# Earthquakes Chapter 19.

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Earthquakes Chapter 19

Earthquakes Natural vibrations of the ground caused by strain along faults in Earth’s crust Faults: the fracture or system of fractures along which movement occurs San Andreas Fault: famous U.S. fault line (California) N.American plate sliding past Pacific plate

Types of Faults Reverse Normal Strike-slip
From compression (convergent boundaries) Normal From tension (divergent boundaries) Strike-slip From shear (transform boundaries)

Types of Faults Reverse Fault Strike-slip Fault Normal Fault

Elastic Rebound Theory
When two plate slide past each other, they eventually ‘snag’ on each other and begin to pull. The ground can only ‘stretch’ so far and when that snag fails, the ground snaps back into place (like a rubber band)

Seismic Waves All Earthquakes release seismic waves
3 types of seismic waves: P-Waves - primary, travel through all material, fastest, back & forth motion S-Waves - secondary, travel through solids only, 2nd fastest, move up & down L-Waves - surface, travel along surface, slowest, move back and forth AND up and down

Seismic Waves P-waves: S-waves:
S-waves: L-waves:

Seismic Waves Used to determine the Earth’s layers
These waves move through Earth & will only pass through certain materials - tells scientists what’s inside the Earth

Locating Earthquakes Seismometer: machine that picks up Earthquake waves Because seismic waves travel at different speeds, they arrive at a seismometer at different times Difference between P and S waves can tell how far away an earthquake occurred

Locating Earthquakes Information from 3 seismometers is needed to plot where an earthquake occurred (triangulation) Focus: point inside the Earth where the EQ begins/starts Epicenter: point on the surface of Earth directly above the focus - reported in the news as the location of the EQ

Locating Earthquakes

Measuring Earthquakes
Richter Scale: measures Earthquake magnitude (energy) Scale from 1-10 Strongest: 9.5 in Chile (1950) Mercalli Scale: measures Earthquake intensity (damage) Scale from I - XII Used by insurance companies

Where do Earthquakes Occur?
Pacific Ring of Fire - 80% Eurasian Belt - 15% Mid-Ocean Ridges - 5% United States Most occur on West coast Strongest Quake: St.Louis (1813)

Earthquake Hazards Buildings fall, glass breaks, roads crack, bridges fall Slope failure (landslides) Tsunami Safest place to be: Away from buildings In a doorway

Earthquake Prediction
Can we predict Earthquakes? NO Best scientists can do is to say they think one may occur in the next 100 years California: Parkfield experiences a magnitude 6 earthquake every years - last one was Sept.2004

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