Submarine earthquake

A submarine earthquake is an earthquake which occurs underwater at the bed of the sea. They can produce tsunamis, also known as "harbor waves" or "tidal waves". The magnitude can be measured scientifically by the use of either the Richter scale or the Mercalli scale [ [http://www.bartleby.com/65/ea/earthqua.html earthquake. The Columbia Encyclopedia, Sixth Edition. 2001-05] URL accessed January 23, 2007]

Understanding plate tectonics helps to explain the cause of submarine earthquakes. The earth's surface or lithosphere comprises tectonic plates which average approximately 50 miles in thickness, and are continuously moving very slowly upon a bed of molten lava in the asthenosphere and inner mantle. The plates converge upon one another, and one subducts below the other. Little movements called fault creep are minor and not measurable. The plates meet with each other, and if rough spots cause the movement to stop at the edges, the motion of the plates continue. When the rough spots can no longer hold, the sudden release of the built up motion releases, and the sudden movement under the sea floor causes a submarine earthquake. This area of slippage both horizontally and vertically is called the epicenter, and has the highest magnitude, and causes the greatest damage.

As with a continental earthquake the severity of the damage is not often caused by the earthquake at the rift zone, but rather by events which are triggered by the earthquake. Where a continental earthquake will cause damage and loss of life on land from fires, damaged structures, and flying objects; a submarine earthquake alters the sea bed floor, resulting in a series of waves, and depending on the length and magnitude of the earthquake, huge tidal waves and tsunamis which bear down on coastal cities causing property damages and loss of life.

Submarine earthquakes can also damage submarine communications cables, leading to widespread disruption of the Internet and international telephone network in those areas. This is particularly common in Asia, where many submarine links cross submarine earthquake zones such as the Pacific Ring of Fire.

Tectonic plate boundaries

The different ways in which tectonic plates rub against each other under the ocean or sea floor to create submarine earthquakes. The type of friction created may be due to the characteristic of the geologic fault or the plate boundary as follows. Some of the main areas of large tsunami producing submarine earthquakes are the Pacific Ring of Fire and the Great Sumatran fault.

Convergent plate boundary

The older, and denser plate moves below the lighter plate. The further down it moves, the hotter it becomes, until finally melting altogether at the asthenosphere and inner mantle and the crust is actually destroyed. The location where the two oceanic plates actually meet become deeper and deeper creating trenches with each successive action. There is an interplay of various densities of lithosphere rock, asthenosphere magma, cooling ocean water and plate movement for example the Pacific Ring of Fire. Therefore the site of the sub oceanic trench will be a site of submarine earthquakes; for example the Marianas Trench, Puerto Rico Trench, and the volcanic arc along the Great Sumatran fault. [ [http://geology.com/nsta/convergent-plate-boundaries.shtml Convergent Plate Boundaries - Convergent Boundary - Geology.com] URL accessed January 23, 2007]

Transform plate boundary

A transform-fault boundary, or simply a transform boundary is where two plates will slide past each other, and the irregular pattern of their edges may catch on each other. The lithosphere is neither added to from the asthenosphere nor is it destroyed as in convergent plate action. For example along the San Andreas fault strike-slip fault zone, the Pacific Tectonic Plate has been moving along at about 5 cm/yr in a northwesterly direction, whereas the North American Plate is moving south-easterly. [http://pubs.usgs.gov/gip/dynamic/understanding.html Understanding plate motions [This Dynamic Earth, USGS] URL accessed January 23, 2007]

Divergent plate boundary

Rising convection currents occur where two plates are moving away from each other. In the gap, thus produced hot magma rises up, meets the cooler sea water, cools, and solidifies, attaching to either or both tectonic plate edges creating an oceanic spreading ridge. When the fissure again appears, again magma will rise up, and form new lithosphere crust. If the weakness between the two plates allows the heat and pressure of the asthenosphere to build over a large amount of time, a large quantity of magma will be released pusing up on the plate edges and the magma will solidify under the newly raised plate edges, see formation of a submarine volcano. If the fissure is able to come apart because of the two plates moving apart, in a sudden movement, an earthquake tremor may be felt for example at the Mid-Atlantic Ridge between North America and Africa. [ [http://geology.com/nsta/divergent-plate-boundaries.shtml Divergent Plate Boundaries - Divergent Boundary - Geology.com] URL accessed January 23, 2007]

List of major submarine earthquakes

*2006 December 26 - 2006 Hengchun earthquake: off the southwest coast of Taiwan, in the Luzon Strait, which connects the South China Sea with the Philippine Sea magnitude 7.1 M_w.

*2004 December 26 Sumatra-Andaman earthquake and Asian Tsunami: off the western coast of Sumatra, Indonesia measuring 9.3 on the Earthquake Magnitude scale. It was the fifth largest earthquake in recorded history and generated massive tsunamis, which caused widespread devastation when they hit land, leaving an estimated 230,000 people dead in countries around the Bay of Bengal and the Indian Ocean.

*1998 May 4 a part of the island of Yonaguni destroyed by a submarine earthquake

*1960 May 22 - Great Chilean Earthquake occurred off the coast of South Central Chile with a magnitude of 9.5 is the strongest earthquake ever recorded.

*1946 21 December - The Nankai earthquake which occurs off the southern coast of Kii Peninsula and Shikoku, Japan with a magnitude of around 8.0.

*1944 December 7 - Tonankai earthquake, Japan occurred about 20 km off the Shima Peninsula in Japan with a magnitude of 8.0.

*1929 November 18 - 1929 Grand Banks earthquake: Off the south coast of Newfoundland in the Atlantic Ocean, a Richter magnitude 7.2 submarine earthquake centered on Grand Banks.

*1896 June 15 An undersea earthquake off the Sanriku coast of northeastern Honshū, Japan measured a magnitudeof 8.5

*1771 4 April - An undersea earthquak occurred near Yaeyama Islands in Okinawa, Japan of estimated magnitude 7.4.

*1700 January 26 - The Cascadia Earthquake offshore from Vancouver Island to northern California, estimated MW 9.3 magnitude was one of the largest earthquakes on record.

ee also

*2004 Indian Ocean earthquake characteristics
*List of plate tectonics topics
*Cascadia subduction zone
*Fracture zone
*Geology
*Sedimentary basin
*List of tectonic plates
*List of tectonic plate interactions
*Metamorphism
*Plate Tectonics
*Triple junction
*Tsunamis

References