You are here: home > geotopics > earthquakes > introduction to earthquakes 1989 San Francisco earthquake ( Source - J. K. Nakata - US Geological Survey) Introduction to earthquakes Earthquakes - An Introduction Cause of Earthquakes Effects of Earthquakes Measuring Earthquakes Current Earthquakes What factors influence the effects and response to tectonic activity? Earthquakes - An Introduction An earthquake is a sudden movement of the earth's surface. You can find out about current earthquakes here. Earthquakes are caused by the movement of the earth's tectonic plates. Earthquakes occur where the earth's plates meet along plate boundaries (see plate tectonics page for more information on this).For example as two plates move towards each other, one can be pushed down under the other one into the mantle. If this plate gets stuck it causes a lot of pressure on surrounding rocks. When this pressure is released it produces shock waves. These are called seismic waves. This is an earthquake. The waves spread out from the point where the earthquake started - the focus. More damage is done near the focus. The point on the earth's surface directly above the focus is the epicentre. The effects of earthquakes vary. This is a result of a variety of factors such as the strength of the earthquake, the level of population in an area and the level of economic development of the area struck. Measuring Earthquakes The magnitude (size) of an earthquake is measured using a seismometer. This is a machine that measures movements in the earth's surface. The Richter Scale measures earthquakes on a logarithmic scale - this means that an earthquake of 6 is ten times more powerful than one with a score of 5. The Kobe Earthquake At 05.46 on 17th January 1995 an earthquake measuring 7.2 on the Richter scale struck the heavily populated city of Kobe, Japan. The earthquake occurred along the destructive plate boundary where the Pacific and the Philippine Plate (oceanic) meet the Eurasian (continental) plate. Many freeways and buildings were destroyed, despite the strict building regulations, and 5000 were killed. Fires spread as a result of broken gas mains. 250,000 people were left homeless. The Turkish earthquake occurred on Tuesday 17th August 1999 at 3.02 a.m. local time (12 am GMT). The earthquake lasted 45 seconds and measured 6.8 - 7.0 on the Richter Scale. See the BBC News web site for detailed case study information and images on the Turkish Earthquake. You can download a fact file about the Turkish Earthquake here. (MS Word 97 - Size - 59kb) Earthquakes in California California lies on the conservative margin where the Pacific Plate slides alongside the North American Plate (timeline of movement). As a result it is susceptible to earthquakes. The 1906 Earthquake almost destroyed San Francisco. Later, in 1989 an earthquake measuring 6.9 on the Richter Scale hit San Francisco.
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