Earthquakes and Seismic Waves

Earthquake – shaking and trebling that results from the movement of rock beneath Earth’s surface. Plate movement causes earthquakes from stress and faults in Earth’s crust. When the rock breaks earthquakes happen

Most Earthquakes start in the lithosphere about 100 kilometers of Earth’s surface. Focus – area beneath Earth’s surface where rock that is under stress breaks, causing an earthquake Epicenter – the point on the surface directly above the focus

Seismic waves are vibrations that travel through Earth carrying the energy released during an Earthquake Seismic waves move out from the focus in all directions through the interior and across the surface

3 main seismic waves P Waves – Primary waves First to arrive They compress and expand the ground Can damage buildings Travel through solid and liquid

S Waves – Secondary waves Seismic waves that vibrate from side to side as well as up and down Travel through solid not liquid

Surface Waves – when P waves and S waves reach the surface, some become surface waves. Move slower than P waves and S waves, produce severe ground movement Can make the ground roll like ocean waves

Measuring Earthquakes Mercalli Scale Rates Earthquakes by the level of damage at a given place 12 steps I-III people notice vibration IV-VI slight damage VII-IX Moderate to heavy damage (buildings off foundations or destroyed) X-XII Great destruction (cracks in the ground, waves seen on surface)

The Richter Scale Magnitude a number geologists assign based on the earthquake size Geologists measure seismic waves and fault movement with a Richter scale Seismic waves are measured by a seismograph More accurate for small, nearby earthquakes

Moment Magnitude Scale Rating system that estimates the total energy release Rates near or far earthquakes Use data from the seismographs, kind of seismic waves, how strong they were, movement along the fault and the strength of the rocks that broke

Locating the Epicenter Use seismic waves to locate an earthquake’s epicenter P waves arrive first, then S waves close behind They use the arrival time between the P waves and S waves, the farther away the greater the time between the arrival They then draw three circles using data from different seismographs, where they all intersect that is the epicenter.

News Report As a television news reporter, you are covering an earthquake rated between IV and V on the Mercalli scale. Write a short news story describing the earthquake’s effects. Your lead paragraph should tell who, what where, when and how. (The Mercalli Scale on page 55 Figure 9 will help)