Measuring Earthquakes

Where Earthquakes Start

Earthquakes start beneath Earth's surface.

Seismogram

a record of an earthquake's seismic waves that is produced by a seismograph. A seismogram appears as a pattern of lines.

Seismograph

an instrument that records the ground movements that seismic waves cause as they move through Earth. ~ Scientists use 2 types of seismographs - one type to measure horizontal (side to side) ground movement and one type to measure vertical (up and down) ground movement.

Surface Waves

are formed when P waves and S waves reach the surface of Earth. They move along Earth's surface instead of its interior. Surface waves produce the most severe ground movements and the most damage. They make the ground roll like ocean waves. Surface waves are the slowest moving waves.

Primary Waves (P waves)

are the1st waves to reach a location after an earthquake. They cause ground to compress and expand. P waves are the fastest moving waves and can travel through solid, liquid, and gas.

Secondary Waves (S waves)

reach a location after the P waves, even though they form at the same time as the P waves. S waves cause the ground to shake side to side or up and down. S waves make buildings rock back and forth. They travel at about ½ the speed of P waves. S waves can only travel through solids.

Focus

the area along a fault where rock begins to move. The focus is where an earthquake starts. The closer the focus is to the surface, the stronger the earthquake.

Epicenter

the point on Earth's surface directly above the focus.

Seismic Waves

vibrations that travel through the Earth carrying the energy released by an earthquake. Seismic waves travel outward from the earthquake's focus.

How Scientists Locate an Earthquake's Epicenter

~ Scientists try to determine the epicenter of an earthquake to help identify places where future earthquakes may occur. ~ Scientists calculate the difference in the arrival time of P waves (arrive 1st) and S waves (arrive 2nd) at 3 or more different seismograph stations. * The time differences are used to determine the distance of each seismograph from the epicenter. * On a map, the distances are plotted as circles around each station. * The epicenter is at the point where all of the circles meet.

Extra Seismogram Facts

~ Straight line - indicates no seismic activity (no earthquake). ~ Zigzag line - indicates seismic activity (an earthquake). ~ P waves (the fastest waves) are shown 1st, then S waves are shown. Surface waves (the slowest waves) are shown last. ~ Greater line height indicates a more severe quake or a quake that is closer to the seismograph station (where a seismograph is located). ~ Aftershocks are indicated by a set of shorter lines that come after the recording of an earthquake.

The Modified Mercali Scale

~ The Modified Mercali Scale rates the level of damage from an earthquake. The ratings are based on people's observations instead of instrument readings ~ The Modified Mercali Scale has 12 steps that describe the affects of an earthquake. The same earthquake can have different ratings in different locations because the earthquake causes different amounts of shaking in those locations. ~ The Modified Mercali Scale is useful in areas that do not have a lot of seismograph stations.

Moment Magnitude Scale

~ The Moment Magnitude Scale measures the magnitude (size) of an earthquake. ~ The Moment Magnitude Scale rates the total energy released by an earthquake. ~ The Moment Magnitude Scale uses data from seismographs and other sources. ~ The Moment Magnitude Scale accurately rates earthquakes of all sizes and at all distances from seismograph stations. ~ The Moment Magnitude Scale assigns a number rating (from 1 - 10) to an earthquake. The rating is based on the types & strength of seismic waves produced, the amount of movement at the fault, the strength of rocks broken when the fault slipped. ~ An increase of 1 number on the Moment Magnitude Scale means 32 times more energy was released by the earthquake.

The Richter Scale

~ The Richter Scale measures the magnitude (size) of an earthquake. ~ The Richter Scale rates small earthquakes based on the size of seismic waves recorded by seismographs. ~ The Richter Scale Is only accurate for small earthquakes near seismograph stations. It does not work for earthquakes that are large or occur far from a seismic station. ~ The Richter Scale assigns a number rating (from 1 - 10) to an earthquake. The rating is based on how fast the ground moves at a seismic station. ~ An increase of 1 number on the Richter Scale means 32 times more energy was released by the earthquake.