Is LA prepared for the BIG one?

The Earth’s crust is split into about 12 pieces called plates. Earthquakes occur due to convection currents radiating out from the liquid core, causing these plates to move. There are regions where the plates drift apart from one-another, which give rise to the creation of new land and crust. However, the world cannot expand, and where land is created in some zones, land has to be destructed in areas where plates converge. It is at such zones where earthquakes are most likely to occur. Where plates collide, friction occurs as the oceanic plate slides beneath the continental crust of the land. Pressure builds up between such converging plates when slipping cannot occur. Eventually the pressure becomes so great that the plates suddenly slip, causing seismic waves to travel through the earth and the pressure released causes earthquakes.

The San Andreas Fault occurs on a conservative plate margin. The oceanic crust, the pacific plate and the continental crust, the North American plate are moving parallel to each other. As the two plates move past each other, friction causes pressure to build up. The pressure is eventually released causing the two plates to jerk forward, sending seismic waves through the earth. These waves are released at the epicentre causing earthquakes. The San Andreas runs 1200km through the length of California. The fault is seen as a potential threat to many large Californian cities, such as Los Angeles and San Francisco. The San Andreas Fault runs 30 miles to the south of Los Angeles, yet is close enough to be largely affected by a seismic event, as proved by the Northridge earthquake in 1994.

The 1994 LA earthquake occurred as a result of a thrust fault. These are shallow dipping reverse faults, which terminate before they break the crust surface. Where they break it may lead to uplift, but it never breaks to cause a surface rupture. This lead to 57 people being killed with over 1,500 seriously injured. This would have been higher had the earthquake not occurred early in the morning. 11 major roads were damaged, 20,000 people were made homeless and over 11,000 landslides were triggered. This led to a total cost of the damage exceeding $30 billion.

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To prepare for future earthquakes, further understanding of past earthquakes is needed. In 1985, on September 19th the city of Mexico experienced a devastating earthquake of magnitude of 8.1. People were trapped in poorly constructed buildings that had collapsed on them, many citizens died as a result. The Mexican government estimated some 5000 people perished; however, international agencies placed the death toll at more than 10,000. Due to the city being situated on the site of a former lake, liquefaction occurred. Liquefaction is a type of ground failure in which water saturated sediment turns from a solid to a liquid as a result of shaking. This causes the magnitude of the damaged cause to increase. After the earthquake, the Mexican Government was confronted on several issues arising, such as the substandard building materials used on lower class housing and the lack of land use planning, which led to many new developments being built on landfills and unstable land.

In January 17th 1995, the Japanese city of Kobe was hit by largest earthquake in Japan since 1923. This earthquake was one of the most powerful to hit Japan with a magnitude of 6.9. The worst affected region, near the main port and docks, was built on soft easily moved rocks and reclaimed ground. Here the ground actually liquefied, and led to buildings toppling sideways, resulting in the huge cranes in the harbour toppling over into the sea. More than 102,000 buildings were destroyed in Kobe, leaving over a fifth of the city population, some 300, 000 people, homeless. The local government‘s estimate of the cost to restore the basic infrastructure of the city was about $150 billion dollars, and that was just for the state owned buildings and services. The area of Kobe was unprepared for an earthquake. The area had no previous history of earthquakes and research didn’t indicate any seismic activity taking place. This brings up the issue that an earthquake can strike at any time, any place so basic knowledge, and preparation is vital.

LA is a well prepared city, in the case of earthquakes. The San Andreas Fault line is regularly monitored, local magnetic fields are measured, groundwater levels, release of random gas and unusual animal behaviour are monitored. However it is very difficult to predict an earthquake precisely. Preventing an earthquake is virtually impossible; however studies have been carried out to keep the plates from sliding past each other.

One of the ways of protection from the earthquake is preparing for the event by modifying the human and built environment to decrease vulnerability. These include:

Hazard Resistant Structures- Buildings that are designed to be aseismic or earthquake resistant. This can be done by

Putting a large concrete weight on the top of a building which will move with the aid of a computer programme, in the opposite direction to the force of the earthquake to counteract stress.
Building large rubber shock absorbers into the foundations to allow some movement in the building.
Adding cross-bracing to the structure to hold it together when it shakes.

This can lead to the prevention of many deaths. A comparison of the 1989 Californian earthquake and Armenia shows that due to the earthquake proof buildings in California, there were only 63 deaths compared with the death toll of 25,000 in Armenia.

Education- This can minimise loss of life by explaining how to prepare for an earthquake. This includes the practice of earthquake ‘drills’ in factories, schools and offices and keeping emergency provisions in case of disaster.

Land Use planning- The most hazardous areas in the event of earthquakes are identified and regulated. And in California, high rise buildings are located in between areas of sufficient open space as a safe are away from fires and aftershock damage to buildings.

With these measures in mind, it is believed that LA will be able to prepare and cope will the arrival of THE BIG ONE!