Earthquakes
Earthquakes• An earthquake is the shaking and
trembling that results from the movement of rock beneath Earth’s surface.
• Not all earthquakes occur at plate boundaries. Sometimes they happen in the middle of a tectonic plate.
• Earthquakes can happen both near the Earth’s surface or far below it.
• Earthquakes always begin in rock below the surface.
• Most earthquakes begin in the lithosphere within 100 kilometers of the surface.
• The focus is the point beneath Earth’s surface where rock that is under stress breaks, triggering an earthquake.
• The point on the surface directly above the focus is called the epicenter.
Seismic Waves• Seismic waves are vibrations that travel through Earth
carrying the energy released during an earthquake.
• Seismic waves that travel through the Earth are called body waves.
• Seismic waves that travel along Earth’s surface are called surface waves.
• Each type of seismic waves travels through Earth’s layers in a different way and at a different speed.
Body Waves
• There are two types of body waves:– P waves– S waves
P-waves
• The first waves detected in an earthquake are p waves, or pressure waves.
• P waves compress and expand the ground like an accordion.
• P waves can travel through solids, liquids, or gases.
Click on picture to view animation
after reading the notes
S waves
• After P waves come secondary waves, or S waves. • S waves are earthquake waves that vibrate from side
to side and thrust the ground up and down, or back and forth.
• When S waves reach the surface, they shake structures violently.
• S waves cannot move through liquids.
Click on picture to view animation
after reading the notes
Surface Waves• When P and S waves
reach the surface, some of them are transformed into surface waves.
• Surface waves move more slowly than P and S waves, but they produce the most severe ground movements.
• They can actually make the ground roll like ocean waves.
• Other surface waves shake the ground from side to side.
• Geologists use a seismograph to record and measure the vibrations of seismic waves.
Seismograph
Seismogram
Detecting Seismic Waves
• When the waves reach a seismograph, the instruments creates a seismogram.
• A seismogram is a tracing of earthquake motion.
• Until recently, scientists used mechanical seismographs, like the one in the picture.
• Today they use electronic seismographs that convert ground movements into a signal that can be recorded and printed.
Finding the Epicenter• Scientists use seismograms to find the earthquakes epicenter.• One method they use is called the S-P Time Method. • They collect readings for the same earthquake from
seismographs stations at different locations.
• They then use this data to determine the distance each station is from the earthquake.
• They can then triangulate the results to find the epicenter.
• It takes a minimum of 3 seismograph readings to find the epicenter of and earthquake.
Richter Scale• There are many ways that scientists can measure an earthquake.
• Magnitude is a measurement of earthquake strength based on seismic waves and movement along faults.
• Charles Richter created the Richter magnitude scale in the 1930s to compare earthquakes by measuring ground motion and adjusting for distance to find their strength.
• When magnitude increases by one unit the measured ground motion becomes 10 times larger on the Richter scale.
• The Richter scale provides accurate measurements for small, nearby earthquakes, but the scale does not work well for large, or distant earthquakes.
Richter Scale no.
No.of earthquakes per
year
Typical effects of this magnitude
< ¾ 800,000 Detected only by seismometers
3.5 - 4.2 30,000 Just about noticeable indoors4.3 - 4.8 4.800 Most people notice the, windows rattle.4.9 – 5.4 1,400 Everyone notices the, dishes may break,
open doors swing.5.5 – 6.1 500 Slight damage to buildings, plaster cracks,
bricks fall.6.2 – 6.9 100 Much damage to buildings; chimneys fall,
houses move on foundations.7.0 – 7.3 15 Serious damage; bridges twist, walls
fracture, buildings may collapse.7.4 – 7.9 4 Great damage, most buildings collapse.
> 8/0 One every 5 to 10 years
Total damage, surface waves seen, objects thrown in the air.
Mercalli Scale
• Seismologists can also measure the intensity of an earthquake.
• The Mercalli scale uses Roman numbers from I to XII to describe increasing earth quake intensity levels.
• The Modified Mercalli Intensity Scale is used to measure the degree to which an earthquake is felt by people and the amount of damage done by it.
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