Living With the Threat- EARTHQUAKES
A couple of days ago an earthquake occurred near California, USA. Most quakes occur around the Pacific, where oceanic plates constantly shift and plunge against the continental plates. This famous circle of coastal belt is called "the Ring of Fire". Volcanic eruptions, quakes and the tsunamis they generate threaten and destroy millions of lives as well as other assets in these regions.
Never before so many people have packed into quake-prone cities such as Tokyo, Istanbul, Los Angeles and Lima. Being near the boundaries of earth's huge, moving crustal plates, these cities face a rising risk of death and economic disaster due to large scale earthquakes.
Forecasting quakes is certainly a difficult issue. After the recent quake that occurred near California I had a feeling that it would be followed up by a larger quake very soon. A computer based numerical model of California's San Andreas Fault system helps scientists estimate where stress pools before a quake strikes and forecast where quakes could happen next. On the San Andreas transform fault, plates grind past each other and stress builds when the rock gets stuck. However the quake that was expecting suddenly delivered not in the Pacific but in Haiti surrounded by Atlantic Ocean.
Where ever it happens the result of a quake is tremendous. The most powerful earthquake ever recorded, a magnitude 9.5, exploded off Chile in 1960, killing 5,700 and leaving two million homeless. Alaska suffers more and bigger quakes than any other U.S. state. In 1964, a magnitude 9.2 quake leveled buildings in Anchorage, Alaska. Leaving the whole world in a shock, in Dec 26, 2004, a ruptured fault cracked the ocean floor and hurled a tsunami across the Indian Ocean, killing more than 220, 000 people instantly. A year later this was followed up by another major quake in Eurasia. Pakistan controlled poor Kashmir lost at least 73, 000 people in 2005 quake.
Since the most of the regions of the world are now densely populated, the number of casualties owing to quakes has remarkably increased and economic disaster has become unbearable. Wealthy nations can reduce the danger by designing quake-resistant structures and adhering to strict building standards. But developing countries often don't have the resources to enforce building codes or buy expensive strengthening measures. This leaves homes, offices and lives of poor at the mercy of the Earth's incessant shaking.
Earthquakes radiate destruction much like bomb blasts: Seismic waves burst from the underground hypocenter. Hypocenter is the central point of the quake. There are two types of seismic waves which come out from the underground. P waves, which compress and stretch rock, deliver the quake's initial punch. Slower but often more destructive S waves follow, slithering side to side. S waves tear buildings off foundations and can churn wet soils into a mixture that acts like quicksand, causing buildings to tilt and sometimes landslides in mountainous regions. At ground level, P and S waves produce surface waves that can flatten bridges, crack windows or simply pass unnoticed. Powerful surface waves can destroy infrastructure and cause a devastation by cutting unprotected electric power lines, water mains and gas lines, adding fire and flooding to a city's miseries.
Traditional and simple building techniques which have been used commonly all over the world are more vulnerable for destruction at a powerful quake. Thus the issue of building for protection emerges. Earthquake-resistant building methods and materials have consistently improved since serious study of earthquakes began in the early 1900s. In cities that enforce strict construction standards, new structures such as bridges, tunnels, stadiums and high-rising buildings are design from the start to withstand at least some shaking. Some of the quake-resistant methods are as follows:
1) Bolted foundation method
2) Isolated foundation method
3) Flexible tunnel joints
4) Protected utility lines
5) Reinforced columns
6) Braced steel skeletons & shock absorbers
