Earthquakes

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. How many earthquakes have already occurred today?

PLATE BOUNDARIES

Earthquake Locations Around the World

What Causes Earthquakes? Earthquakes are caused by movement along faults. When stress is placed on rocks it deforms, or changes. This is called Elastic Deformation (remember faults and folds, tension and compression) Rock is stretched and bent until it can no longer take the stress. When enough stress builds up in the rock it, it slips and energy is released. The rock then returns to its original shape. This is called Elastic Rebound. Think of a rubber band, you can only stretch it so far until it breaks and return to its original shape. This energy is felt as an earthquake.

Elastic ELASTIC REBOUND

Earthquakes in Alabama?? Do we live near a plate boundary? Do we live near a fault?

Several of the largest earthquakes ever recorded in the United States occurred in the Midwest, far from any plate boundary. These earthquakes took place in an area called the New Madrid Fault Zone, named after the town of New Madrid, Missouri.

New Madrid Fault Over a three-month period in the winter of 1811 to 1812, the New Madrid Fault Zone was struck by three huge earthquakes estimated to be greater than magnitude 8.0 The New Madrid Fault is what causes Alabama to experience earthquakes.

The map shows the earthquakes recorded in Alabama since 1886.

3 Types of Faults associated with Earthquakes PLATE MOTION FAULT TYPE Transform Strike –Slip Fault Convergent Reverse Fault Divergent Normal Fault

Strike-Slip Fault occurs at a Transform Boundary

Reverse Fault occurs at a Convergent Boundary

Normal Faults occur at Divergent Boundaries

Chapter 8: Sections 1: Earthquakes and Faults: Organizer PLATE MOTION FAULT TYPE Transform Strike –Slip Fault Convergent Reverse Fault Divergent Normal Fault Plates move past each other Fault blocks move past each other Plates move together Fault blocks move together Plates move apart Fault blocks move apart

How do Earthquake Waves Travel? Energy released from moving plates and faults travels through the Earth as waves. These waves are called seismic waves.

Depending on what kind of material they are moving through. 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: 1.At different speeds and 2 In different ways 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.

1.P-Waves

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.

S-Waves

3. Surface Waves Surface waves move along the surface of the Earth. Surface waves can move up, down and around or side to side. Surface waves move the slowest and cause the most destruction.

Surface Waves

Seismic Wave Arrival First- P-waves Second- S-waves Third- Surface Waves

Seismograph A seismograph is an instrument used by scientists to measure earthquakes. Seismologists who study earthquakes can determine when an earthquake started by noting the arrival times of P-waves and S-waves. A seismograph records vibrations in the Earth and determines the strength and location of an earthquake. Ancient Chinese Seismograph. The ball would drop from the dragon to the frog. It told the people which direction the earthquake come from.

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: Richter Scale 2. Mercalli Scale

Richter Scale/Measures Magnitude The Richter Scale measures magnitude. The measurements are given in numbers. Measures the energy released by an earthquake Earthquake Severity-Energy released by an earthquake. Richter Earthquake Magnitudes Effects 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 over small regions. 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.

Mercalli Scale/Measures Intensity In seismology a scale of seismic intensity is a way of measuring or rating the effects of an earthquake at different sites. The Modified Mercalli Intensity Scale is commonly used in the United States by seismologists seeking information on the severity of earthquake effects. Intensity ratings are expressed as Roman numerals between I at the low end and XII at the high end.

I. People do not feel any Earth movement. II. A few people might notice movement if they are at rest and/or on the upper floors of tall buildings. III. Many people indoors feel movement. Hanging objects swing back and forth. People outdoors might not realize that an earthquake is occurring. IV. Most people indoors feel movement. Hanging objects swing. Dishes, windows, and doors rattle. A few people outdoors may feel movement. Parked cars rock. V. Almost everyone feels movement. Doors swing open or close. Dishes are broken. Pictures on the wall move. Small objects move or are turned over. VI. Everyone feels movement. People have trouble walking. Objects fall from shelves. Pictures fall off walls. Furniture moves. Plaster in walls might crack. Trees and bushes shake.

VII. People have difficulty standing. Drivers feel their cars shaking VII. People have difficulty standing. Drivers feel their cars shaking. Some furniture breaks. Loose bricks fall from buildings. Damage is slight to moderate in well-built buildings; considerable in poorly built buildings. VIII. Drivers have trouble steering. Houses that are not bolted down might shift on their foundations. Tall structures such as towers and chimneys might twist and fall. Well-built buildings suffer slight damage. Poorly built structures suffer severe damage. Tree branches break. Hillsides might crack if the ground is wet. IX. Well-built buildings suffer considerable damage. Houses that are not bolted down move off their foundations. Some underground pipes are broken. The ground cracks. X. Most buildings and their foundations are destroyed. Some bridges are destroyed. Dams are seriously damaged. Large landslides occur. Water is thrown on the banks of canals, rivers, lakes. The ground cracks in large areas. Railroad tracks are bent slightly. XI. Most buildings collapse. Some bridges are destroyed. Large cracks appear in the ground.. Railroad tracks are badly bent. XII. Almost everything is destroyed. Objects are thrown into the air. The ground moves in waves or ripples. Large amounts of rock may move. As you can see from the list above, rating the Intensity of an earthquake's effects does not require any instrumental measurements. Thus seismologists can use newspaper accounts, diaries, and other historical records to make intensity ratings of past earthquakes.

Mercalli Intensity Scale What is the intensity at Monterey? What is the intensity at the epicenter? What is the intensity at San Jose? What is the intensity at Santa Cruz? What is the intensity at Smith? Smith

Earthquakes in Alabama? Largest Earthquake in Alabama: 1916 South of Birmingham, in Irondale 5.1 on the Richter Scale VII (7) on Mercalli Scale

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?_______________