Geological Hazards, Risk Assessment and Planning
By Ashraf Hussain Shigri (Geologist)
Changes in the earth’s atmosphere that define climate and weather can also spawn hurricanes, tornadoes, floods, drought, and other forms of severe weather. These natural disasters are the result of the ever changing nature of the earth and the environment in which people live. Experts differentiate between natural hazards and natural disasters. A hazard is something that has the potential to cause harm. Natural disasters vary widely in both cause and effect. Some disasters, such as an earthquake or landslide, happen with no warning, but scientists often track the formation of a tropical storm or hurricane for days prior to its landfall. Similarly, the damage done by an earthquake orvolcano tends to be fairly local, especially when compared to the devastation caused by a far-moving hurricane or cluster of tornadoes. Furthermore, no two stormsor volcanoes or earthquakesare alike. Each responds to the unique environment in which it exists.
A geological hazard is one of several types of adverse geologic conditions capable of causing damage or loss of property and life. These hazards consist of sudden phenomena and slow phenomena:
Sudden phenomena include
- Earthquakes – Liquefaction (soils), Tsunamis
- Volcanic Eruptions – Lava Flows, Ash Fall, Lahars
- Landslides – Rock Falls or Slides, Debris Flows, Mud Flows
- Floods – Inundation, Erosion
- Snow Avalanches
- Sand Blasting (Windblown)
Gradual or slow developing geologic phenomena include:
- Ground Settlement
- Ground Subsidence or Collapse
- Sinkholes
- Erosion (stream or shoreline
Earthquakes
Some disasters, such as earthquakes, are sudden and violent, starting and ending within minutes. “Earthquakes are so scary because you don’t have any warning,” says Allan Lindh, a scientist with the U.S. Geological Survey (USGS). “It’s the only thing besides a nuclear war [in which] one minute you’re living in a beautiful city and ten seconds later it’s flat.” Experts say that more than a million earthquakes occur throughout the world each year. Most are minor tremors caused by rocks breaking far below the ground or under the ocean floor, where they cannot be felt by humans. But roughly once a year a major earthquake causes catastrophic devastation.
Following a quake, scientists measure the shaking and give the earthquake a number that shows its intensity. This is called the earthquake’s magnitude. The magnitude scale starts at 1.0. At magnitude 1.0, the trembling is so minor that it cannot be felt by humans. Each number up from 1 represents 10 times as much power or force. Shaking that reaches a magnitude of 5.0 or 6.0 is strong enough break dishes and cause cracks in walls. Major earthquakes are typically defined as those that are of magnitude 8.0 or higher. Major earthquakes release energy equivalent to several nuclear bombs. On average, the world witnesses one or two earthquakes of this size each year. An earthquake’s magnitude tells nothing about the damage that results. A magnitude 6.0 earthquake under a population center may be far more devastating than a magnitude 8.0 quake in a deserted area. In addition, some earthquakes take place farther below the earth’s crust than others do. All else being equal, a shallower earthquake will cause more damage than a deeper one. Earthquakes are among the deadliest of all natural disasters. Most deaths from an earthquake are caused by collapsing buildings. Aftershocks small quakes caused by the initial disturbance often trigger additional fires, landslides, or floods. Sometimes aftershocks occur within hours or minutes of the initial quake, but with a large earthquake aftershocks sometimes continue for days, weeks, or even months.
Plate Tectonic Theory
Earthquakes are a reminder that the earth is constantly changing. Studying the history of the earth over several millennia pointed to clues that the earth’s crust was moving, a theory that became known as continental drift. A century ago, a German scientist named Alfred Wegener theorized that the collision of continents or other pieces of the earth’s crust led to the formation of mountains. When the leading edge of the continent encountered resistance, it folded upward to form mountain ranges and volcanoes, he theorized. Wegener’s ideas were not well received by the scientific community at the time, but more recent study of the earth has proved his theory to be sound. In the 1960s scientists developed instruments that allowed them to map the surface of the earth under water. These maps revealed a system of oceanic ridges where molten rock rises from below the crust and hardens into new crust. At these ridges is where volcanoes are located.
Today, scientists have shown that the earth’s crust is made up of enormous slabs of rock called tectonic plates that fi t together like a jigsaw puzzle. Under the earth’s crust lies the mantle, a layer of hot rocks. The rocks cool near the edge of the plates, and the hotter rock in the middle begins to rise. This causes the plates to move in different directions. When the plates collide, they release energy. The waves of energy called seismic waves cause the ground to shake.
The world’s most violent earthquakes occur at the fault, which is where the tectonic plates meet.
Landslides and slope movements
Landslide hazard refers to the probability of a landslide of a given size occurring within a specified period of time within a particular area. Most landslides occur in areas previously affected by instability, and few occur without prior warning. Hence, it is important to carry out careful surveys of areas that appear potentially unstable. The associated risk relates to the loss of lives or damage to property. Consequently, landslide hazard has to be assessed before landslide risk can be estimated.
Generally, the purpose of mapping landslide hazards is to locate problem areas and to help understand why, when and where landslides are likely to occur. Most landslide investigations are local and site specific in character, being concerned with establishing the nature and degree of stability of a certain slope or slope failure or all of these.
There are a number of data sources from which background information may be obtained. These include reports, records, papers, topographical and geological maps, and aerial photographs and remote sensing imagery. Field investigation, monitoring, sampling and laboratory testing may provide more accurate and therefore more valuable data.
Landslide prevention may be brought about by reducing the activating forces, by increasing the forces resisting movement or by avoiding or eliminating the slide. Reduction of the activating forces can be accomplished by removing material from that part of the slide that provides the force that will give rise to movement. Complete excavation of potentially unstable material from a slope may be feasible; however, there is an upper limit to the amount of material that can be removed economically. Although partial removal is suitable for dealing with most types of mass movement, it is inappropriate for some types. For example, removal of head has little influence on flows or slab slides
Glacial Hazards
Although the potential hazards of glaciers may be appreciable, their impact on man is not significant since less than 0.1% of the world’s glaciers occur in inhabited areas.
Glacial hazards divided into two groups, that is, those that are a direct action of ice or snow, such as avalanches, and those that give rise to indirect hazards. The latter include glacier outbursts and flooding. The time involved in the different types of glacial hazards varies significantly, the rapid movement of masses of snow or ice down slopes as avalanches can pose a serious hazard in many mountain areas. For example, avalanches, particularly when they contain notable amounts of debris, can damage buildings and route ways, and may lead to loss of life. Two types of avalanches are recognized; firstly, the dry snow or wet snow avalanches. Slab avalanches (an avalanche formed by a sheet of snow breaking along a fracture line) represent the second type, and these take place when a slab of cohesive snow fails. These tend to be the more dangerous type.
Avoiding Disaster
Obviously, one of the aspects of planning that intimately involves geology is the control or reduction of the effects of geological processes or geohazards that work against the interests of humans. Geological hazards can be responsible for devastating large areas of the land surface and so can pose serious constraints on development. However, geological processes such as volcanic eruptions, earthquakes, landslides and floods cause disasters only when they impinge upon people or their activities
Whether a natural event is a disaster often depends on how well people are prepared for a disaster and how they react when one occurs. People need to take care to avoid hazards. Sometimes this may mean avoiding building in areas that are susceptible to a wildfire or flood. At other times, it may mean building structures to withstand an earthquake or high winds. It also means taking appropriate steps when the signs show that disaster is imminent. In order to take these steps, people need to understand the hazards and warning signs.
Scientific understanding of the hazards is a critical first step. “The less we know, the more we have to guard against,”
According to my lack of knowledge, GB as animportant placein spite of the geologically important place, there has not been any sort of geological survey conducted in many places except in some areas.Are we waiting for anotheraccident like Attadabad Lake Hunza should be somewhere else? Then we should take steps and take a survey after taking the damages?
God Forbid, if there is any such accidentoccurs;we do not have so many resources, that we could quickly perform any rescue activities in the affected areas.
The need is that the Government of Gilgit-Baltistan and Geological Survey of Pakistanshould take practical steps to consider the seriousness of the matter.Apart from this, NGOs should play their role in awareness programs in this regard.
Email: ashraf.geologist@gmail.com
