1. the eathquake What are earthquakes ? Why do they occur ? and Why can't we predict them ? Although we still can't predict when an earthquake will happen, we have learned much about earthquakes as well as the Earth itself from studying them. We have learned how to pinpoint the locations of earthquakes, how to accurately measure their sizes, and how to build flexible structures that can withstand the strong shaking produced by earthquakes and protect our loved ones. So press the Next Button and come learn more about Earthquakes......

2. What happen An earthquake is a sudden shaking of the ground. They generate seismic waves which can be recorded on a sensitive instrument called a seismograph. The record of ground shaking recorded by the seismograph is called a seismogram. This early eastern seismoscope consisted of a copper vessel with eight dragon heads attached to it, positioned above eight frogs. Each dragon head held a ball in its mouth, which, when dropped due to the strong shaking of an earthquake, would fall into the open mouth of the frog directly below it. By noting which frogs contained balls after a strong earthquake, it was possible to determine how the Earth had moved in response to the earthquake.

3. .then after. Earthquakes generate seismic waves which can be detected with a sensitive instrument called a seismograph. Advances in seismograph technology have increased our understanding of both earthquakes and the Earth itself. Perhaps the earliest seismograph was invented in China A.D. 136 by a m an named Choko.

4. After that wow By the end of the 19th century, several European inventors had constructed different seismographs. Most were electromagnetic and operated by suspending a magnetic mass, or pendulum, within an electric coil. Because a magnet moving inside a coil creates a current within the coil, the movement of the ground during an earthquake can be converted into an electrical signal. This signal could then be used to modify the projection of light onto photographic paper, or to move a needle across paper and trace out the wiggles of the Earth's shaking.

5. After this is coming.. Today's high-technology, digital seismographs record ground shaking over a large band of frequencies and seismic amplitudes. Today's seismometers are called broadband because they are able to sense ground motion over a wide range of frequencies, from thousands of seconds to less than a hundredth of a second. The amplitude of the signals recorded by old seismometers was limited by the amount of movement possible between the mass, or pendulum, and the seismometer housing. Today's seismometers operate by measuring the amount of electrical energy needed to keep the mass centered in the housing in the presence of strong ground shaking. Modern seismometers can record a wide range of seismic signals, both very small and very large.

6. More coming Beginning in the 1960s, significant strides were made in the study of earthquakes and the Earth's structure with the deployment of the World-Wide Seismographic Station Network (WWSSN). This network consists of over 120 seismographs in 60 countries. The seismographs were put in vaults or old mine shafts deep in the Earth in order to make sensitive recordings of seimic signals free from the "noise" created by cars and other environmental factors. This "global observatory" illustrates the international nature of global seismology and the essential cooperation in data exchange needed to study earthquakes. To better understand how and why earthquakes occur, we must understand the theory of Plate Tectonics.

7. The fact The Earth's outermost surface is broken into 12 rigid plates which are 60-200 km thick and float on top of a more fluid zone, much in the way that icebergs float on top of the ocean. Along the boundary separating any two plates, the relative motion between the plates can be classified into one of 3 categories: Click Below to find out more

8. The less but not last the end The movement between plates and along faults is not smooth. They move in jerks, giving rise to earthquakes. The locations of earthquakes throughout the world delineate the major tecton Faults are narrow zones in the Earth, usually extending no more than about 10 miles deep, which separate rigid crustal blocks. A well known fault is the San Andreas Fault which separates the Pacific plate from the North American plate. The Pacific plate has San Francisco and Los Angeles on it, while the North American plate contains the rest of California and the U.S. The Pacific plate is moving to the northwest at a rate of about 4 inches per year ic boundaries