FAULT LINES AND EARTHQUAKES

Taipei, September 21, 1999.

Cracks in the earth’s crust along which movement takes place.

Faults

There are three different kinds of faults!

At diverging boundaries, the force stretches the crust. When the stress is too much, the rock breaks and one block moves down a sloping crack. This is called a dip-slip fault.

Dip Slip Fault

Reverse FaultAt 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 as plates move horizontally past each other. When pressure builds and then releases, it is called a strike-slip fault.Faults in action

A violent shaking of the Earth’s crust caused by the release of built up energy at a plate boundary. What are earthquakes?

Earthquakes

Earthquakes happen when stress builds, plates push, pull and strain against each other. When plates slip or the lithosphere breaks, there is a sudden jolt of energy that moves through the crust. These waves of energy are called seismic waves.

FOCUS

The point underground where the faulting occurs

Seismic Waves

Plate Movement

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.

There are three types of seismic waves – P, and S waves (body waves) and surface waves.

The initial jolt comes from a “P” or primary wave. It travels in a fast longtitudal fashion. They can pass through both solid and liquid rock. They are generally felt as a bang or thump

The slower and stronger secondary or shear wave arrives after the “P” wave, shaking the ground in a crosswise and vertical motion. “S” waves cannot travel through the outer core because they cannot exist in molten rock.

Surface waves, the slowest wave, move close to or on the surface. The Raleigh wave shown above has similar properties to an ocean wave and can cause severe damage to larger structures. Surface waves

How do scientist measure seismic waves?Scientists use an instrument called a seismograph.How do scientist express an earthquake’s size or magnitude?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 7.6 on the Richter Scale. The epicenter was in Nantou County in central Taiwan, 150 km south of Taipei. The whole island suffered catastrophic damage.

More information on 921 EarthquakeSome more information on the 921 earthquake