NSSS2.2

toc =2.2 Measuring Earthquakes=

Vocabulary

 * Focus**: The focus is the point beneath the surface where rock breaks because it is under stress and then triggers an earthquake.
 * Epicenter**: The point that is exactly above the focus is called the epicenter.
 * Seismic waves**: These waves are vibrations that travel through the earth at various speeds carrying the energy that is released by an earthquake.
 * P waves**: These waves are earthquake waves that compress and also expand the surface like an accordion.
 * S waves**: These waves are earthquakes waves which vibrate from side-to-side as well as up-and-down making a lot of damage are called S waves.
 * Surface waves**: These waves move more slowly than P waves and S waves but surface waves create the most severe ground movement.
 * Seismograph**: A seismograph is an instrument that records the ground movements that are caused by seismic waves as they travel through the Earth.
 * Magnitude**: The strength of an earthquake.
 * Mercalli scale**: This is a scale developed to rate earthquakes by their intensity.
 * Richter scale**: This is a scale developed to rate the size of the seismic waves.
 * Moment magnitude** **scale**: This is a rating system used to estimate the quantity of the energy that is released by an earthquake.

Introduction

 * Every day there are at least 8,000 earthquakes occurring everywhere.
 * Some are way too small to notice and some that we notice can create little or severe destruction.
 * Earthquakes begin in the lithosphere.
 * They begin within 100 kilometers of the Earth's surface.

Seismic Waves

 * Seismic waves are vibrations that travel through the earth at various speeds carrying the energy that is released by an earthquake.
 * They move in the form of ripples carrying the large amount of energy of the earthquake.
 * The energy of the seismic waves is the greatest at the epicenter.
 * The seismic waves carry the energy of the earthquake away from the focus.
 * There are three types of seismic waves:
 * P waves
 * Also known as primary waves
 * Compress and expand the surface like an accordion.
 * Move through solid and liquid
 * Arrive first during earthquake
 * S waves [[image:Nikhil.jpg width="279" height="156"]]
 * Also known as secondary waves
 * Arrive second in an earthquake and are very violent
 * Move from side to side as well as up and down
 * Cannot move through liquids
 * Surface Waves
 * Make the ground roll like an ocean wave or shake severely

Detecting Seismic Waves

 * In order to record seismic waves, scientists use seismographs.
 * Siesmographs are of two types
 * Mechanical seismographs which have a drum that vibrates when an eathrquake occurs and a stationary pen causes these vibrations o be recorded on paper
 * Electronic seismographs which convert seismic waves to a signal into a signal which can be recorded

Measuring Earthquakes

 * Over 20 different measures are used to rate earthquakes
 * The three most common rating systems are
 * The Mercalli scale
 * Developed in the early 1900's
 * Used to rate earthquakes based on their intensity
 * Not considered accurate,
 * Its 12 steps describe how the effects of the earthquake affect people
 * An earthquake can have different ratings because it can cause different amounts of damage at locations
 * The Richter Scale
 * Rates based on the size of the seismic waves
 * Developed in the 1930's
 * Moved from the manual seismograph to the electric seismograph
 * Accurately measures small and close-by earthquakes
 * The Moment Magnitude scale
 * Can accurately measure a small, distant and earthquakes of all sizes
 * Geologists combine the strength of the rock and how much movement occurred on the fault

Locating the Epicenter

 * The difference between the arrival of P waves and S waves is indicative of hte distance of the epicenter from the seismograph
 * The farther away an earthquake, the greater the arrival time of P and S waves
 * At least 3 circles are drawn with 3 different seismographs as centers
 * The point of intersection of these circles is the epicenter.