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Download An earthquake is the vibration of the Earth that results in a
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An earthquake is the vibration of the Earth that results in a sudden release of energy in the Earth’s crust that creates seismic waves. Earthquakes are also known as quakes, tremblors or tremors. At the Earth's surface, earthquakes manifest themselves by shaking and sometimes displacing the ground. The point deep within the crust where plate contact is made is called the focus. The point on the Earth’s surface (directly above the focus) is called the epicenter. A fault is a fracture in rock in which the rock on one side of the fracture has moved with respect to the rock on the other side. Earthquakes are caused by the energy release during rapid slippage along a fault. Slippage Along a Fault What happens to the plate during an earthquake? When plate boundaries move (either transform fault, convergent, or divergent), they will move freely and easily if the two plates are smooth. But plate boundaries are not smooth and the edge of the plates can get “caught.” When the edges get “caught,” great pressure gets exerted on the plate. This pressure translates into potential energy. When the energy becomes too great and the strength of the rock is exceeded, the plate slips or cracks and a rapid release of potential energy (stored in rock) occurs. Potential Energy Earthquakes are recorded with a seismograph using one of two scales called the Moment Magnitude or the Richter scale (mostly obsolete among seismologists). Seismograph and Seismograms Seismograms are traces of amplified, electronically recorded ground motion made by seismographs. A seismogram shows all three types of seismic waves—surface waves, P waves, and S waves along a timeline. Most mechanical seismographs have been replaced by digital recorders that store the seismogram electronically. Surface or “L” Wave Transverse or “S” Wave Push Pull or “P” Wave Seismograph Stations in California Wave Energy The energy found in an earthquake is called wave energy. There are two types of waves: surface waves and body waves. Surface Waves Surface waves are seismic waves that travel along Earth’s surface (many P and S waves turn into surface waves when they reach the surface). These are the rolling waves you feel during an earthquake. Body Waves The two types of body waves are: P waves and S waves. P(primary) waves! P waves are push-pull waves that push (compress) and pull (expand) in the direction that the waves travel. P waves travel through solids, liquids, and gases. They can also temporarily change the volume of the material they are passing through. P waves have the greatest velocity of all earthquake waves. About 1.7 times faster than “S” waves. S (secondary) waves! S waves are seismic waves that travel through the Earth. S waves shake particles at right angles to the direction that they travel. S waves travel only through solids. S waves have slower velocity than P waves Seismogram Moment Magnitude and Richter Scale The moment magnitude scale is used by seismologists to measure the size of earthquakes in terms of the energy released. This scale was developed in the 70’s and is used by seismologist as opposed to the 30’s era Richter scale. The moment magnitude scale is based on the moment of the earthquake, which is equal to the rigidity of the Earth multiplied by the average amount of slip on the fault and the size of the area that slipped. Both the Moment Magnitude and the Richter scales are based on multiples of ten. They are logarithmic scales. So each whole number (greater or less) is ten times (more or less) than the next. SO…. How much stronger is a 5.0 earthquake than a 4.0? 10 times How much weaker is a 4.5 than an 8.5? 10,000 times How much stronger is an 8.8 (Chile) than a 6.5 (Northridge)? About 100 times. Richter Scale Richter magnitudes Description Less than 2.0 Micro Earthquake effects Microearthquakes, not felt. Frequency of occurrence About 8,000 per day 2.0-2.9 Generally not felt, but recorded. About 1,000 per day Often felt, but rarely causes damage. 49,000 per year (est.) Light Noticeable shaking of indoor items, rattling noises. Significant damage unlikely. 6,200 per year (est.) 5.0-5.9 Moderate Can cause major damage to poorly constructed buildings over small regions. At most slight damage to well-designed buildings. 800 per year 6.0-6.9 Strong Can be destructive in areas up to about 160 kilometres 120 per year (100 mi) across in populated areas. 7.0-7.9 Major Can cause serious damage over larger areas. 18 per year Can cause serious damage in areas several hundred miles across. 1 per year Minor 3.0-3.9 4.0-4.9 8.0-8.9 Great Devastating in areas several thousand miles across. 9.0-9.9 10.0+ Epic Never recorded; see below for equivalent seismic energy yield. 1 per 20 years Extremely rare (Unknown) Results of Earthquakes Tsunami (from underwater earthquakes) Landslides Volcanoes (like Mt. St. Helens) Urban Fires Floods Fissures Avalanches Loss of human life and property Causes of Earthquakes Earthquakes are caused mostly by rupture of geological faults, but also by volcanic activity, landslides, mine blasts, dams (waterquakes), and nuclear experiments. Remember: Faults are fractures in Earth where movement has occurred. An aftershock is a small earthquake that follows the main earthquake. A foreshock is a small earthquake that often precedes a major earthquake. Largest Earthquakes in the World! Rank 1 2 3 4 4 4 5 5 5 6 6 6 7 7 7 Date May 22, 1960 December 26, 2004 March 27, 1964 November 4, 1952 August 13, 1868 January 26, 1700 November 25, 1833 February 27, 2010 January 31, 1906 February 4, 1965 November 1, 1755 July 8, 1730 March 28, 2005 March 9, 1957 August 15, 1950 Location Valdivia, Chile Sumatra, Indonesia Prince William Sound, AK Kamchatka Arica, Chile Cascadia subduction, Canada/USA Sumatra, Indonesia Maule, Chile Ecuador/Colombia Rat Islands, Alaska Lisbon, Portugal Valparaiso, Chile Sumatra, Indonesia Andreanof Islands, Alaska Assam, India /Tibet, China Magnitude 9.5 9.3 9.2 9.0 9.0 9.0 8.8 8.8 8.8 8.7 8.7 8.7 8.6 8.6 8.6 Deadliest Earthquakes in the World Rank Date Location Fatalities Magnitude 1 January 23, 1556 China 830,000 8.0 2 July 28, 1976 China 255,000 7.5 Estimated death toll as high as 655,000 3 January 12, 2010 Haiti 240,000 7.0 4 December 16, 1920 China 234,117 7.8 5 December 26, 2004 Indonesia 230,210 9.3 Deaths from earthquake and resulting tsunami. 6 October 11, 1138 Syria 230,000 8.5 7 September 1, 1923 Japan 142,000 7.9 8 October 6, 1948 Turkmenistan 110,000 7.3 Long Valley (SUPERVOLCANO!) Earthquakes! Los Angeles Earthquakes! Depth of the Recent Earthquakes Chile’s earthquake’s depth was 21.7 miles. Haiti’s earthquake depth was 8.1 miles.
