What are Earthquakes?

Earthquakes are a shaking or trembling of the Earth's crust caused by the release of huge stresses due to underground volcanic forces, the breaking of rock beneath the surface, or by sudden movement along an existing fault line. The latter type result from constant gradual movement of the tectonic plates that make up Earth's crust which causes stress to build up in its rock layers. Small earthquakes sometimes result from human activity (e.g. filling of large reservoirs).

Characteristics and Measurement

• Variability - Earthquakes are unpredictable and strike without warning. They range in strength from slight tremors to great shocks lasting from a few seconds to as long as five minutes. They could come in a series over a period of several days.
• Magnitude - Energy released by (or the magnitude of) an earthquake is expressed on a recording device called a seismograph, using the Richter Scale. This scale is open-ended, as there is no upper limit to the amount of energy an earthquake might release. The most severe earthquakes so far have not exceeded 9.5 on this scale. It is not a simple arithmetic scale; for instance, a magnitude 7.0 creates 10 times the ground motion of a magnitude 6.0 uplift as in the Hokkaido, Japan, 1993 earthquake; and the total energy release is about 30 times greater. This in turn is 30 times greater than a 5.0 and so on.
• Intensity - Another scale used to describe earthquakes is the Modified Mercalli (MM) Scale. It rates the amount of shaking felt and damage caused, (or intensity) and uses Roman numerals. On this Scale, I = a barely detectable tremor, and XII = total damage.

Earthquake Survival and Damage Reduction

Know Your Local Earthquake Risk - Ask your State or Territory Emergency Service, council and insurance company for this information:

• Whether tremors or earthquakes have occurred in your area and what damage resulted.
• Ask your State or Territory Emergency Service for a free pamphlet or poster showing Australia's earthquake hazard zones.
• Even if you're in a slight-risk zone, ask your council how to make your house safer.
• Check that your insurance covers earthquake damage.

Emergency Kit and Plan - Needed for during and after an earthquake:

• Have candles, matches, a torch and a portable radio with fresh batteries.
• Have containers of fresh water, a first aid kit and basic first aid knowledge.
• Know safe areas to shelter, and danger areas to avoid.

Watch for Possible Warning Signs

• Erratic Animal Behaviour - Watch for frightened or confused pets running around, or a bird-call not usually heard at night.
• Ground-Water Levels - Watch for sudden changes of water level in wells or artesian bores.

During the Earthquake

• If indoors, stay there. There could be falling debris outside. Take cover under an internal door frame, sturdy table, bench or bed. Keep away from windows, mirrors, chimneys, overhead fittings and tall furniture.
• In high-rise buildings, stay clear of windows and outer walls. Get under a desk near a pillar or internal wall. Do not use elevators. In crowded areas, do not rush for doors. Stay clear of roof and wall fittings.
• If outside, keep well clear of buildings, walls, power lines, trees etc. In a city street, with tall buildings, shelter from falling debris under strong arch ways or doorways of buildings. Don't stand under awnings or parapets as they may collapse.
• If in a vehicle, stop in the open until shaking stops. Beware of 'downed' power lines and damage to roads, overpasses or bridges.
• Listen to radio for warnings before moving.

After the Earthquake - Tend to injuries and watch for hazards as follows:

• Check for injuries. Apply first aid. Do not move the seriously injured unless in danger.
• Do not use telephones (avoid congestion) unless there is a serious injury or fire.
• Turn off electricity, gas and water.
• Check for gas and fuel leaks before lighting matches.
• Check for water or sewage leaks, broken electrical wiring etc.
• Check for cracks and damage, including roof, chimneys and foundation.
• Be prepared for aftershocks. Evacuate if the building is badly damaged.
• Do not waste food and water as supplies may be interrupted.
• Collect emergency water from heaters, ice cubes, toilet tanks and canned foods.
• Listen to local radio and heed warnings and advice on damage and service disruptions.
• Avoid driving unless for emergency - keep streets clear for emergency vehicles.
• Do not go sight-seeing or enter damaged buildings.
• Stay calm and help others if possible.

What Are Your Chances of Survival

You can survive an earthquake and minimise its damage by being aware of, and prepared for, potential hazards. An earthquake will be over before you can do much about it.

Most people are killed or injured as they attempt to move prematurely during the earthquake and are struck by falling or flying objects.

Your chances of avoiding serious injury are high if you remain calm and take cover as recommended above.

Follow this advice and by being well-prepared, you could be a potential resource for your community, not a potential casualty requiring help.

Modified Mercalli Scale

The Modified Mercalli Intensity value assigned to a specific site after an earthquake has a more meaningful measure of severity to the nonscientist than the magnitude because intensity refers to the effects actually experienced at that place.

The lower numbers of the intensity scale generally deal with the manner in which the earthquake is felt by people. The higher numbers of the scale are based on observed structural damage. Structural engineers usually contribute information for assigning intensity values of VIII or above.

The following is an abbreviated description of the 12 levels of Modified Mercalli intensity.

I. Not felt except by a very few under especially favorable conditions.

II. Felt only by a few persons at rest, especially on upper floors of buildings.

III. Felt quite noticeably by persons indoors, especially on upper floors of buildings. Many people do not recognize it as an earthquake. Standing motor cars may rock slightly. Vibrations similar to the passing of a truck. Duration estimated.

IV. Felt indoors by many, outdoors by few during the day. At night, some awakened. Dishes, windows, doors disturbed; walls make cracking sound. Sensation like heavy truck striking building. Standing motor cars rocked noticeably.

V. Felt by nearly everyone; many awakened. Some dishes, windows broken. Unstable objects overturned. Pendulum clocks may stop.

VI. Felt by all, many frightened. Some heavy furniture moved; a few instances of fallen plaster. Damage slight.

VII. Damage negligible in buildings of good design and construction; slight to moderate in well-built ordinary structures; considerable damage in poorly built or badly designed structures; some chimneys broken.

VIII. Damage slight in specially designed structures; considerable damage in ordinary substantial buildings with partial collapse. Damage great in poorly built structures. Fall of chimneys, factory stacks, columns, monuments, walls. Heavy furniture overturned.

IX. Damage considerable in specially designed structures; well-designed frame structures thrown out of plumb. Damage great in substantial buildings, with partial collapse. Buildings shifted off foundations.

X. Some well-built wooden structures destroyed; most masonry and frame structures destroyed with foundations. Rails bent.

XI. Few, if any (masonry) structures remain standing. Bridges destroyed. Rails bent greatly.

XII. Damage total. Lines of sight and level are distorted. Objects thrown into the air.