Earthquake: Natural Disasters

• Aaron Leo, Grade 10
• Short Story
• 2011

An earthquake is a shaking of the ground caused by the sudden breaking and movement of large sections (tectonic plates) of the earth's rocky outermost crust. The edges of the tectonic plates are marked by faults (or fractures). Most earthquakes occur along the fault lines when the plates slide past each other or collide against each other. The shifting masses send out shock waves that may be powerful enough to alter the surface of the Earth, thrusting up cliffs and opening great cracks in the ground and cause great damage ... collapse of buildings and other man-made structures, broken power and gas lines (and the consequent fire), landslides, snow avalanches, tsunamis (giant sea waves) and volcanic eruptions.
Although the ground we walk on seems solid, the earth is actually made up of huge pieces of flat rock called tectonic plates. These move very slowly, and where they meet is called a fault. When the plates rub together, the movement forces waves of energy to come to the earth's surface. This causes tremors and shakes and this is what causes earthquakes.
A seismograph produces wavy lines that reflect the size of seismic waves passing beneath it. The record of the wave, called a seismogram, is imprinted on paper, film, or recording tape or is stored and displayed by computers. The Richter scale is a standard scale used to compare earthquakes. It is a logarithmic scale, meaning that the numbers on the scale measure factors of 10. So, for example, an earthquake that measures 4.0 on the Richter scale is 10 times larger than one that measures 3.0. On the Richter scale, anything below 2.0 is undetectable to a normal person and is called a micro quake. Micro quakes occur constantly. Moderate earthquakes measure less than 6.0 or so on the Richter scale. Earthquakes measuring more than 6.0 can cause significant damage. The maximum quake rating ever measured is about 8.9.
To be earthquake proof, buildings, structures and their foundations need to be built to be resistant to sideways loads. The first thing is to make the highest bit, the roof, as light as possible. This can also have double skin with spacers and insulation. It can have a roof slope between 3 and 15 degrees. If it is required to have a 'flat' roof, this could be made with a galavanised steel decking and solid insulation boards, and topped with a special membrane. If it is required to have a 'flat' concrete roof, then the best solution is to have steel joists at about 2m and over these to have composite style roof decking. As with the roof, the floors should be made as light as possible. The first way is to use traditional timber joists and timber or chipboard or plywood flooring.
An Earthquake moves the ground. It can move the land up and down, and it can move it from side to side. Small cracks appear in the concrete. The bonding of the ‘stirrups’ (the small steel bars which bind the main reinforcement together) to the concrete weakens, the outer concrete crumbles, the main reinforcing bars can bend outwards away from the column and all strength disappears. This was beautifully demonstrated under the Oakland Freeway, where huge round concrete columns crumbled and crumpled. They have now been reinforced with massive belts around them.
Some tall buildings can stay almost intact but fall over in their entirety. The taller the building, the more likely this is to happen, particularly if the building can oscillate at the frequency of the shock waves, and particularly if some liquefaction of soft soil underneath has allowed the building to tilt.
There are many ways that a community can do to prepare and stay safe during an earthquake. Make sure shelves are securely fastened to the walls. Place large, heavy objects on lower shelves or on the floor. They might fall. Place breakable items (bottles and glass etc...) in closed cabinets. Ensure that they have latches. Heavy pictures and mirrors should be hung away from beds, couches, and anywhere that someone might sit. During an earthquake, try to stand between door frames or go under the table. Never run around the house or outside.

Aaron Leo