Expert: Keith Patton - 1/16/2006

Question
Dear sir:
we are a group of geology teachers following a Professional Development course .We would be happy if you answer our questions regarding the subject we teach .
1- how does the eartheqick takeplace?
2-mentioning three of the most destrictive erthequicks in the history of humanity
3- what are the most recent tools to know about erthquicks before they strike  

Answer
Ismaiel:

Earthquakes occur when movement takes place along one of three types of faults.

There are normal faults where one block moves up the other down.  There are reverse faults where one block is thrust up over the other.  the third kind is transverse or strike slip, where the movement, is along the fault, where if a road cut across the fault it would be offset, _ - like that. You can illustrate this with your hands placed flat on a table next to each other.  The seam between your hands is the fault.  Now push one up and over the other so that your index fingers overlap, that is the reverse fault.  Lift one hand and rest the edge of the index finger just on the edge of the other with no overlap, that is a normal fault, the keeping them flat, push one ahead of the other, that is strike slip fault, movement is along the fault.

Earthquakes result when this movement occurs in rocks and the earth shakes.  It is made worse if there is a deep layer of sediment or unlithified material over the rock that is moving.  If the material contains a lot of water it is made even worse, as the waterlogged earth shakes like jello or pudding.  This jiggling is what destroys buildings.  Buildings are built to withstand vertical compressional forces, e.g. their own weight, not laterial stress such as occurs when the ground under then moves from side to side.  This is why houses made of rock, mud or brick collapse.

Notable earthquakes were the 1906 San Francisco Earthquake, the huge quake in Japan that leveled Tokyo in the 1920's, the New Madrid fault in Missouri, it caused the Mississippi river to change course.  Other large quakes have occured in Turkey, and Pakistan.

We can now use lasers to sense very small movements along faults.  Seismographs can measure small quakes, and when their frequency increase, it means stress along the fault might be increasing and the likelihood that a large quake might be imminent becomes greater.  There are also studies to learn animal behavior prior to quakes as it is believed that they are sensitive to changes that we are not.  Particularly insects.

Keith