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The earthquake I have chosen happened in China and it occurred in the Provence of Sichuan, the earthquake measured 7.9 magnitude on the Richter scale, this type of magnitude causes lots of structural damage and had very high death toll. 69,000 were killed missing or presumed dead, the epicentre Wenchaun which is North West of - Chengou the capital of Sichuan. There were aftershocks which measured 6 magnitude and caused further damage to an all ready devastated area, China has already seen many devastating earthquakes and has lots of tectonic zones which are divided into tectonic regions.3
What is an Earthquake?
An earthquake occurs when brittle rock is stressed and fractures- this is the epicentre, the break along a fault which already pre-exist weakened along the rocks, slips cause vibrations and seismic waves and depending on the magnitude can be very disruptive .When this happens the stress is reduced and the rock strength is returned this a cyclic event.1
Henry Field Reid explained what an earthquake was by using The Elastic Rebound Theory - The plates are being pushed and pulled apart by movement of the plates by convection current heat from the mantle. The rocks are pushed down by the weight of the rocks above causing friction which locks into the fault .The rocks are stretched and strained along the fault like an elastic band so if the pressure is released rocks become undeformed .Rocks have been displaced and the distance of the displacement is called a fault slip.1
http://www.uwsp.edu/geO/faculty/ozsvath/images/elastic_rebound.jpg Figure 1 Elastic Rebound Theory http://www.uwsp.edu/geO/faculty/ozsvath/images/elastic_rebound.jpg
Earthquakes can happen along major fault lines which already pre-exist and they can vary in direction -normal faulting forces that are pulled apart, reverse faulting forces that push the fault together, thrust fault is a reverse fault with a dipping fault, strike slip horizontal shearing forces the ground to move the ground left or right, transform faults different plates move past one other.1
http://rst.gsfc.nasa.gov/Sect2/FaultTypeX.jpgFigure 2 Types of Faults http://images.google.co.uk/imgres?imgurl=http://rst.gsfc.nasa.gov/Sect2/FaultTypeX.jpg&imgrefurl=http://rst.gsfc.nasa.gov/Sect2/Sect2_7.html&usg=__zMN3VKAOxfbU9nMLVmZhA9p2veE=&h=251&w=340&sz=55&hl=en&start=9&um=1&itbs=1&tbnid=GFPFk28njOI_BM:&tbnh=88&tbnw=119&prev=/images%3Fq%3Dtypes%2Bof%2Bearthquake%2Bfaults%26um%3D1%26hl%3Den%26sa%3DN%26rlz%3D1T4SMSN_enGB361GB361%26tbs%3Disch:1
There are many definitions to describe an earthquake: earthquakes themselves are the shaking of the earth underneath the ground and release of energy and the release of waves S and P waves that travel along the ground in all directions, foreshock- are small tremors which usually happen before a large earthquake this can be a warning of an earthquake and can occur days or weeks before the earthquake happens, aftershock- come after the earthquake and can occur days ,weeks and months after the event. Magnitude- is the release of energy by the earthquake and its strength and its effects can be measured using the Richter scale, micro earthquakes- these are earthquakes which can be described as small as there magnitudes is less than 2 on the Richter scale, Harmonic tremors- is a continual release of energy caused by a fault slipping.4
How are the Earthquakes Recorded?
The study of earthquakes is seismology and a seismometer is used to record the vibrations and the waves that are recorded on a seismograph, the seismometer are instruments which is either attached to the earth e.g. pole and bar is placed at the end attached to a spring, a weight is attached at the other end along with a pen which record any movement of the graph. The older techniques are now combined with more up to date methods; many of the functions have been separated into different types of measuring: semioscopes -give indications that the ground has moved. Modern equipment has electronic sensors and recording devices these seismoters record the frequency of the motion of the earthquake.2
http://www.cartage.org.lb/en/themes/Sciences/Earthscience/Geology/Earthquakes/MeasuringEarthquakes/Seismographs/seismograph.GIFFigure 3 Seismograph http://www.cartage.org.lb/en/themes/Sciences/Earthscience/Geology/Earthquakes/MeasuringEarthquakes/Seismographs/seismograph.GIF
There are two types of waves P and S they go through the centre of the earth which generates another wave at the surface, this is called surface waves. P waves can move through solid gaseous or liquid they move by compression and expansions, S waves are slower and cause displacement at right angles along the line of the wave. The surface waves only occur at the surface of the earth ,they roll along the ground and shakes the surface these waves are felt, measuring earthquakes can help decide the size of the seismic wave's strength and its potential to cause damage. The scale used to measure magnitude is the Richter Scale named after the Charles Richter 1935 a seismologist, this scale measures the distance from the seismograph to the fault as seismic waves move from the epicentre.1 &6
cartoon of particle motion and propagation of P and S waves
Figure 4 P and S waves
Plate Tectonic Movement and the earthquake modified the land
The plate tectonic movement was the Northeast thrust fault the earthquake occurred in North-western margin of the Sichuan basin it measured 7.9 on the Richter Scale, the earthquake happened on the Longmenshan (Dragons Gate Mountain) fault. It erupted 300 km along the fault and slipped 9m on the day of the earthquake, the stress increased along other points along this fault line Xianshuihe, Kunlun and Min Jiang.The tectonic stresses carried material from the crust and this converged at the Tibetan Plateau and moved West, the plate is under the Sichuan Basin and South East China. In Central and Eastern Asia there is a Northern Convergence of two plates, the Indian and the Eurasia Plate. 3
Figure5 Convergence of two plates http://www.prism.gatech.edu/~gh19/b1510/f1india.gif
The earthquake changed the land many buildings collapsed, landslides also occurred because of aftershocks and rain increased this causing further problems - Quake Lakes form when heavy rain has fallen and rivers are blocked from falling debris e.g. trees so when an aftershock occurs these can cause overflow causing more fatalities and further damage .There was an impact on the natural environment as many animals habitats were destroyed, this was because the quake happened in Minshan-Hengduan Mountains this area is very important as some endangered species live freely many reserves also suffered permanent damage.7
Properties were destroyed rubble from collapsed buildings caused a health and safety problem, because of where the debris was being dumped, some of the debris was from health centres and the material would have been hazardous. Clean drinking water was a problem as many of the water systems were contaminated due to ruptured pipes, landslides and dangerous material from factories. Rice fields have been damaged, because of landslides and farmland was used to build temporary residential areas for survivors.7
Figure 6 Landslide Damage Figure 7 Structural damage
Figure 6 http://1.bp.blogspot.com/_a2JvElU8gh4/Sc2iF_HKKMI/AAAAAAAABX4/JfGY62DeP6o/s400/09_03+rockslide+overview+with+damage.JPG
Figure 7 http://images.google.co.uk/imgres?imgurl=https://www.e-education.psu.edu/files/geog588/image/sichuan9.jpg&imgrefurl=https://www.e-education.psu.edu/geog588/book/export/html/1834&usg=__y3Vfl95oEX5Hh-0tCZe1X6JUhAU=&h=758&w=990&sz=230&hl=en&start=5&um=1&itbs=1&tbnid=9dd1co52b2tguM:&tbnh=114&tbnw=149&prev=/images%3Fq%3Ddamage%2Boccurred%2Bsichuan%2Bearthquake%26um%3D1%26hl%3Den%26rlz%3D1T4SMSN_enGB361GB361%26tbs%3Disch:1
Long and short impact on the economy
The economic lost was $86 billion and affected many areas of the local economy, the short term economic problem for this area of the local economy, tourism as this contributes 8% of tourism revenue in China, and Sichuan also relies on Hydropower this affected income because the earthquake affected the power supply as this disturbed communication and transportation in the Provence.9
The long term economical impacts the area needed $145.7 billion to rebuild the Provence of Sichuan to ensure the houses and buildings were built to strict codes of standard in the long term more money may be needed to ensure completion of the reconstruction work.9
What can be done to assess the risk of earthquakes in the Provence Sichuan?
To assess the risk of scientist must be able to establish a difference between a hazard and risk. Seismic hazard is the degree of shaking and how much the ground is disturbed what the effects are over a long time and a specific location. The hazard indicates how the active the fault is and how they cause earthquakes; these can be shown in seismic hazard maps. Seismic risk assessment is the amount of damage the earthquake could cause over a long period in a specific country or region. The seismic risk is also how the earthquake (seismic damage) affects the population and the infrastructure. To reduce further damage in the future there are measures which have helped hazard characterisation - this is the learning of the particular fault how it erupts, this can take decades to learn because they become characterised as new data becomes available every time the an earthquake occurs . Land use policies ensues infrastructure is not built on a land that is exposed to any seismic risk, the policy helps earthquake engineers construct buildings that can withstand the shaking from an earthquake .Engineers must also follow a building code which gives specifics in how the building must be built to withstand the expected shaking this information will be from seismic hazard assessment.1
http://neic.usgs.gov/neis/eq_depot/2008/eq_080512_ryan/neic_ryan_w.jpg Figure 8 Seismic Hazard Map http://neic.usgs.gov/neis/eq_depot/2008/eq_080512_ryan/neic_ryan_w.jpg
During my research this area the Longmenshan fault was not seen as danger because it had never shown any significant slipping, the geologists concentrated on Biechuan fault which had been more active especially Anning and Xiashui faults slipping more than 10mm per year9 .14th April 2010 four hundred miles from Sichuan an earthquake struck Qinghai , this earthquake killed at least 10,000 people and occurred on Yushu fault and measured 6.9 on the Richter Scale .This area is very active and more earthquakes will be seen in the future as the different faults release more stress.9&10