Earthquakes happen along the known fault lines in the Earth's crust (see my page on plate tectonics). Although they usually last for less than one minute, earthquakes can be felt over considerable distances. Their effects can vary from a gentle shake, with a noise no more than the sound of a door slamming, leaving you wondering what it was, to the almost total destruction of a town and all of it's buildings.

There are two main causes for the collapse of buildings, Vibration and Liquefaction (also caused by vibration). The vibration causes the whole building to shake until it disintegrates. With liquefaction, the earth, especially if it is wet, turns into a semi-liquid with the high frequency vibrations, and the buildings sink and topple under their own weight.

Engineers and architects have made great strides in building design, with many innovations like huge shock absorbers, and massive bearings upon which the buildings stand, allowing controlled movement of the structure during the course of an earthquake. Unfortunately, throughout the world, and especially in the high risk zones, many poorly designed and constructed buildings still remain, with the attendant risk to all in those areas.

1st November 1755 - It had been 200 years since the last major earthquake, when just off the coast, a seven-minute earthquake that was to be the first of the triple disasters to hit Lisbon occurred. Thousands of people were killed, and many thousands of buildings were destroyed. A 5 m (16 ft) wide fissure opened up, running right through the centre of the City. A tsunami was triggered by the quake, with waves up to 15 m (50 ft)above sea level, drowning thousands of people who had gathered on the quay, trying to escape the effects of the earthquake, and flooded the lower half of the City. The wave went on to break over the walls of Cadiz in Spain, hit Gibraltar, and the island of Madeira. In Lisbon, around noon, fires that had been started by overturned cooking-fires and broken oil lamps, were raging through the City, killing many of those who had managed to survive the earthquake and the tsunami. After three days, all that remained of the City was no more than rubble and smouldering ashes.

100,000 people were killed, and 18.000 buildings demolished.

7:18am 18th September 1985 - In 120 seconds, Mexico City, built upon the sedimentary bed of an ancient lake, was torn apart by an earthquake of the magnitude of 8.1 on the Richter Scale. Because of the soft ground, liquifraction paid a large part in the destruction that was to follow. Over 500 buildings were shaken to pieces, including a seven-story wing of the General Hospital, killing 280 patients and staff. The twelve-story Juirez Hospital also collapsed, killing up to 1.000 patients and staff. The Nueva Leon apartment block, housing over 1,000 residents was flattened killing almost every person within the block. A 60 second aftershock registering 7.5 occurred thirty-six hours later, destroying many more buildings, and killing hundreds of people who had survived the initial quake, and rescue teams who had been trying to excavate for survivors.

5,500 people were killed, 40,000 were injured, and more than 30,000 made homeless.

22nd. June 1990 - Measuring 7.7 on the Richter scale an earthquake hit the farming provinces of Zanjan and Gilan in northern Iran, near the border with Russia. The earthquake ravaged 260 sq km (100 sq miles) in 60 seconds. 45,000 people were killed in the first shock and 100,000 people were injured. These injured people, and the rest of the survivors had to endure a further 36 hours of aftershocks. Dozens of towns were rased to the ground, burying many thousands of people. 400,000 people were left without shelter, and those villages that survived the earthquake were isolated, and cut off from supplies of food, water, etc.

5:12 am 18th. April 1906 - An earthquake, registering 8.25 on the Richter scale hit San Francisco, in the USA. It lasted for no more than 40 seconds, but as a result of that much of the city was destroyed, and an estimated 700 - 2,500 people lost their lives.

16th December 1920 - An earthquake killed 200,000 and made 20,000 homeless, who froze to death due to exposure to the severe winter weather.

28th July 1976 - An earthquake lasting only 23 seconds, turned 95 percent of the town to rubble. It was felt 320 km (200 ml) away, and destroyed buildings in many Cities within the radius of the shock, including Beijing, where 100 people were killed. No less than 125 aftershocks were recorded in the region. The total number of deaths was 750,000.

It is estimated that there are several million earthquakes in the world each year. Many of these earthquakes go undetected because they occur in remote areas or have very small magnitudes. The USGS Earthquake Info Centre locates 12 000 to 14 000 earthquakes each year (about 35 per day).