Taipei, September 21, 1999.FAULT LINES AND EARTHQUAKES
FaultsCracks in the earth’s crust along whichmovement takes place.
There are threedifferent kinds of faults!
Dip Slip FaultAt diverging boundaries, the force stretches thecrust. When the stress is too much,the rock breaks and one block moves down a slopingcrack. This is called a dip-slip fault.
Reverse Fault At a convergent boundaries the crust is compressed. When the stress is too much the rock breaks and one block moves up a sloping crack. This is called a reverse fault.
At transform boundaries the crust grinds against itself asplates move horizontally past each other. When pressurebuilds and then releases, it is called a strike-slip fault.Faults in action
EarthquakesA violent shaking of the Earth’s crust causedby the release of built up energy at a plateboundary. What are earthquakes?
Earthquakes happen when stress builds, platespush, pull and strain against each other. Whenplates slip or the lithosphere breaks, there is asudden jolt of energy that moves through the crust.These waves of energy are called seismic waves.
Plate MovementSeismic Waves Epicenter The point on the surface directly above the focus. The seismic waves are the most intense at this point because of their close proximity to the focus. FOCUS The point underground where the faulting occurs There are three types of seismic waves – P, and S waves (body waves) and surface waves.
Surface waves, theThe initial jolt comes slowest wave, move The slower and strongerfrom a “P” or secondary or shear wave close to or on theprimary wave. It arrives after the “P” surface. The Raleightravels in a fast wave, shaking the ground wave shown abovelongtitudal fashion. in a crosswise and vertical has similarThey can pass motion. “S” waves properties to anthrough both solid cannot travel through the ocean wave and can outer core because they cause severeand liquid rock. They cannot exist in molten damage to largerare generally felt as a structures. Surfacebang or thump rock. waves
How do scientist measure seismic waves?Scientists use an instrument called aseismograph.How do scientist express an earthquake’s size ormagnitude?Scientist use a scale called the Richter Scale. An earthquake measuring 7.5 on the Richter Scale is a major earthquake.
The September 21, 1999 earthquake that hit Taiwan measured 7.6on the Richter Scale. The epicenter was in Nantou County incentral Taiwan, 150 km south of Taipei. The whole island sufferedcatastrophic damage.More information on 921 EarthquakeSome more information on the 921 earthquake