These form by plates colliding; the denser of the two plates moves down through the asthenosphere and is then melted back into the mantle.
There are three types of destructive plate margins:
1. Continental – oceanic
The denser oceanic plate sinks or is subducted beneath the continental plate in a subduction zone . The continental plate is compressed to form a mountain range and deep ocean trench e.g. Peru-Chile trench and Andes
2. Oceanic – oceanic
When oceanic plates collide, a volcanic island arc is produced as the denser plate melts and magma rises to the surface. e.g. Kurile, Aleutian and Tonga islands
3. Continental – continental
If the oceanic crust is completely subducted the two remaining continental plates will collide to form fold mountains as the sediment on the old sea floor is compressed and uplifted.
This type of boundary has no volcanic activity and earthquakes are mainly shallow focus. E.g. Himalayas, formed by the collision of the Indian plate with the Eurasian plate.
http://earthquake.usgs.gov/eqcenter/recenteqsww/ DISTRIBUTION OF EARTHQUAKES .
The main zones of earthquakes are not randomly distributed but closely mark out the boundaries of the lithospheric plates . The world-wide earthquake locator
Most earthquakes result from movements along fractures = faults . Faults occur in zones which can vary from a metre to several km.
Movement occurs due to stresses created by crustal movement. The stresses build up until they become so great that the rocks shift suddenly along a fault.
The point of the break is the focus = hypocentre
If stresses are released in small stages there may be a series of small earthquakes.
Focus, origin, hypocentre, epicentre hypocentre The hypocentre is the point within the earth where an earthquake rupture starts. The epicentre is the point directly above it at the surface of the Earth. Also commonly termed the focus . epicentre The epicentre is the point on the earth's surface vertically above the hypocenter (or focus), point in the crust where a seismic rupture begins source
The compressional forces along a destructive margin cause crustal stresses.
Intermediate and deep earthquakes occur in a narrow zone indicating the subducting plate - this is called a Benioff Zone . These areas are subject to major earthquakes and represent areas of major hazard .
e.g. Alpine Himalayan chain. Shallow focus earthquakes occur in a relatively broad zone resulting in a high risk hazard. 1990 Iran Arabian plate collided with the Eurasian plate 35,000 killed, 100,000 injured 400,000 made homeless. 1988 Armenian earthquake resulted in 25,000 deaths
Lateral crustal movement in the continental regions producing; mainly shallow depth earthquakes e.g. San Andreas fault
About 15% of earthquakes occur in the relatively stable continental crust. These earthquakes are caused by stresses created in the crustal rocks which may be due to plate movement or stresses caused by other factors e.g. isostatic recoil.
Human activity has also been attributed to some intraplate earthquakes:
Subsidence associated with deep underground mining
the abstraction of underground water leading to sudden pressure changes.
1993 Killari India – reservoir and dam had recently been completed. Some seismologists suggest the weight of water in the reservoir and increase in the pressure of water in the pores of the rocks can lubricate a fault line so that it can move more easily.
Seismometers are used to record the different intervals of the waves to produce seismograms
Liquefaction is when a solid material turns into a liquefied state due to an increase in pore water pressures as a result of ground shaking during an earthquake. It affects unconsolidated sediments at depths of less that 10m which are saturated with water. These ground failures can destroy or severely damage building foundations and cause them to sink or collapse.
Structures such as bridges, dams and subsurface pipes will also be damaged
Sudden mass movements can result from causes other than earthquakes. The stress resulting from the ground shaking of an earthquake can result in slope failure on even gentle slopes.
Landslides, rock and snow avalanches can overrun people and structures, cause building damage or collapse, break underground pipes and disrupt rescue efforts by blocking roads. In many earthquakes the land sliding has caused as much or more damage than the ground shaking
Population density – high population densities lead to more potential victims
Strength of the earthquake – the stronger the earthquake the greater the damage
Nature and types of buildings – are they earthquake proof?
Time of day – where are people?
Distance from the epicentre – the closer the epicentre, the greater the damage and the less warning time there is
The type of rocks and sediments – loose unconsolidated materials are liable to liquefy
Secondary hazards such as mudslides and tsunami – often cause more damage that the earthquake itself.
7.4 3000 Turkey 1999 6.1 4000 Afghanistan 1998 7.2 5,477 Kobe, Japan 1995 6.6 57 Los Angeles 1994 6.5 10 000 Latur, India 1993 7.3 50 000 Iran 1990 7.1 67 San Francisco 1989 6.9 25 000 Armenia 1988 8.1 9 500 Mexico City 1985 7.6 240 000 Tangshan, China 1976 6.5 65 San Fernando USA 1971 7.8 66 000 Peru 1970 8.3 131 Alaska, USA 1964 7.9 156,000 Tokyo, Japan 1923 8.2 450 San Francisco 1906 RICHTER SCALE DEATHS PLACE YEAR 7.9 69,197 Sichuan, China 2008 7.6 19,000 Pakistan 2005 6.6 15,000 Bam 2003
Produce a fact sheet summarising the Sichuan Earthquake of May 2008
Both the primary and the secondary hazards associated with tectonic activity are more predictable and therefore less serious in MEDCs than in LEDCs. Examine the validity of this statement with reference to either earthquakes of volcanoes that you have studied
Distinguish between primary hazards (shaking ground) and secondary hazards (liquefaction, landslides, tsunamis, floods and fires)
The level of predictability depends on the monitoring technology available, financial resources, vulnerability and mitigation of effects. Risk perception, preparation and planning also play a significant role.
How serious the hazards are should be examined in terms of the injuries and loss of life that may result from tectonic activity and also in terms of economic damage and costs. While tectonic processes result in greater economic losses in MEDCs than in LEDCs the reverse is often true as far as loss of life is concerned.