A natural hazard is a threat of a naturally occurring event will have a negative effect on humans. This negative effect is what we call a natural disaster. In other words when the hazardous threat actually happens and harms humans, we call the event a natural disaster. Natural Hazards (and the resulting disasters) are the result of naturally occurring processes that have operated throughout Earth’s history. Effects of Hazards
Hazardous process of all types can have primary, secondary, and tertiary effects. Primary Effects occur as a result of the process itself. For example water damage during a flood or collapse of buildings during an earthquake, landslide, or hurricane. Secondary Effects occur only because a primary effect has caused them. For example, fires ignited as a result of earthquakes, disruption of electrical power and water service as a result of an earthquake, flood, or hurricane, or flooding caused by a landslide into a lake or river. Tertiary Effects are long-term effects that are set off as a result of a primary event. These include things like loss of habitat caused by a flood, permanent changes in the position of river channel caused by flood, crop failure caused by a volcanic eruption etc. Vulnerability to Hazards and Disasters
Vulnerability refers the way a hazard or disaster will affect human life and property Vulnerability to a given hazard depends on: Proximity to a possible hazardous event
Population density in the area proximal to the event
Scientific understanding of the hazard
Public education and awareness of the hazard
Existence or non-existence of early-warning systems and lines of communication Availability and readiness of emergency infrastructure
Cultural factors that influence public response to warnings
Construction styles and building codes
There are three main types of “earthquake proof” building structures, all used in Japan over the past decade. The first has a heavy concrete weight on the top of a building that, activated by computer-controlled dampers, is shifted across the roof to counteract the force of the earthquake; however, a power cut could stop this sophisticated system working.
The second employs shock absorbers, normally a sandwich rubber composition that acts as a form of suspension; this is suitable for buildings up to 15 storeys. The third method, represented by Foster’s Century Tower in Tokyo, is the eccentrically braced frame. “This”, says Ed Booth of the engineers Ove Arup and Partners, “has steel braces providing stiffness for moderate earthquake motions, but a sacrificial ductile shear link between braces designed to yield in an intense earthquake, absorbing seismic energy and acting as a fuse which prevents the braces from buckling.” “But, earthquake engineering”, adds Mr Booth, “is still a relatively new field.
During this century more than 1.5 million people have lost their lives as a result of earthquakes and the vast majority of this toll because of buildings that have collapsed through unsuitable design.” “I telephoned my parents in Kobe”, says “Mog” Morishima, a Japanese architect working in London, “and they tell me that many of the new buildings on Port Island, a major land reclamation project in the Eighties, have survived. These include the Port Opia Hotel, the tallest building in the area. This is where you can find many new fashionable buildings designed by architects like Tadao Ando and Frank Gehry; it seems that new methods of construction and the new building regulations established in 1971 and revised in 1980 have saved many buildings and, so, many lives.” (The Atlantic, MAR 11, 2011)
In general, less developed countries are more vulnerable to natural hazards than are industrialized countries because of lack of understanding, education, infrastructure, building codes, etc. Poverty also plays a role – since poverty leads to poor building structure, increased population density, and lack of communication and infrastructure.
Assessing Hazards and Risk
Hazard Assessment and Risk Assessment are2 different concepts! Hazard Assessment consists of determining the following when and where hazardous processes have occurred in the past. the severity of the physical effects of past hazardous processes (magnitude). the frequency of occurrence of hazardous processes. the likely effects of a process of a given magnitude if it were to occur now and, making all this information available in a form useful to planners and public officials responsible for making decisions in event of a disaster.
Risk Assessment involves not only the assessment of hazards from a scientific point of view, but also the socio-economic impacts of a hazardous event. Risk is a statement of probability that an event will cause x amount of damage, or a statement of the economic impact in monetary terms that an event will cause. Risk assessment involves hazard assessment, as above,
location of buildings, highways, and other infrastructure in the areas subject to hazards potential exposure to the physical effects of a hazardous situation the vulnerability of the community when subjected to the physical effects of the event. Risk assessment aids decision makers and scientists to compare and evaluate potential hazards, set priorities on what kinds of mitigation are possible, and set priorities on where to focus resources and further study.
Prediction and Warning
Risk and vulnerability can sometimes be reduced if there is an adequate means of predicting a hazardous event. Volcanic eruptions are usually preceded by a sudden increase in the number of earthquakes immediately below the volcano and changes in the chemical composition of the gases emitted from a volcanic vent. If these are closely monitored, volcanic eruptions can be often be predicted with reasonable accuracy. In the prediction of earthquakes, the word forecast is used in a much less precise way – referring to a long-term probability that is not specific in terms of the exact time that the event will occur.
For example: Prior to the October 17 1989 Loma Prieta Earthquake (also know as the World Series Earthquake) the U.S. Geological Survey had forecast a 50% probability that a large earthquake would occur in this area within the next 30 years. Even after the event, the current forecast is for a 63% probability that a major earthquake will occur in this area in the next 30 years.