into intense fires and explosions. In Japan, due to tsunami of 2011 various oil tankers at ports and gas cylinders at industrial complexes were damaged and caused massive fire and explosions. Fukushima Daiichi nuclear power plant, 150 miles northeast of Tokyo, was severely damaged by the earthquake and tsunami impairing its cooling systems resulting in a series of explosions, meltdowns – and the world’s worst nuclear accident in 25 years.

Fig. 1.11: Radiation clearance at Fukushima, Japan (Post-tsunami event)

Monetary loss, Diseases and Psychological Problems: It also causes lot of monetary loss to individuals, family and government. The victims may also suffer from various diseases due to stagnant water and decomposing dead bodies of humans and animals. Many people also tend to develop psychological problems after the event.

Risk Reduction

The most important aspect of tsunami preparedness is its detection and early warning. Indian Ocean tsunami compelled world scientist to develop widespread tsunami warning system. Following activities were undertaken by the international community to develop early warning system:

Seismic stations started collecting data on undersea earthquake and transmit it to the monitoring centres such as Pacific Tsunami Warning System (PTWS) situated at Hawaii.

Tsunami watch is released for regions which are going to be affected later by the tsunami event.

Tide gauges were monitored to detect the changes in the waves.

NOAA (US National Oceanic and Atmospheric Administration) developed bottom pressure sensors to measure the wave characteristics and pressure changes.

NOAA through DART (Deep Ocean Assessment and Reporting of Tsunami) using sea surface Buoys transmits signals to satellite, which transfer data to shore-based Regional Warning Systems situated in Pacific and Indian ocean.

Other measure for risk reduction includes:

Site planning and management – designating or zoning tsunami prone areas and change the landuse accordingly.

Constructions of structures and coastal homes at higher elevations. Water breakers to minimize velocity of waves

Construction of community halls and shelters.

Landslides

The term “landslide” describes wide variety of processes that results in the downward and outward movement of slope forming materials including rocks, soil, artificial fill or combination of these (USGS). The areas of World prone to landslides are mountain and hills, particularly deforested mountainside, areas with coarse-grained soil or lack of vegetation. Several studies have shown that more than 12 per cent of the land area of India is susceptible to landslides. The major landslide prone areas of India includes Western Ghats (Nilgiris) Konkan region (Tamil Nadu, Karnataka, Kerala, Maharashtra and Goa), Eastern Ghats (Araku region of Andhra Pradesh), Eastern Himalayas (Darjeeling, Sikkim and Arunachal Pradesh), North-west Himalayas (Uttrakhand, Himachal Pradesh and Jammu & Kashmir). Landslides have been declared as third most fatal disasters in the World. In world, around 300 people die every year due to landslides and $400 billion are annually spent on landslide mitigation and disaster management.

Types of Landslides (USGS)

Slides: It is a type of mass movement where the sliding material detaches or breaks from the underlying stable material. It can be divided into rotational slides and translational slides.In rotational slide the movement is rotational, and its axis is parallel to ground surface and transverse across the slide. Translational slides do not show any rotation, for example block slide, where single unit slides as a coherent mass.

Falls: When rock, soil and debris break away from cliffs and slopes and start moving suddenly.

It may be the result of earthquake, weathering or gravity.

Flows:It is divided into five basic categories that vary from each other in fundamental manner.

Debris Flow:Rapid mass movements of combination of loose soil, organic matter, water that flow downslope.

Debris Avalanche:It is a type of extremely rapid debris flow.

Fig. 1.13: Types of Landslides

Earth Flow: It is mainly found in rocks that are primarily composed of clay and fine-grained materials. The materials flow after liquefaction. In some cases dry flow may also be possible.

Mud Flow: It is a type of earth flow, where material is more saturated with water and contains half sand and remaining silt and clay sized particles. Mud flow and Debris flow are also called as “Mudslides”.

Creep: It is a very slow movement. This is caused by shear stress. There are three types of creeps – i) seasonal, ii) continuous, iii) progressive.

Topples: It includes forward spinning and movement of huge masses of earth, debris and rocks from a slope, It occurs when topples fails.

Spreads: They are little distinctive because it takes place on a very gentle slope or flat terrain. It is caused by shear force or tensile fractures, leading to lateral extensions.

Causes of Landslides

Earthquakes: It is linked with tectonic forces. It is a major contributor to the global landslide events. The 2011 earthquake of Sikkim led to several landslides and mudslides.

Climate: The most important component of climate is precipitation. Intense rainfall leads to ground saturation and increase in ground water table that ultimately leads to soil run-off. Heavy rainfall specifically in upper reaches of Himalaya causes frequent landslide of this nature in Nepal, Uttrakhand and Himachal Pradesh.

Weathering and Erosion (weathered Material): Disintegration of rocks develops weak regolith that is more susceptible to landslides. Erosion wipes out lateral and latent support and facilitates landslides.

Volcanic Eruptions: It can also trigger landslides. If the eruption occurs and conditions are wet, the ash and mud coming out of volcanoes may start flowing.

Gravity: Steep slopes in combination with gravitational pull can cause massive landslides.

Human Interferences: It includes mining and excavation using blasting techniques, cutting and clearing of forest areas, construction of roads, landuse and land-cover changes, building reservoirs and water leakage from the reservoirs may also lead to landslides.

Landslide Hazard

Major landslides in the past have occurred in the Andes Mountains, Pacific Ring of Fire, tropical regions of Central America, Africa and Asia. Landslide hazard refers to the potential occurrence of a damaging landslide within a given area such damage could include loss of life or injury, property damage, social and economic disruption or environmental degradation.

Loss of Human Life: In Ningxia (China) (1920) due to 8.5 Magnitude earthquake caused 675 major loess-linked landslides that killed more than 100, 000 people. In June, 2013 mudslides in Kedarnath (India) killed around 5000 people. One of the worst tragedies took place in Malpa (Uttrakhand) in August, 1998, when nearly 380 people were killed due to massive landslides. This included 60 pilgrims going to Mansarovar in Tibet.

Fig. 1.14

Decimation of Infrastructure and Economic Loss:It can cause serious damage to property. It can totally decimate roads, railways, telephone lines, buildings, homes and other infrastructure. The rehabilitation also involves heavy capital investment that puts extra burden on already cash-crunched state governments in India.

Risk Reduction Measures

Hazards mapping and preparation of hazard Zonation Maps Afforestation and prevent deforestation

Strengthening land-use regulations Relocation of vulnerable settlements Retention wall and nets

Strengthening of weak structures Drainage Control Measures

Community education and awareness

   Avalanches

Avalanche is a type of slide where any amount of snow comes sliding down a mountain slope. It is also known as “snowslide”. The avalanche moving downslope when reaches bottom tends to gain power and speed, this can transform a small snowslide into a full blown disaster.

Types of Avalanches

Avalanches can be categorised into two types on the basis of its depth:

Surface Avalanche: occurs when a layer of dry but loosely packed snow slides over wet but dense layer of snow.

Full Depth Avalanche: It occurs, when the full snow cover (top to bottom surface) starts sliding.

Avalanche can also be categorised on the basis if snow mass and snow type:

Slab Avalanche: when a plate of snow slides as a cohesive unit. The slabs are very huge in size.

Loose Snow Avalanche: when snow that is loose slides downwards on a mountain slope. When loose slides are small in nature, they are termed as “sluffs”. Sluffs are not dangerous, as the fatalities due to sluffs are rare.

Ice Fall Avalanche: When glaciers slides over a cliff, an ice-equivalent of waterfall.

Cornice Fall Avalanches: They are girder like snow structures formed by drifting of snow due to winds. The weight of falling cornice produces an avalanche on the slope, or cornice may break into pieces and transform into an avalanche.

Wet Avalanche: they usually occur when warm air temperature cause water to seep-in beneath the snowpack and reduces its strength.

Glide Avalanche: It is quite similar to glaciers. In this case, entire snowpack slowly slides as a unit. It is a very slow process.

Slush Avalanches: They are unusual type because of its occurrences on gentle slopes. It mainly occurs in the permafrost soil that permits water to pile up and snowpack gets saturated, as a consequence snowpack loses its strength and slushes on a gentle plain.

        Fig. 1.15: Slab Avalanche                                            Fig. 1.16: Sluff Avalanche

Factors Causing Avalanches

Terrain: It constitutes slope profile, angle of slope and ground surface. Slope ranging between 25 to 45 degree is prone to snow movement. The ruggedness and smoothness of surface rocks determines the pace of the movement of snow. Convex slopes allows more tension to develop hence augments the chances of slab avalanche.

Climate: Due to excessive snowfall, the snow-built-up can be very rapid (2cm/hour), it can create very unstable conditions. Sudden change in temperature, wind speed and direction may also influence stability of the snowpack. Himalayan region becomes vicious from January to March. Generally, winters with heavy snowfall are associated with major avalanche in Himalayas.

Earthquakes:Himalaya is tectonically very active and any tremors or earthquakes can result in hazardous avalanche by breaking off large masses of snow, ice and rock.

Vibration or Movement: Vibrations produced by vehicles coupled with the gravitational pull, it is one of the quickest ways to cause an avalanche. Construction work where use of explosives is involvedtends to weaken the snowpack and may trigger avalanches.

Human Interaction: Human interference is the reason behind 90 per cent of the avalanches. The avalanche area of India lies along the northern part of our country covering Jammu and Kashmir

Himachal Pradesh and the hills of Uttaranchal, extending up to Sikkim in the eastern region. The problem, however, is more acute in the western part of the Himalaya where there is frequent interaction of troops and civilians with avalanches.

Avalanche Hazard

Major avalanche prone areas of the World are located in higher latitudes or sub-tropical regions of high altitude mountain regions. They occur frequently in France, Swiss, German, Austrian and Italian parts of Alps mountains. Other regions are western Canada, Utah, Alaska, Colorado and Himalayan mountains. Following are the major impacts of avalanches:

Death and Fatalities: Victims of avalanche who are buried under snow die due to asphyxiation (suffocation), hypothermia and serious wounds. In February, 2016 very heart rendering incident 10 soldiers of Indian army buried under avalanche died near the 19000 feet high Siachen Glacier. In World, most number of fatalities took place in France, followed by Austria, USA, Switzerland and Italy.

Damage to Property: It damages Infrastructure and cause blockage that can adversely impact the livelihood of several people.

Flash floods: Flash floods are seen to happen after avalanches. It brings down all the debris with it and can cause havoc in low lying areas.

Economic Impact: Various ski resorts depend on tourists to run their business. Ski resorts and other businesses are forced to close due to avalanches.

Risk Reduction

These are some of the passive methods adopted for Avalanche Hazard Mitigation in India:

Trying to increase the general awareness of the affected population through:

Preparation and Publication of Hazard Map and Avalanche Atlas

Training: Training in avalanche safety and rescue methods would go a long way to bring down the avalanche casualties. SASE has been training army personnel in safety and rescue methods.

In India, the Snow Avalanche Study Establishment (SASE) has been forecasting and issuing warning for snow avalanches, this is mostly done for the movement of Indian army in the glaciated region. SASE has been using satellite imageries, Digital Terrain Model (DTM), Stress

Distribution Model (SDM) for better forecasting.

Some active methods have also been adopted in India to minimize the damage caused by avalanches. It includes:

Structural controls such as snow bridges, snow rakes and snow nets. Afforestation

Artificial Triggering method inhibits the disastrous build-up of snow cover on slopes

References

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