101 Amazing Facts about Natural Disasters by Jack Goldstein & Frankie Taylor

Author:Jack Goldstein & Frankie Taylor
Language: eng
Format: epub
ISBN: 9781785382093
Publisher: Andrews UK
Published: 2015-07-21T04:00:00+00:00

Australian Bushfires in 2003

Earthquakes

The Pacific Ring of Fire

An earthquake is a shaking of the surface of the earth that is strong enough to be perceptible to humans. Also known as quakes, tremors or temblors, some have been so violent that they have destroyed whole cities, whereas others are barely even felt. The exact point at which an earthquake’s rupture occurs is called the hypocentre. The more commonly-used term epicentre is actually the point at ground level which is directly above the hypocentre.

The strength of an earthquake is today described by the Moment Magnitude Scale, which measures the size of the event in terms of energy released. This replaced the well-known Richter Scale in the 1970s and many media reports today still confuse the two (often announcing a quake is a particular number ‘on the Richter scale’). The new scale was in fact designed to class magnitudes similarly to the older and more widely recognised one, so any difference as a result of using the wrong scale is actually quite minor.

Earthquakes are generally the result of natural causes, the most common being the rupture of geological faults, however the word does still apply to seismic events that are the result of alternative causes, such as volcanic activity, landslides and even nuclear blasts. The ‘standard’ type of earthquake is called a tectonic earthquake and is caused by the release of stored strain energy where the sides of a fault plane ‘break through’ after having been stuck against each other whilst their respective plates continue to move.

There are three main fault types: normal, thrust (also called reverse) and strike-slip. Normal and thrust faults occur when the displacement along the fault is in the direction of the dip - this results in the movement having a vertical element to it. On the other hand, strike-slip faults occur when the two sides of the fault slip horizontally past each other. Of course, many earthquakes are not exclusively just one type of these, and therefore have components of both normal/thrust and strike-slip - we call these oblique slip faults.

Scientists believe that around half a million detectable earthquakes occur each year, around a fifth of which are felt by occupants of the areas in which they occur. Many places such as California, Mexico, Pakistan and others experience almost constant earthquakes, although most are at a very low level. There are thousands of seismic stations positioned around the world which detect earthquakes, most of which (in fact almost 90%) occur in a horseshoe-shaped zone forty thousand kilometres long known as the Pacific Ring of Fire.

The Great Lisbon Earthquake stuck Portugal during the morning of the 1st of November 1755, a day celebrated among the Roman Catholic Church (and a number of protestant denominations) as All Saints Day. At the time the earthquake struck, many were attending mass; sadly the historic churches in which they were worshipping were unable to withstand the force and collapsed, leaving those inside either dead or injured. The quake (which had an estimated magnitude of around 8.

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