What are earthquakes ? Why do they occur? Where do they occur?
Where do Earthquakes Occur? Earthquakes can occur near the Earth’s surface or far below the surface. Most earthquakes occur at plate boundaries But some happen at faults located in the middle of tectonic plates.
What Causes Earthquakes ? Earthquakes are caused when STRESS FORCES build along faults and then are suddenly released. Tension forces / Tensile forces – either side of a fault is pulling away from each other. This happens at divergent plate boundaries. Compression forces – either side of a fault is pressing against each other. This happens at convergent plate boundaries. Shearing forces – either side of a fault is sliding past each other. This happens at transform boundaries.
STRESS CAUSES ROCK TO CHANGE Elastic Deformation Rock folds and stretches until it deforms. Rock is stretched and bent until it can no longer take the stress. EARTHQUAKE! Elastic Rebound No longer under stress, the rock returns to its original shape
The New Madrid Fault Under the Mississippi??!! The New Madrid Fault Zone is part of an ancient plate boundary. The New Madrid Fault Zone is part of an ancient plate boundary. In this area, the North American Plate tried to form a divergent plate boundary about 500 million years ago. The splitting stopped before new plates could form. In this area, the North American Plate tried to form a divergent plate boundary about 500 million years ago. The splitting stopped before new plates could form. Earthquakes occur because the North American Plate is still "settling down". Earthquakes occur because the North American Plate is still "settling down". The faults in the New Madrid Zone do not reach the Earth’s surface but are connected to other faults closer to home! The faults in the New Madrid Zone do not reach the Earth’s surface but are connected to other faults closer to home!
East Coast Fault and Ramapo Fault Systems The ancient Ramapo Fault System --- runs through the eastern part of the PA. Smaller modern faults in Pennsylvania and along the Ramapo Fault Line are forming and becoming active!
Chapter 8: Sections 1: Earthquakes and Faults: Organizer PLATE MOTIONFAULT TYPE Transform Strike –Slip Fault Convergent Reverse Fault Divergent Normal Fault Plates move past each other Plates move together Plates move apart Fault blocks move past each other Fault blocks move together Fault blocks move apart
How do Earthquake Waves Travel? Energy released from moving plates and faults Travels through the Earth in Seismic waves.
Types of Seismic Waves 1. BODY WAVES: Seismic waves that travel through the Earth. 2. SURFACE WAVES: Seismic waves that travel along the surface. WAVES TRAVEL: WAVES TRAVEL: 1.At different speeds and 2 In different ways Depending on what kind of material they are moving through. Depending on what kind of material they are moving through.
Body Waves There are two types of body waves: –P-waves –S-waves
P-waves P-waves are pressure waves. P-waves travel through solid, liquids and gases. P-waves are the fastest waves. P-waves are the first waves to be detected and the first waves to arrive. P-waves are also called primary waves because they arrive first. P-waves cause rock to squeeze and stretch. Imagine a slinky! The area of the Earth that does not receive seismic energy is called the shadow zone.
2. S-waves S-waves are the second fastest waves. S-waves are also called secondary waves because they arrive second. S-waves are slower than P-waves. S-waves move rock from side to side. S-waves can’t travel through parts of the Earth that are completely liquid.
Shadow Zone The shadow zone results from S waves being stopped entirely by the liquid core and P waves being bent (refracted) by the liquid core.
Moho Zone The Moho is the boundary between the crust and the mantle in the earth. This is a depth where seismic waves usually increase velocity, or speed and direction.
Quiz 1. Most Earthquakes occur along ________________. 2. The first seismic waves to arrive are______________. 3. The second seismic waves to arrive are _____________. 4. The last seismic waves to arrive are_______________. 5. Which seismic waves travel the fastest?___________ 6. Which type of seismic wave can move through a solid, liquid or a gas?________________ 7. Which seismic wave cannot travel through material that is completely liquid?______________ 8. Which seismic waves are the slowest and the most destructive?_______________
Seismograph A seismograph is an instrument used by scientists to measure earthquakes – determines the strength and location! Ancient Chinese Seismograph. The ball would drop from the dragon to the frog. It told the people which direction the earthquake come from.
Seismograms Time in Minutes 0 1 2 3 4 5 6 7 8 1. How many minutes did it take for the P-Waves to arrive? 2. How many minutes did it take for the S-waves to arrive? 3. How long did the surface waves last?
Epicenter The epicenter is the point on the Earth’s surface directly above an earthquake’s starting point.
Focus The focus is the point inside the Earth where the earthquake begins. The epicenter is located directly above the focus.
Measuring Earthquakes There are two major ways earthquakes are measured: 1.Richter Scale 2. Mercalli Scale
Richter Scale Richter Scale measures magnitude or energy released by an earthquake Richter Scale measures magnitude or energy released by an earthquake Less than 3.5-Generally not felt, but recorded. 3.5-5.4- Often felt, but rarely causes damage. Under 6.0-At most slight damage to well-designed buildings. Can cause major damage to poorly constructed buildings Can cause major damage to poorly constructed buildings 6.1-6.9- Can be destructive in areas up to about 100 kilometers across where people live. 7.0-7.9- Major earthquake. Can cause serious damage over larger areas. 8 or greater- Great earthquake. Can cause serious damage in areas several hundred kilometers across.