Earthquakes and Prediction

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Seismology

This is the study of earthquakes. As with much of earth sciences there is alot of research being done on the structure of the earth and how it affects us. Much of what is known today about earthquakes and how they begin has been learnt within the last half of the century. So there are still some big questions to be answered, like how do you pridict an earthquake-evidently there is no easy answer.

There is a vested intrest in some countries to study earthquakes as they are life-threatning, a warning of half an hour could make enough difference to save many lifes.

Why?

The map above shows earthquakes (black dots). It also shows the different plates. Notice the concentration of quakes in the Pacific.

The areas where earthquakes occur are known as subduction zones. this where the denser, oceanic crust sinks beneath a continental plate. The friction caused by the crusts moving past one another and the friction of the plate being forced into the earth causes the earthquakes. The diagram below shows this (although is a little over complicated).

The dots on the subducted plate shows where the quakes form. The map showed that there where many quakes forming around the boundaries of the pacific especially next to Japan. This is the site of the worlds deepest trench caused by the subducting plate, the magma rising from the melting plate also makes this an active volcanic area. Quakes and volcanoes go hand in hand because they're formed by the same process, this is also why the area is also known as the Pacific Ring of Fire.

What causes the damage?

The elastic rebound theoryshows how rocks act plastically up to a certain point which differs for the type of rock, and then the rock snaps releasing the energy as waves.

There are two types of waves released-body waves and surface waves. As the names suggest the body waves travel through the earth while the surface waves travel at the surface of the crust and cause all the damage.

Colapsed subway and bridge in Japan.

The surface waves can also cause faults to become unstable and suddenly move as shown below, again from Japan.

Links to quake sites

There are hundreds of sites, alot are university departments (some of the best being from America because of the infamous San Andreas Fault!)

Earthquake prediction

Much money is spent on a fail safe method of earthquake detection but still none has been found. Much of the problem seems to be that earthquakes can't be directly observed so that quite possibly there are other reasons or contribting factors that are not being taken into account. However progress never seems to stop and there seems to be no limit on ideas. Unfortunately there are lies, damn lies and then there's statistics and earthquake prediction seems to rely to heavily on statistics. Knowing how litely a quake is isn't enough and so hard beaten earth scientists carry on manfully!

Ashtech technology -the company here is working in conjunction with scientists in America. The idea is to record the crustal deformation which occurs before an earthquake, when the crust is acting plastically. To dectect the deformation the system needs to be able to notice slight changes in movement of the crust over distances of at least 100 meters. The technology relies on GPS(global positioning system) which uses satilites to find an accurate possitioning. The first idea is to use two use two poles stuck in the ground and see if one moves relative to the other. But the idea being used currently is to put a pole deep into the crust and stick a GPS at the top of pole and see if the instruments record any movement in the pole from day to day. The key is here that this is an experiment not only earthquakes cause crustal deformation. The problem now is using it to predict earthquakes by finding which movements are significant.

Earthquake prediction -This link is a favourite of mine with quite a few usefull links and some books for further reading. Again research here relies on crustal deformation. The theories are do with the fact that when the crusts shape is being changed tiny fractures form in some rocks. The first is that when the cracks open up water is able to wash through the rock. Naturally occuring in rocks is radon gas which is water soluble so that when the rocks crack and water moves through the rock radon emmisions and there for radioactivity will increase. Another theory is that because of the water and because of the new fractures forming new electrical pathways the electrical resistance decreases in the rock, this has been recorded the difference is upto 15%. There is also the idea that when the rock fractures the water movement should have some effect on the water table so fluctuations should be recorded. The problem with all the idea is the fact that all these differences in the rock behaviour could happen with out the influence of an earthquake and so again it's being able to deduce when recordings are normal every day occurences.

Earthquake research breakthrough -the idea here is that earthquake happen in cycles. These cycles are 19 years, or there abouts, which is caused by the moon passing over some of the major faults and the gravity having an effect. I'm extremely sceptical of this theory and apart from anything else it does nothing to explain the more frquently daily quakes around the world. (check out the upto date site-you'd be amazed how many quakes are recorded daily)

Sites worth a look at-

Made April '97 by Paul Milton.