Earthquakes occur along faults, which are large cracks in the earth’s crust. Most of these are associated with the larger plate boundaries, along which the largest earthquakes usually occur. They are caused by the sudden jerking movements of the fault, either laterally or vertically, and are almost impossible to predict.Earthquakes are measured in two ways:
The Richter scale measures the magnitude of an earthquake using an instrument called a seismograph. The Richter scale is logarithmic, meaning that an earthquake measuring 7 is 10 times more powerful than one measuring 6, and 100 times more powerful than one measuring 5.
The mercalli scale measures the damage caused by an earthquake. It rates each quake from I to XII, depending on how much damage was done, and is dependent not only on the magnitude of depth of the earthquake.
The point at which an earthquake actually begins, deep below the earth’s surface is called the focus. If the focus is deep then the effects of the earthquake may be less as the shockwaves have more rock to move through. Obviously this also depends on what type of rock it is. The point directly above the focus, on the earth’s surface, is called the epicentre. The effects of the earthquake are usually worst here, and then radiate out from this spot.
The effects of an earthquake can be easily split up into two sections. Primary effects are those that occur immediately as the earthquake happens. These include buildings collapsing, roads and bridges being destroyed and railway lines being buckled. All occur due to the shaking of the ground.
Secondary effects are the subsequent effects of the quake, and can be even more devastating then the primary ones. The main secondary effects are:
Fires: usually from ruptured gas lines. This was the main cause of death and damage after the San Francisco earthquake in 1906.
Tidal waves: A tidal waves caused by an earthquake is called a tsunami. They can travel very quickly across entire oceans, before engulfing land 1000’s of miles away. The 1964 Alaskan earthquake caused considerable damage in several Californian coastal areas. Although Los Angeles has escaped so far, its is still considered to be a tsunami hazard prone area. Landslides can often be triggered by earthquakes, causing huge amounts of material to be moved very quickly. This is actually what occurred just before the volcanic eruption on Mt. St. Helens. They are most likely to occur where the land is steep, saturated or weak.
Diseases can spread very quickly in the unsanitary conditions often left behind by massive earthquakes. Water becomes contaminated very quickly, and in Less Economically Developed Countries (LEDC’s) especially; access for the medical services can be badly hampered by the damage caused by the quake. The most common diseases to be associated with earthquakes are therefore water-borne ones like cholera and typhoid.
Turkey —August 1999
Centred around Izmit in Northwest Turkey, the earthquake occurred on Tuesday August 17, 1999. It affected an area over a radius of 80km.
Primary effects included a death toll of approximately 5 500, with another 30 000 injured and 250 000 made homeless. Over 100 000 buildings collapsed. Infrastructure damage included a 1km stretch of elevated road, numerous railway bridges, and 120 of the city’s 150 quays.
Of 1 200 people rescued on the day of the quake, only 300 survived because of the totally inadequate hospital facilities.
Secondary effects included outbreaks of cholera and typhoid, due to the polluted water supply. People made homeless by the quake had no escape from the intense heat, dust and stench of rotting bodies. A few days after the quake there was no water, electricity or sewage systems working.
The threat of disease was so great that rescue workers had to wear protective gloves and masks, and the cities Gocuk, Yalova and Izmit had large areas quarantined.
It took the Government in Ankara 48 hours to organise emergency troops to go to the area. Locals were totally unprepared for the disaster, despite it being a common earthquake area.
Kobe, Japan — January 1995
The earthquake occurred at 5.46am on January 17, 1995. It measured 7.2 on the Richter Scale and lasted 20 seconds.
Kobe lies on the Nojima fault, a destructive boundary, where the Philippine plate dives below the Eurasian plate. This plate boundary is the reason for Japan’s existence but also means that there is a constant earthquake threat.
Kobe was unlucky in the sense that the focus of the earthquake was very close to the surface and the epicentre was right beside the city.
Primary effects included a death toll of approximately 5 500, with another 30000 injured and 250 000 made homeless. Over 100 000 buildings collapsed. Infrastructure damage included a 1km stretch of elevated road, numerous railway bridges, and 120 of the city’s 150 quays.
Secondary effects included the fact that electricity, gas, water and sewage systems were all hugely disrupted. Emergency services found it very difficult to get into the city due to the massive destruction of the roads. Many temporary shelters were required, as well as food and medicines. Cold weather meant that diseases spread quickly.
A week after the earthquake fires still were burning, 2 million homes still were without power and 1 million were without water. The fires destroyed over 7 000 more homes. Hundreds of aftershocks, 74 strong enough for people tofeel, meant people were too afraid to return to their homes for weeks after the event.
Tough new laws, building codes and emergency plans were brought in after criticism of the Japanese Government. Work is continuing to try to predict future earthquakes, but as yet there is very little way of giving any significant warning time.
The formation of fold
Form along both destructive and collision plate boundaries, in other words where two plates are pushing towards each other.
The best examples are the Himalayas, the Rockies, the Andes and the Alps, all of which are huge fold mountain ranges caused by the collision of two plates.
The general theory is that as two plates, with land masses on them, move towards each other they push layers of accumulated sediment in the sea between them up into folds. Thus most fold mountains will continue to grow, as the plates constantly move towards each other.
As already seen, at a destructive plate boundary the oceanic plate is subducted beneath the continental one. The molten material then rises to the surface to form volcanoes, either in an island arc (e.g. the West Indies) or on the continental land mass (e.g. the volcanoes of the Andes). In both cases Fold Mountains can be formed.
When the Nazca plate dives under the South American one, their motion forward also has been pushing sediment together. This, over millions of years, has been pushed up into huge fold mountains: The Andes. Within them there are also volcanoes as the mountains are above the subduction zone.
If an island arc has been formed, the same idea occurs. Over millions of years the movement of the two plates together will push the island arc nearer to the continent. As this occurs the sediments on the seabed are folded up to become huge mountains.
The formation of fold mountains at collision margins:
These occur less frequently, but two excellent examples are the Himalayas, where the Indian plate is moving North and East towards the stationary European plate, and the Alps, formed by the collision between the African and Eurasian plates.
In these examples both plates are continental ones, and so can neither sink nor be destroyed. The material between them is therefore forced upwards to form the mountains.
For the Himalayas the material that now forms the mountains was originally on the bottom of the non-existent Tethy’s Sea. As the Indian plate pushed towards the Eurasian one, the sediments were folded up to form the Himalayas, leaving the only trace of the sea to be the fossilised shells that you can find high up in the mountains.
Human uses of fold mountains
Humans use fold mountains for a wide variety of purposes:
Farming is a primary activity in all of the fold mountain ranges around the world. Mainly, due to the height and steepness of many of the slopes, this is restricted to cattle and sheep farming. However, in the foothills of the Himalayas the Nepalese people use terraces in the mountainside to help them grow crops, and some southern facing Alpine slopes are used for vines and fruits.
In the Alps a system called transhumance was used. This basically is the seasonal movement of grazing animals between the high ground in the warmer summer months and the valley floors in the colder autumn months. Nowadays transhumance is a little outdated as modern technology has meant that farmers can stay in one place all year.
Tourism is another major use of the Fold Mountains of the world. Because they are in more economically developed countries,the Alps and the Rockies are perhaps the best examples of the impact of tourism. However, it is an increasing industry in both the Andes and the Himalayas,as people look for less crowded places to go to.
The main tourist attraction in the Rockies and the Alps is skiing. Hundreds of thousands of people ski each year and this has brought great changes and problems to the main areas.
The increase in tourism has meant much-improved infrastructure, a huge increase in hotels and restaurants and the development of entire resorts. It has brought a large amount of much needed money into these areas and allowed local people to diversify from farming into many other jobs.
Fold mountains have a lot of other things to attract visitors. These include hill walking, the attractive scenery, river rafting, and climbing. All these have contributed to areas in the Alps and the Rockies becoming all year round holiday resorts.
Forestry is another big business in these mountainous regions. Examples of cultivated coniferous forests can be seen in the Alps, where the trees have been deliberately planted as crops. However, in the foothills of the Himalayas large-scale deforestation is also taking place, with logging companies cutting down vast tracts of the deciduous rainforest there.
Many of the fold mountain regions of the world are prime spots for the generation of hydroelectric power (HEP). They have a plentiful supply of water; deep, narrow valleys with quick flowing rivers, and they are sparsely populated, meaning that few people are displaced when a reservoir is created.
The HEP is then used either for electricity in cities some distance away, or as a power source for local industries, such as saw and paper mills.
The problems of living in
fold mountain areas
Mountainous regions are particularly difficult to build in due to the steep sided valleys and cold climate. Roads and other communications links have to snake their way up wherever they can, and often these roads are not big enough to adequately service a large community.
The climate is very cold and wet, meaning that most industrial and agricultural activity is difficult. For farmers they have a very short growing season, and it is difficult to use machinery on the steep slopes.
Avalanches are a constant threat, as was seen to devastating effect in Ranrahirca, Peru, in 1962. Huge amounts of money are spent each year to try and combat the avalanche threat, especially with the large amount of tourists using the mountains. — s-cool.uk