Earthquakes occur when stress from plate margin movement causes rock to break beneath the surface and is suddenly released. As the rock breaks, energy waves travel outwards. These energy waves can be detected by seismographs and, used in conjunction with extremely accurate clocks, scientists can pinpoint the earthquake╒s focus - the point at which the rock broke and therefore the centre of the shock waves. The point on the earth╒s surface immediately above the focus is known as the epicentre.
Several different kinds of energy waves are released during an earthquake:
Primary or P waves (sometimes called pressure waves) are the fastest and the first to be felt, shaking the earth backwards and forwards.
Secondary or S waves (sometimes called shear waves) are stronger, slower and move through the crust with an up and down motion.
L waves (Love waves) or Rayleigh waves are ground motion surface waves which travel more slowly but cause the most damage.
Study of these waves is called seismology.
MEASUREMENT OF EARTHQUAKE INTENSITY
The Modified Mercalli Scale (named after the Italian seismologist and volcanologist, Giuseppe Mercalli in 1902) provides a subjective way of measuring an earthquake╒s intensity based upon observation of the damage caused rather than the strength of the energy waves - expressed in Roman numerals, from I to XII.
The Richter Scale (originated in 1931 by K. Wadati in Japan and developed by Charles Richter, the U.S. seismologist in 1935 in California) measures the exact amount of energy that an earthquake or volcano releases on a logarithmic scale from 0 to over 8 - the greater the magnitude, the higher the number. The largest magnitude recorded was in Chile in 1960 which reached 8.9 on the Richter Scale. An increase of one point in the Richter Scale is ten times stronger than the value below it. It is estimated that there are over 620 earthquakes each year which measure 5.0 and at least 62,000 each year measuring between 4 and 4.9 on the Richter Scale. The 1906 San Francisco quake measured 8.3, killing 450 people and destroying 250,000 homes as fire swept through the city for over 12 hours as the main water lines were destroyed. The Los Angeles earthquake of 17th January 1993 (6.4 on the Richter Scale) caused damage which amounted to $10 billion.
The Richter Scale - a rough guide:
Magnitude 1-2 (500,000 detected per year): Undetectable by humans. Observed only by using seismographs or tiltometers.
Magnitude 2-3 (100,000 to 500,000 per year): Sometimes detected by humans but normally only by instruments.
Magnitude 3-4 (10,000 to 100,000 per year): Tremors slightly felt - ceiling lights might swing but probably no damage caused.
Magnitude 4-5 (1,000 to 10,000 per year): Earthquake strongly felt by humans. Windows crack and some damage to buildings.
Magnitude 5-6 (200 to 1,000 per year): Very strongly felt. Walls crack and there will be some panic.
Magnitude 6-7 (20 to 200 per year): Severely felt. Damage to buildings, some collapse. Chimneys fall and some people panic.
Magnitude 7-8 (10 to 20 per year): Severely felt. Buildings collapse, the ground cracks and there is widespread panic.
Magnitude 8-9 (up to 10 per year): Buildings and bridges collapse, roads and railways buckle. Total destruction and severe human hardship.
Task D
SURVIVING AN EARTHQUAKE
To reduce the effects of volcanic disaster, special building regulations must be adhered to in earthquake zones. Existing buildings can be reinforced with cross-bracing. Liquefaction can be a massive problem in certain areas with a loose soil texture such as sandy or land-fill sites. This process occurs when soil particles are literally shaken apart so that the ground no longer has any cohesive strength and acts like a thick, muddy liquid causing buildings to sink and fall over. Engineers have found that structures which rest on concrete ╥rafts╙ that float when energy waves pass through can withstand the effects of an earthquake much more successfully. Some areas have installed flexible gas mains that are less likely to break and cause fires. To minimise fire risk, many modern municipal buildings install earthquake alarms that automatically cut off gas supplies when they register shock waves above a certain level. Flying objects may cause death, so furniture is fixed to walls and heavy objects screwed to tabletops or the floor. Windows are sometimes covered with clear film so as not to injure when shattered and earthquake drills are an everyday part of life in the main earthquake zones. After the event, fires are often the main hazard to life, made worse by the possible lack of available water when water mains fracture. Lack of water and normal hygiene services may also lead to disease although this is less common in modern times. Buried survivors are rescued using sensitive sound detectors and sometimes sniffer dogs.
SOME NOTABLE EARTHQUAKES
San Francisco
On 18th April 1906, an earthquake measuring 8.3 on the Richter shook the city of San Francisco, California, on the San Andreas Fault - a conservative boundary. Tall buildings collapsed and the fire hazard was complicated by lack of water due to pipes being fractured. Fires raged for three days. More than 28,000 buildings were destroyed and 1,000 people perished. The entire city centre lay in ruins.
Anchorage
On 27th March 1964, a massive earthquake at Prince William Sound, 129 km east of the city of Anchorage, Alaska, USA shook the ground for more than seven minutes. Damage was considerable in the densely populated south central area of Alaska - there were landslides and cracks in the ground, and surface shock waves reshaped the coastline. Tsunamis followed and raced across the Pacific Ocean, eventually reaching as far south as Antarctica.
Tangshan
On 27th July 1976, one of the greatest natural disasters in history hit Tangshan in China. The city had been built above a massive coalmine which was riddled with a network of tunnels. 650,000 people died when the earthquake struck, as the coalmine collapsed underneath them.
Spitak
Registering 6.9 on the Richter Scale, the Armenian earthquake of 7th December 1988 left the towns of Spitak, Kirovakan and Leninakan in ruins. 25,000 died but an amazing 15,000 survivors were rescued alive.
Loma Prieta
On 17th October 1989, movement in a segment of the San Andreas Fault - a conservative boundary - caused an earthquake at Loma Prieta in California. Many comuters were trapped, injured or killed when pieces of the Californian freeway collapsed. Most of the damage in the San Francisco area affected those buildings built on unstable land-fill sites. Ironically, some of these sites had been used to bury rubble and debris from the 1906 San Francisco disaster.
Japan
On 1st September 1923, the Great Kwanto earthquake shook Japan, a complex subduction zone. The epicentre was situated 80 km south of Tokyo, in Sagami Bay, Yokohama. Measuring 8.3 on the Richter Scale, the quake shook the islands for 5 minutes and even split the floor of Sagami Bay. A staggering 100,000 people perished and 300,000 buildings were shattered. Fires raged in the wood and paper houses which ironically had been thought to be safer than brick in an earthquake zone. Then, to compound the devastation, the quake was followed by a tidal wave (or tsunami) which swept homes and people out to sea, then a second major tremor only 24 hours later.
In October 1994, a major earthquake (estimated at 7.9 on the Richter Scale) occurred in the ocean off Hokkaido, causing the collapse of many buildings. Then, three months later, another earthquake (estimated at 7.2 on the Richter Scale) lasting only 30 seconds, devastated the city of Kobe and the surrounding villages where more than 5,500 lost their lives and 171,000 were rendered homeless. This was Japan╒s worst disaster since Kwanto in 1923. Broken gas mains led to fires which swept quickly through the flimsy wooden houses and, due to broken water mains, firefighters had no water to quench the flames. Most people were still in their beds at the time but many early morning commuters were trapped or tragically killed when expressways collapsed.
Mexico City
On 19th September 1985, a crack 20 km down inside the mantle, caused by the subduction of the Cocos plate under the North American plate, produced a surge of energy 1,000 times more powerful than an atomic bomb. Its shock waves inflicted devastation on Mexico City (350 km away), doubly vulnerable to damage due to its being built on a sandy lake bed which amplified the shock waves and destabilised the buildings. This process is known as liquefaction. Although rescue workers searched nonstop for days and saved the lives of 4,000 (including a four day old baby boy), still 8,000 died, 30,000 were injured and 50,000 left homeless.
