Tuesday, May 18, 2010
"Anatomy of an Earthquake." (Part 3)
Earthquakes can cause fires by damaging electrical power or gas lines. In the event of water mains rupturing and loss of pressure, it may also become difficult to stop the spread of a fire once it has started. For example, more deaths in the 1906 San Francisco earthquake were caused by fire than by the earthquake itself.
Tsunamis are long-wavelength, long-period sea waves produced by the sudden or abrupt movement of large volumes of water. In the open ocean the distance between wave crests can surpass 100 kilometers (62 miles), and the wave periods can vary from five minutes to one hour. Such tsunamis travel 600-800 kilometers per hour (373-497 miles per hour), depending on water depth. Large waves produced by an earthquake or a submarine landslide can overrun nearby coastal areas in a matter of minutes. Tsunamis can also travel thousands of kilometers across open ocean and wreak destruction on far shores hours after the earthquake that generates them.
A flood is an overflow of any amount of water that reaches land. Flood occur usually when the volume of water within a body of water, such as river or lake, exceeds the total capacity of the formation of the body. However, floods may be secondary effects of earthquakes, if dams are damaged. Earthquakes may cause landslips to dam rivers, which then collapse and cause flood.
Earthquakes may lead to disease, lack of basic necessities, loss of life, higher insurance premiums, general property damage, road and bridge damage, and collapse or destabilization (potentially leading to future collapse) of buildings.
Earthquakes can also precede volcanic eruptions. The largest earthquake that has been measured was the 9.5 magnitude one in Chile in 1960.
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All tectonic plates have internal stress fields caused by their interactions with neighbouring plates and sedimentary loading or unloading (e.g. deglaciation). These stresses may be sufficient to cause failure along existing fault planes, giving rise to intraplate earthquakes.
Earthquakes often occur in volcanic regions and are caused there, both by tectonic faults and the movement of magma in volcanoes. Such eartquakes can serve as an early warning of volcanic eruptions, like during the Mount St. Helens eruption of 1980. Earthquake swarms can serve as markers for the location of the flowing magma throughout the volcanoes.
Most earthquakes form part of a sequence, related to each other in terms of location and time. Most earthquake clusters consist of small tremors which cause little to no damage, but there is a theory that earthquakes can recur in a regular pattern. There are around 500,000 earthquakes each year. 100,000 of these can actually be felt. Minor earthquakes occur nearly constantly around the world in places like California and Alaska in the U.S., as well as in Guatemala, Chile, Peru, Indonesia, Iran, Pakistan, the Azores in Portugal, Turkey, New Zealand, Greece, Italy and Japan, but earthquakes can occur almost anywhere, including New York City, London and Australia. Larger earthquakes occur less frequently.
While most earthquakes are caused by movement of the Earth´s tectonic plates, human activity can also produce earthquakes. Four main activities contribute to this phenomenon: construction large dams and buildings, drilling and injecting liquid into wells, and by coal mining and oil drilling. Most earthquakes form part of a sequence, related to each other in terms of location and time.
Shaking and ground rupture are the main effects created by earthquakes, principally resulting in more or less severe damage to buildings and other rigid structures. The severity of the local effects depends on the complex combination of the earthquake magnitude, the distance from the epicenter, and the local geological and geomorphological conditions, which may amplify or reduce wave propagation. The ground-shaking is measured by ground acceleration.
Ground rupture is a visible breaking and displacement of the Earth´s surface along the trace of the fault, which may be of the order of several metres in the case of major earthquakes. Ground rupture is a major risk for large engineering structures such as dams, bridges and nuclear power stations and requires careful mapping of existing faults to identify any likely to break the ground surface within the life of the structure.
Earthquakes, along with severe storms, volcanic activity, coastal wave attack, and wildfires, can produce slope instability leading to landslides, a major geological hazard. Landslide danger may persist while emergency personnel are attempting rescue.
Part 3 will follow...
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Monday, May 17, 2010
Recent severe earthquakes have occurred in the world like the ones in Haiti and Chile in 2010.
Contemporary depictions of earthquakes in films are variable in the manner in which they reflect human psychological reactions to the actual trauma that can be caused to directly afflicted families and their loved ones. Disaster mental health response research emphasizes the need to be aware of the different roles of loss of family and key community members, loss of home and familiar surroundings, loss of essential supplies and services to maintain survival. Particularly for children, the clear availability of care giving adults who are able to protect, nourish, and clothe them in the aftermath of the earthquake, and to help them make sense of what has befallen them has been shown to be even more important to their emotional and physical health than the simple giving of provisions. As was observed after other disasters involving destruction and loss of life and their media depictions, such as those of the 2001 World Trade Center Attacks or Hurricane Katrina, and has been recently observed in the 2010 Haiti and Chile Earthquakes.
An earthquake, a quake, tremor, temblor or seismic activity is the result of a sudden release of energy in the Eath´s crust that creates seismic waves. Earthquakes are measured with a seismometer; a device which also records is known as a seismograph. The moment magnitude (or the related and mostly obsolete Richter magnitude) of an earthquake is conventionally reported, with a magnitude 3 or lower earthquakes being mostly imperceptible, and magnitude 7 causing serious damage over large areas.
At the Earth´s surface, earthquakes manifest themselves by shaking and sometimes displacing the ground. When a large earthquake epicenter is located offshore, the seabed sometimes suffers sufficient displacement to cause a tsunami. The shaking in earthquakes can also trigger landslides and occasionally volcanic activity.
In its most generic sense, the word earthquake is used to describe any seismic even, whether a natural phenomenon or an event caused by humans that generates seismic waves. Earthquakes are caused mostly by rupture of geological faults, but also by volcanic activity, landslides, mine blasts, and nuclear experiments. An earthquake´s point of initial rupture is called its focus or hypocenter. The term epicenter refers to the point at ground level directly above the hypocenter.
Where plate boundaries occur within continental lithosphere, deformation is spread out over a much larger area than the plate boundary itself. In the case of the San Andreas fault continental transform, many earthquakes occur away from the plate boundary and are related to strains developed within the broader zone of deformation caused by major irregularities in the fault trace (e.g. the "Big bend" region).
Part 2 will follow...
Thanks for reading!!