Sunday, 27 February 2011

10 Things You Didn't know about earthquakes - Part 1

Notes taken from the Iain Stewart video seen in my upper sixth classes this week:

Earthquakes are among the deadliest forces known to man, capable of releasing vast amounts of energy over a short period of time, crumbling buildings and turning solid ground into a mushy liquid that swallows houses whole. For the exam this summer you need toe able to comment on a range of earthquake case studies how we have approached management and how close we are to predicting the worst of these disasters.

This video took you through ten lesser known facts about earthquakes, mentioned some useful anecdotal evidence you can use in your essays and covered the surprising ways in which we have learned about this natural hazard.

1) Greece - how earthquakes affected our earliest civilizations

Ancient Greece is widely regarded as the birthplace of modern civilization, yet it sits upon the most earthquake prone region in Europe. Although today it is still littered with the remains of Classical Greece, many thousands more buildings have been lost either through natural weathering and decay or removed by antiquities hunters in the 18th century. By far the largest number have been lost to earthquakes, many of the most magnificent temples have been destroyed including the Temple of Apollo at Corinth, and the frequent seismicity led Aristotle o form the earliest "rational theory"on earthquakes (By rational we mean not based on the God Poseidon who liked to cause carnage and ruin). Aristotle likened the rumblings of the earth to the rumblings of the human stomach, and said earthquakes were caused by subterranean winds, much like trapped gas!

2) San Fransisco 1906 - the Birth of seismology

This was the worst earthquake in American history, a magnitude 7.9 struck, lasting 45 seconds, enough to nearly flatten most of American Boom Town built on the Gold industry (which is a by product of tectonic activity and rivers - the concentration of rare earth materials). Out of a city of 400 000 inhabitants, 6000 were dead and 200 000 homeless, fires from ruptured gas mains raged for three days.

This earthquake had significant impacts on shaping America as we see it today, the rebuilding effort saw the diversion of trade and population from San Fransisco to Los Angeles, which is now the larger city, it was the first natural disaster in America to be both photographed and filmed and led to significant further study of the topic. San Fransisco was due to hold the Panama-Pacific International Exposition - a significant draw for future industry and investors, and as such the move to rebuild was rushed. By far the worst affected area in the 1989 quake was the Interstate 880, a dual tier motorway which collapsed killing 42 people.
The problem following the breach of the new building codes, was that it took until the 1950's for buildings rebuilt after the quake to reach the quality demanded by 1906 building codes. This remains a significant problem and it is estimated that a large earthquake would have a significant impact on many of the buildings from this era, and potentially cause many thousands of deaths.

One positive outcome of this was that Henry Fielding Reid, an American Geophysicist, formed a new scientific theory on earthquakes, this is the theory of elastic rebound and strain, and was the first to set out how rocks are strained along fault lines to a point at which they snap, releasing energy which is the cause of earthquakes. This theory remains one of the most important developments in tectonics and is studied world wide.

3) Paranoid Eureka Moments - Seismology and Nuclear Testing

During the height of the cold war and the nuclear arms race, America developed the means to remotely spy on Russian test sites for nuclear weapons. As the two nations developed nuclear capability, America's desire to know how large the bombs were led them to place over 120 seismometers worldwide to record the seismic shock traces left by weapons testing.

Novaya Zemlya was the designated Russian test site, in total 224 Nuclear detonations took place here, the largest of which registered as a 6.97 on the Richter scale and generated an 80 million tonne avalanche which blocked several streams and created a lake 2km long.
In total the tests at Novaya Zemyla held the explosive energy equivalent to 265 megatons of TNT.For comparison, all explosives used in the Second World War, including the detonations of two U.S. nuclear bombs, amounted to only two megatons.

The unexpected side effect of this remote spying, was that not only were all the Russian tests recorded, but the location of earthquake epicentres could also be located and plotted, and for the first time, earthquake foci were observed to follow straight lines, or late boundaries as we know them today. It is worth remembering that although Wegner proposed Continental Drift in 1908, we still lacked an understanding of the potential mechanism for movement, and this was still regarded as a fairly crazy theory until the early 60's.
The final thing the nuclear seismographs showed, was that the plates were moving in specific directions. One of the final pieces of evidence needed for continental drift to be accepted and linked to hazard management.

4) Why loose rock is dangerous.

The Shaanxi earthquake was the deadliest quake in history, for one reason - the rock type. Called Loess it is loose windblown sediment, common in China and a vital proxy for understanding climate change (dont worry thats in the next module). Wen shaken, it collapses, and in this region, it had been used to carve out cave dwellings for centuries. Over a million people died, and the damage spread as the L waves amplified through the soft sediments causing significant ground roll and destruction. It is worth noting that there are many other areas built on soft rock that have not experienced an earthquake in the recent past (ie since we have developed the region/urbanised).

5) Mexico's Tallest tower

Mexico City is built on the dried up bed of an old lake, first drained by Spanish Settlers. As such is it notoriously unstable, with foundations of loose, unconsolidated sediment, on a subduction zone, hence prone to large earthquakes, and located Right next door to the monstrous Popocatepetl. Yet, completed in 2003, the Torre Mayor is Mexico's tallest building, a towering 200m and 55 storeys high, it is one of the most high tech buildings in the word. A series of interlocking diamonds with colossal shock absorbers at the overlaps holds the structure in place, and should counteract the motion of the ground during an earthquake.

It is built on one of the most unstable parts of the city, the ground here liquefied in the 1985 quake,with the soft sediments amplified the waves and turned much of the ground to liquid. The video referenced in the documentary was the Niigita Earthquake of 1964, where liquefaction was first filmed

It has since been experienced at many locations world wide, notably last year in Christchurch, and this February as well, but there will be a separate post on this coming soon. So, it is possible to earthquake proof buildings, this is by no means the only example, the Taipei 101 tower is another, and there are many buildings in LA with shock absorbers or counteraction weights. It is also possible to retrofit older buildings to make them earthquake proof, however, the recent quake in Christchurch has demonstrated the fact that large aftershocks on already weakened buildings, even if they are retrofitted, can only withstand a certain amount of shaking.

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