# Earthquakes

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## Earthquakes

#### Main Concepts

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.

#### Effects of Earthquakes

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.

#### Examples

Turkey. August 1999

• Centred around Izmit in Northwest Turkey, the earthquake occurred on Tuesday 17th August 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 1200 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 the 17th January 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 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.
• 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.