1985 Mexico City Earthquake

      At 7:18 in the morning Mexico city got a 8.1 magnitude earthquake, one of the strongest to ever hit the area. They effects were very bad because Mexico city is surrounded by mountains and volcanoes. The region was covered in lakes in ancient times to the city is on dirt and sand instead of bed rock and is very volatile during an earthquake. The earthquakes epicenter was located 50 km off the coast of mexico and 250 miles west of the city but unstable ground worsen the effects. Old hotels and factories made out of shoddy materials collapsed quickly and the tremors caused gas mains to break causing fires and explosions through out the city.
The mercalli intensity was IX (violent) and killed 10,000, injured 50,000 and made 250,000 people homeless. the average of 66-242,000 buildings and homes were destroyed in the earthquake. An aftershock hit on September 20 and had a magnitude on 7.5. The cost of the damage was estimated to be around 5 billion dollars.


    Coco's plate subducton zone slid under the north american plate. Volatile trenches along the Coco's plate generally have had seismic events 30 to 70 years before 1985. However this particular section of the subduction zone had not had an event for a much longer time. Off the coast of Michoacan and Guerrero the earthquake caused a rupture in the sea bed 240 km long and 70 km wide, located between the subduction trench and the coast line. This is an inter tidal zone and the event caused widespread mortality in a number of species living in the area such as algae and shell fish.

MEXICO'S MANY PLATE TECTONICS

    Mexico is located on one of earths subduction zones, where the ocean floor of the Cocos tectonic plate is sub-ducting beneath the continental plate of the north american plate. Subduction zones get earthquakes and sometimes they stick together (locking), the lower plate pulling the upper plate down with it and therefore causing stress to build up. When the stress builds up to the breaking point, the upper plate breaks free and springs back to where it was before. The lower plate moves very slowly, only around 6 cm per year which is as fast as finger nails grow. It takes many hundreds of years for stress to build up before causing a large earthquake.

      Most of Mexico is on the north american plate but is also on several other plates. The Baja California Peninsula is on the gigantic pacific plate, which is moving northwest and under the north american plate. The intersection of these plates under the Gulf of California causes parallel faults which are part of the famous San Andreas Fault system.
     The small Rivera plate, between Puerto Vallorta and the southern tip of Baja California is moving in a southeast direction and rubbing against the pacific plate  it is also moving under the north american plate. The Coco's plate and tiny Orozco plate are ocean crust plates located off the south coast of Mexico. The collision of the Coco's plate and the north american plate had caused the disastrous 1985 earthquakes. ( Mexico is one of the most seismological active regions on earth. The motion of these plates causes earthquakes and volcanic activity.) Mexico is being carried northwest by the Coco's plate. The ocean floor is fairly dense, when it strikes the lighter granite of the Mexican land mass, the ocean floor id forced under the land mass, creating the deep middle american trench that is at Mexico's southern coast.
         The westward moving land a top the north american plate is slowed and crumpled where it meets the Coco's plate, creating the mountain ranges of southern Mexico. The subduction of the Coco's plate accounts for the frequency of earthquakes near mexico's southern coast. As the rocks on the ocean floor are forced down, they melt and the molten material is forced up through weaknesses in the surface rock, creating the volcanoes in the cordillera Neovalcanica across central Mexico. Motion along the San Andreas Fault in the past pulled Baja California away from the coast, creating the Gulf of California. Continued motion along this fault is the source of earthquakes in western Mexico. September 1985 Mexico city was hit with a Richter scale of 8.1 magnitude earthquake from the subduction zone off Accpulco and killed 4,000 people in the city and Volcan de Calima, south of Guadalajara, erupted in 1994.

   The Impact of Fishing

  Fishermen- if fish stocks crash then fishermen will be out of a job. 

Consumers- Fish costs will rise and then only the rich can afford to eat fish. people will lose an important part of their diet.

Government- fishing is economically important and if the fish stalks crash then the economy will suffer. Also the fishermen will have no work and the government will have to give out welfare to them and find jobs for them.

The environment- over fishing has killed many species of fish and has disrupted the equilibrium of the ocean. fish can not repopulate faster then the rate of fishing to the population of fish keeps on lowering. As certain species are hunted down other fish that eat those fish will lose a very important part of there diet and die as well.

Mexico's fishing industry

     The shallow waters of the pacific coast  and large amount of plankton in waters cooled by the Californian current makes for good fishing in the northwest. Together Sinaloa (23%) and Sonora (22%) make for about 45% of the national total. Fishing economically important in Veracruz (8%) and Baja California (6%). 72% of the total annual catch of 1.5 million metric tons is landed at pacific coast ports such as Guaymas, Mazatlan, Tampico. and Campeche ( golf coast parts) account for a further 25% each. The last 3% is in inland lakes and rivers and fish farms.

    value wise the most important species are shrimp, tuna, and sardines. Fresh water fish farms are becoming more common, many are specializing in the production of high value species like trout and indigenous white fish.

    At the sea of cartes ( gulf of California) is one of the worlds top five seas in terms of ecological productivity and biological diversity. In the 1990's a long line fishing fleet began to operate off of Ensenada. A single long line boat may have 5 km of line with 600 to 700 baited hooks in total. The sword fish population outside of the 80 km protected zone were quickly depleted, leading fishing boat owners to apply for permits to catch shark inside the 80 km limits. Decades of commercial over fishing are causing total collapse of fishing stalks. As late as 1993 the area less then 5% of all Mexico's territorial waters produced about 75% of the nations total fish catch of 1.5 million tons has declined the fish population by 90% since then. 

Mining

The pros and cons of strip mining and open pit mining

Strip mining

PROS- Mine is largely hidden under ground so surface land can easily be used for other uses
         -  A process called ' back filling ' reduces cost of hauling and helps avoid fall of rock bursts.

Cons- Higher risks of cave ins
        - higher risk of poisonous gas release
        - AMD sulfide rocks are exposed to to surface air and water and give off acidic chemicals that drain into near by streams.
         - less safe

Open Pit Mining

PROS- more efficient
          - sampling each level of open pit mining is easier for surveyors to find the highest yield and avoid safety hazards
           - can use larger trucks so can increase the yield per day.
           - efficiency improvements reduce costs
            - more safe

CONS- Very large environmental costs because vegetation is wiped out and lots of pollution


Spouse of a miner - choose the open pit mine because it is much safer then strip mining

Owner of mining company- choose open pit mining because it costs less and there is a higher yield and 40 percent of mining is on the surface

Enviomentalist- choose strip mining because it does not distroy surface life and there is a nuetralizer for the AMD.

Owner of near by ski resort- choose strip mining because it is not as visible from the surface so it will not get ride of costumers

Politician- choose open pit mining because it costs less and there is a higher yield for shipping.



Mexico mining

  Mexico is the worlds leading world producer of silver and produces one fith to one third of the whole worlds silver