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What is an earthquake?• Earthquakes are natural geologic phenomena caused by the sudden and rapid movement of a large volume of rock. The violent shaking and destruction caused by earthquakes are the result of rupture and slippage along fractures in Earth’s crust called fault.
Terms to know:• Fault- A break in a rock mass along which movement has occurred.• Focus- The zone within the Earth where rock displacement produces an earthquake. (Also referred to as the hypocenter)• Epicenter- The point at the surface directly above the focus.• Seismic waves- A form of elastic energy that causes vibrations in the material that transmits them.• Elastic rebound- The sudden release of stored strain in rocks that results in movement along a fault.• Aftershock- A smaller earthquake that follows the main earthquake.• Foreshocks- small earthquakes that often precede a major earthquake.• Seismograph- an instrument that records and measures earthquake waves.
The cause of earthquakes…• Tectonic stresses acting over tens to hundreds of years slowly deform the crustal rocks on both sides of a fault. When deformed by differential stress, rocks bend and store elastic energy. Eventually, the frictional resistance holding the rocks in place is overcome. Slippage allows the deformed rock to “snap back” to its original shape.• Earthquakes are produced by the rapid release of elastic energy stored in rock that has been deformed by differential stresses. Once the strength of the rock is exceeded, it suddenly ruptures causing the vibrations of an earthquake.
Faults faults and more faults• There are 3 basic types of faults: normal, reverse and strike-slip.• Normal faults occur where the crust is stretched and elongated, therefore most earthquakes along normal faults are associated with divergent plate boundaries.• Reverse faults are most often found along convergent plate boundaries. These faults are generated by compressional forces associated with subduction zones and continental collisions.• Strike-slip faults are the result of stress that cause large segments of Earth’s crust to slip horizontally past each other. (Large strike-slip faults are called transform faults)
How are earthquakes measured? a measure of the degree • Intensity is of earthquake shaking at a given locale based on observed damage. • Magnitude relies on data gleaned from seismic records to estimate the amount of energy released at an earthquake’s source. • In 1902 Giuseppe Mercalli developed the Modified Mercalli Intensity Scale to measure intensity. • The Richter Scale is a scale of an earthquake magnitude based on the amplitude of the largest seismic wave. • Richter used a logarithmic scale to express magnitude, in which a tenfold increase in wave amplitude corresponds to an increase of 1 onThe Richter Scale the magnitude scale.
Earthquake Facts• The largest recorded earthquake in the world was a magnitude 9.5 in Chile on May 22, 1960.• In 1663 the European settlers experienced their first earthquake in America.• The largest recorded earthquake in the US was a magnitude 9.2 that hit Prince William Sound, Alaska on March 28, 1964.• The first “pendulum seismoscope” to measure the shaking of the ground during an earthquake was developed in 1751, and it wasn’t until 1855 that faults were recognized as the source of earthquakes.• It is estimated that there are 500,000 detectable earthquakes in the world each year. 100,000 of those can be felt, and 100 of them cause damage.• From 1975-1995 there were only four states that did not have any earthquakes. They were: Florida, Iowa, North Dakota, and Wisconsin.• Most earthquakes occur at depths of less than 80 km (50 miles) from the Earth’s surface.
BIBLIOGRAPHY• http://earthquake.usgs.gov/learn/facts.php• http://geosun.sjsu.edu/~june/june/geol101/in dex.htm• http://www.roebuckclasses.com/105/regions/ seasiaspac/oceania/plateboundaries.htm• http://www.greenscreen.org/articles_sr/FONE arthquakes.htm• Earth an introduction to physical geology- textbook