Earthquakes
Our planet has a rock crust, but some distance beneath it, in Earth’s mantle, there is a hot and moving mass of molten rock. Sometimes the Earth’s crust will break, and the magma will force its way up and onto the surface as lava and volcanic ejecta. When this happens volcanic earthquakes are caused by friction between the rising magma and the surrounding bedrock.
But most earthquakes, 90%, are so-called tectonic earthquakes which are formed when portions of bedrock in the Earth’s crust quickly slide against each other in a fault. The larger the fault movement, the stronger the resulting earthquake will be. The higher up the movement occurs, the greater the effect will be on the surface, but as we move away from the point directly above the quake—the epicenter—we will feel the effects lessening. Earthquakes occur on the ocean floors as well as on the continents. Large quakes on land may not be a hazard if they happen in unpopulated areas, but a large quake on the ocean floor may cause a tsunami with vast reach.
Earthquakes are accompanied by loud low-frequency noises and sometimes by light phenomena. Some minerals that are under pressure may emit light. Gas also leaks from the ground during an earthquake, which is sometimes flammable and catches on fire. Carbon dioxide, sulphur dioxide and hydrogen sulphide are common gases. In southeast France there is a problem of radon leaking from the ground—Provence is a quake-prone area.
The effects of earthquakes are always worst in cities. In addition to the primary effects of collapsed houses there are secondary effects: severed water, sewage, electricity, and gas lines. Often fires can rage freely. Lisbon 1755, San Francisco 1906 (when 28 000 buildings were burned), Messina 1908, and the firestorm after the Kanto earthquake in Japan in 1923 when 40 000 died in a storehouse in Tokyo.
The history of Brazil before 1755 will always remain hazy, as the records kept by Portuguese colonial officers were stored in the Portuguese National Archive in Lisbon. Everything in the Archive was consumed by fire following the 1755 earthquake. Sadly there are many examples of how earthquakes have caused the loss of irreplaceable cultural treasures.
After large earthquakes, construction companies are faced with huge and long-term replacement building projects. Cities that have been destroyed should not be rebuilt in the same place—geologists and geophysicists can provide information on safer areas. Too many areas are built without information on frequency of earthquakes and other natural hazards, even though expertise exists.