Many major cities are located in earthquake zones, such as Los Angeles, Mexico City and Tokyo.
Severe earthquakes can collapse buildings and rip up flyovers.
When freeways collapsed in the 1989 San Francisco quake, some cars were crushed to 0.5 m thick.
The 1906 earthquake in San Francisco destroyed 400 km of railway track around the city.
Some of the worst earthquake damage is caused by fire, often set off by the breaking of gas pipes and electrical cables.
In 1923, 200,000 died in the firestorm that engulfed Tokyo as an earthquake upset domestic charcoal stoves.
In the Kobe earthquake of 1995 and the San Francisco earthquake of 1989 some of the worst damage was to buildings built on landfill — loose material piled in to build up the land.
The earthquake that killed the most people was probably the one that hit Shansi in China in 1556. It may have claimed 830,000 lives. Earthquake damage 0
The most fatal earthquake this century destroyed the city of Tangshan in China in 1976. It killed an estimated 255,000 people.
The worst earthquake to hit Europe centred on Lisbon, Portugal, in 1755. It destroyed the city, killing 100,000 or more people. It probably measured 9.0 on the Richter scale (see earthquake measurement) and was felt in Paris. Volcanoes and earthquakes Earthquakes can begin as much as 700 km below the Earth’s surface. The damage they cause can be devastating, ranging from the collapse of buildings to huge cracks in the road.
Earthquakes are measured with a device called a seismograph.
The Richter scale measures the magnitude (size) of an earthquake on a scale of 1 to 10 using a seismograph. Each step in the scale indicates a tenfold increase in the energy of the earthquake.
The Richter scale was devised in the 1930s by an American geophysicist called Charles Richter (1900-85).
The most powerful earthquake ever recorded was in Chile in 1960, which registered 9.5 on the Richter scale. The 1976 Tangshan earthquake registered 7.8.
Between 10 and 20 earthquakes each year reach 7 on the Richter scale.
The Modified Mercalli scale assesses an earthquake’s severity according to its effects on a scale of 1 to 12 in Roman numerals (I–XII). 268 Volcanoes and earthquakes * * *
The Mercalli scale was devised by the Italian scientist Guiseppe Mercalli (1850-1914).
A Mercalli scale I earthquake is one that is only detectable with special instruments.
A Mercalli scale XII earthquake causes almost total destruction of cities and reshapes the landscape.
The Moment magnitude scale combines Richter readings with observations of rock movements.
The Richter scale measures the strength of the shock waves and energy produced by an earthquake.
One way to predict earthquakes is to study past quakes.
If there has been no earthquake in an earthquake zone for a while, there will be one soon. The longer it has been since the last quake, the bigger the next one will be.
Seismic gaps are places in active earthquake zones where there has been no earthquake activity. This is where a big earthquake will probably occur.
Seismologists make very accurate surveys with ground instruments and laser beams bounced off satellites (see earthquake measurement). They can spot tiny deformations of rock that show strain building up.
A linked network of four laser-satellite stations called Keystone is set to track ground movements in Tokyo Bay, Japan, so that earthquakes can be predicted better.
The level of water in the ground may indicate stress as the rock squeezes groundwater towards the surface. Chinese seismologists check water levels in wells.
Rising surface levels of the underground gas radon may also show that the rock is being squeezed.
Other signs of strain in the rock may be changes in the ground’s electrical resistance or its magnetism.
Before an earthquake dogs are said to howl, chickens flee their roosts, rats and mice scamper from their holes and fish thrash about in ponds.
Some people claim to be sensitive to earthquakes.
Earthquakes are a shaking of the ground. Some are slight tremors that barely rock a cradle. Others are so violent they can tear down mountains.
Small earthquakes may be set off by landslides, volcanoes or even just heavy traffic. Big earthquakes are set off by the grinding together of the vast tectonic plates that make up the Earth’s surface.
Tectonic plates are sliding past each other all the time, but sometimes they stick. The rock bends and stretches for a while and then snaps. This makes the plates jolt, sending out the shock waves that cause the earthquake’s effects to be felt far away.
Tectonic plates typically slide 4 or 5 cm past each other in a year. In a slip that triggers a major quake they can slip more than 1 m in a few seconds.
In most quakes a few minor tremors (foreshocks) are followed by an intense burst lasting just one or two minutes. A second series of minor tremors (aftershocks) occurs over the next few hours.
The starting point of an earthquake below ground is called the hypocenter, or focus. The epicenter of an earthquake is the point on the surface directly above the hypocenter.
Earthquakes are strongest at the epicenter and become gradually weaker farther away.
Certain regions called earthquake zones are especially prone to earthquakes. Earthquake zones lie along the edges of tectonic plates.
A shallow earthquake originates 0-70 km below the ground. These are the ones that do the most damage. An intermediate quake begins 70-300 km down. Deep quakes begin over 300 km down. The deepest-ever recorded earthquake began over 720 km down.
Earthquake waves are the vibrations sent out through the ground by earthquakes (see earthquakes). They are also called seismic waves.
There are two kinds of deep earthquake wave: primary (P) waves and secondary (S) waves.
P waves travel at 5 km per second and move by alternately squeezing and stretching rock.
S waves travel at 3 km per second and move the ground up and down or from side to side.
There are two kinds of surface wave: Love waves and Rayleigh waves.
Love, or Q, waves shake the ground from side to side in a jerky movement that can often destroy very tall buildings.
Rayleigh, or R, waves shake the ground up and down, often making it seem to roll. Volcanoes and earthquakes
In solid ground earthquake waves travel too fast to be seen. However, they can turn loose sediments into a fluid-like material so that earthquake waves can be seen rippling across the ground like waves in the sea.
When waves ripple across loose sediment they can uproot tall buildings. A The city of Los Angeles in the USA lies on the Andreas Fault. Because of its high earthquake risk, many of its buildings are now built with reinforcements to protect against earthquake shock waves.
Some earthquake waves travel at 20 times the speed of sound.
Surface waves travel much slower than deep waves, but they are usually the ones that cause the most damage.