Seismic Hazard Management Flashcards
Prediction
The prediction of earthquakes is very difficult.
Regions at risk can be identified through plate tectonics, but attempts to predict a few hours before the event are questionable.
Such attempts are based upon monitoring groundwater levels, release of radon gas and unusual animal behaviour.
Fault lines such as the San Andreas can be monitored and the local magnetic fields measured.
Areas can also be mapped on the basis of geological information and studies made into ground stability in order to predict the impact of an earthquake and to produce a hazard zone map that can be acted upon by local and even national planners.
Case Study of Prediction
Close studies of fault lines can sometimes indicate the point along the fault where the next earthquake could be due.
A study of the pattern of the events along the San Andreas Fault between 1969 and 1988 indicated the existence of a ‘seismic gap’ in the area of Loma Prieta (that is, the area had not had any real seismic activity for the past twenty years).
This area had suffered an earthquake in October 1989 that measured 6.9 MMS and was the worst to hit the San Francisco region since 1906.
In total, 63 people died as a result of the event with over 3,700 seriously injured.
Because of the seismic survey, this event was not entirely unexpected, but like all earthquakes, it was not possible to predict it precisely.
Such a system, however, would not work for events such as the one at Northridge, as this took place on an unknown fault line.
Prevention
Trying to prevent an earthquake is thought by almost all to be impossible.
This, however, has not stopped studies into the feasibility, of schemes to keep plates sliding past each other, rather than ‘sticking’ and then releasing, which is the main cause of earthquakes.
Suggestions so far as to lubricating this movement have focused on water and oil.
Some people have even gone so far as to suggest nuclear explosions at depth.
Protection
Since earthquakes strike suddenly, violently and without warning, it is essential that everyone from civil authorities to individuals are prepared.
Case Study of Protection
In the USA, the Federal Emergency Management Agency’s (FEMA) programme has the following objectives:
- To promote understanding of earthquakes and their effects.
- To work to better identify earthquake risk.
- To improve earthquake-resistant design and construction techniques.
- To encourage the use of earthquake-safe policies and planning practices.
Protection therefore means preparing for the event by modifying the human and built environments in order to decrease vulnerability.
It also includes attempts to modify the loss by insurance and aid.
Some of the ways include:
- Hazard-resistant structures,
- Education,
- Fire prevention,
- Emergency services,
- Land-use planning,
- Insurance,
- Aid,
- Tsunami protection.
Hazard-resistant Structures
Buildings can be designed to be aseismic, in that they can be earthquake resistant.
There are 3 main ways in which this can be achieved:
- By putting a large concrete weight on tope of the building which will move, with the aid of a computer programme, in the opposite direction to the force of the earthquake in order to counteract stress.
- Putting large rubber shock absorbers in the foundations which will allow some movement of the building.
- By adding cross-bracing to the structure to hold it together better when it shakes.
Older buildings and structures, such as elevated motorways, can be retrofitted with such devices to make them more earthquake proof.
A comparison between the 1989 Loma Prieta earthquake (6.9 MMS) and the 1988 event in Armenia (6.8 MMS) shows the effects of different types of buildings.
The greater earthquake-proof buildings of California resulted in 63 deaths, whereas in Armenia, over 25,000 people died, many inside buildings that collapsed as a result of soft foundations and no earthquake-proofing features.
In the town of Leninakan, for example, over 90% of more modern 9-12 storey pre-cast concrete frame buildings were destroyed.
Education
For many areas, this is the main way that loss of life can be minimised.
Instructions are issued by the authorities in how to prepare for such events by securing homes, appliances and heavy furniture, and assembling ‘earthquake kits’.
Children have earthquake drills at school as do people in offices and factories.
Government offices and many companies in Japan observe Disaster Prevention Day (1st September) which marks the anniversary of the Tokyo (Kwanto Plain) earthquake in 1923.
Following the Loma Prieto event (1989), the American Red Cross issued a list of supplies that people should keep at hand in case of an earthquake. These include water (at least a three-day supply for all people in the house, and pets); a whole range of foodstuffs (particularly canned and high energy foods); clothing and bedding; first aid kit; and tools and supplies (to include radio, torch batteries, can opener, matches, toilet paper, small fire extinguisher, pliers and aluminium foil).
Fire Prevention
‘Smart meters’ have been developed that can cut off the gas if an earthquake of sufficient magnitude occurs.
In Tokyo, the gas company has a network that transmits seismic information to a computer which then informs employees where to switch off major pipelines, so reducing the number of fires.
Emergency Services
These need careful organisation and planning.
Heavy lifting gear needs to be available and many people should be given first aid training, as it could be some time after the event that trained medical personnel arrive.
Much of the preparation in California involves the establishment of computer programs that will identify which areas the emergency services should be sent to first.
Land-use Planning
The most hazardous areas in the event of an earthquake can be identified and then regulated in terms of land use.
Certain types of buildings should be put in areas of low risk, such as schools and hospitals.
It is also important to have sufficient open space, as this forms a safe area away from fires and aftershock damage to buildings.
Insurance
In richer areas, people are urged to take out insurance to cover their losses, the only problem being that for individuals, this is very expensive.
In the Kobe earthquake in Japan in 1995, for example, only 7% of the people were covered by earthquake insurance.
Aid
Most aid to poorer countries has generally been to help in the few days after the event, providing medical services, tents, water purification equipment, search and rescue equipment, etc.
Aid over the longer-term is much more problematical; it is something which is needed for the reconstruction of the built environment and redevelopment of the economy.
Tsunami Protection
Tsunamis cannot be entirely protected, even if the magnitude and location of an earthquake is known.
Certain automated systems can be installed to give warnings, the best of which uses bottom pressure sensors, attached to buoys, which constantly measure the pressure of the overlying water column.
Regions with a tsunami risk use warning systems (such as a klaxon) to warn the population before the wave reaches land.
The Pacific Warning System is based on Hawaii. It monitors earthquake activity and issue warnings to countries around the Pacific edge if tsunamis are likely.
Some countries have built prevention walls up to 12 m in height. These have not proved very effective, as large tsunamis are likely to overwhelm them.
