Earthquakes are the breaking and cracking of the rocks inside the continental plates. The breaks happen after stress has built up in the surrounding area. There are usually very few or no quakes when the plates move slowly. When there is a fast movement of the plate, there is a snap (like breaking a cracker).
BASIC EARTHQUAKE FACTS #1 (study jams)
Earthquakes occur when vast plates, or rocks, within the Earth suddenly break or shift under stress, sending shock waves rippling.·
Sudden movement along the fault causes the ground to shake.·
Most earthquakes occur along fractures in the Earth's crust called faults. ·
Intraplate quakes occur far from plate edges and happen when stress builds up and the Earth's crust is stretched or squeezed together until it rips.·
There are several different types of faults. Each can be a few inches or many hundreds of miles long. They can be horizontal, vertical, or at an angle.
Earthquake waves are measured on sensitive instruments called seismographs.·
The Richter scale assigns quakes a number based on the power of its seismic waves.·
Thousands of quakes occur every day around the globe, most of them too weak to be felt.·
Every year about 10,000 people, on average, die as a result of earthquakes.
Where Do Earthquakes Happen?
BASIC EARTHQUAKE FACTS #1 (study jams)
Earthquakes occur when vast plates, or rocks, within the Earth suddenly break or shift under stress, sending shock waves rippling.·
Sudden movement along the fault causes the ground to shake.·
Most earthquakes occur along fractures in the Earth's crust called faults. ·
Intraplate quakes occur far from plate edges and happen when stress builds up and the Earth's crust is stretched or squeezed together until it rips.·
There are several different types of faults. Each can be a few inches or many hundreds of miles long. They can be horizontal, vertical, or at an angle.
Earthquake waves are measured on sensitive instruments called seismographs.·
The Richter scale assigns quakes a number based on the power of its seismic waves.·
Thousands of quakes occur every day around the globe, most of them too weak to be felt.·
Every year about 10,000 people, on average, die as a result of earthquakes.
Where Do Earthquakes Happen?
Earthquakes occur all the time all over the world, both along plate edges and along faults.
Most earthquakes happen within the ‘Ring of
Fire’ around the Pacific Ocean. Alaska experiences a 7.0 earthquake at least
once a year. California gets about 10,000 quakes a year, but most are unnoticed
except by the seismograph machines.
It is thought that
there are as many as 500,000 earthquakes every year, although only 100,000 can
be felt at all and only about 100 of those actually cause any damage.
All
around the world people are using their better understanding of earthquakes to
design safer buildings and transport systems so, although we can’t do anything
to stop the Earth shaking, we can minimize the damage caused.
Along Plate Edges
Most earthquakes occur along the edge of the oceanic and continental plates. The earth's crust (the outer layer of the planet) is made up of several pieces, called plates. The plates under the oceans are called oceanic plates and the rest are continental plates. The plates are moved around by the motion of a deeper part of the earth (the mantle) that lies underneath the crust. These plates are always bumping into each other, pulling away from each other, or past each other. The plates usually move at about the same speed that your fingernails grow. Earthquakes usually occur where two plates are running into each other or sliding past each other.
Earthquakes can also occur far from the edges of plates, along faults. Faults are cracks in the earth where sections of a plate (or two plates) are moving in different directions. Faults are caused by all that bumping and sliding the plates do. They are more common near the edges of the plates.
Why Do Earthquakes Happen?
Earthquakes are usually caused when rock underground suddenly breaks along a fault. This sudden release of energy causes the seismic waves that make the ground shake. When two blocks of rock or two plates are rubbing against each other, they stick a little. They don't just slide smoothly; the rocks catch on each other. The rocks are still pushing against each other, but not moving. After a while, the rocks break because of all the pressure that's built up. When the rocks break, the earthquake occurs. During the earthquake and afterward, the plates or blocks of rock start moving, and they continue to move until they get stuck again. The spot underground where the rock breaks is called the focus of the earthquake. The place right above the focus (on top of the ground) is called the epicenter of the earthquake.
Earthquake-like seismic waves can also be caused by explosions underground. These explosions may be set off to break rock while making tunnels for roads, railroads, subways, or mines. These explosions, however, don't cause very strong seismic waves. You may not even feel them. Sometimes seismic waves occur when the roof or walls of a mine collapse. These can sometimes be felt by people near the mine. The largest underground explosions, from tests of nuclear warheads (bombs), can create seismic waves very much like large earthquakes. This fact has been exploited as a means to enforce the global nuclear test ban, because no nuclear warhead can be detonated on earth without producing such seismic waves.
More than buildings collapse when an earthquake hits. The land itself is totally changed. You can see scars across the landscape. Those scars appear when one block of land has moved compared to another. Roads often change their placement. They either become uneven or just crack. Streams can also change course. Sometimes rocks can fall and block the stream. Other times, the land is even lowered in certain areas. When it's lower, it's easier for the water to flow in the new direction.
Changes also happen on larger scales. Fault valleys and troughs can be created. These areas have large amounts of fracturing (the fracture zone is large). After the land has opened up, weather begins to act on the area and erosion follows. Slowly, new valleys are created.
More than buildings collapse when an earthquake hits. The land itself is totally changed. You can see scars across the landscape. Those scars appear when one block of land has moved compared to another. Roads often change their placement. They either become uneven or just crack. Streams can also change course. Sometimes rocks can fall and block the stream. Other times, the land is even lowered in certain areas. When it's lower, it's easier for the water to flow in the new direction.
Changes also happen on larger scales. Fault valleys and troughs can be created. These areas have large amounts of fracturing (the fracture zone is large). After the land has opened up, weather begins to act on the area and erosion follows. Slowly, new valleys are created.
Waves Across The Land
You might think, "If it only happens in one place, why can you feel them hundreds of miles away?" The crack happens where the quake starts. Ripples then move out in waves across the plate. Those waves are called seismic waves. Those waves shake everyone up within a specific area.
When scientists analyze an earthquake they look at several parts. They find out where the focus is. The focus is the exact point where the earthquake started. It is usually many miles/kilometers below the surface of the Earth. Scientists also look to see where the epicenter was. The epicenter is the point on the surface directly above the focus. When you think about the epicenter, try to remember there is actually a crack in the crust many kilometers below.
Primary Waves (P Waves)
Surface Waves
Several types, travel along the earth's surface or on layer boundaries in the earth. The slowest waves but the ones that do the damage in large earthquakes.
•A
type of seismic wave that moves the ground up and down or side to side
How do scientists calculate how far a location is from the epicenter of an earthquake?
Scientists calculate the difference between arrival times of the P waves and S waves
MODIFIED MERCALLI SCALE INTERACTIVE #1 - click on the graphic
Built For Quakes
After an earthquake, you can turn on the television and see all sorts of footage showing buildings that didn't quite survive. Buildings are built for up and down forces. If you jump on your floor a bunch of times your house will stay up. Earthquakes have forces that move side to side. Houses aren't built for that direction of energy. When the Earth moves side to side, the house can't wobble and it snaps. The walls collapse and roof falls in. Really tall skyscrapers are well designed for earthquakes. They are built to withstand strong winds. When strong winds happen, they can move side to side and sway if needed.
EARTHQUAKE - INTERACTIVE #2
You might think, "If it only happens in one place, why can you feel them hundreds of miles away?" The crack happens where the quake starts. Ripples then move out in waves across the plate. Those waves are called seismic waves. Those waves shake everyone up within a specific area.
When scientists analyze an earthquake they look at several parts. They find out where the focus is. The focus is the exact point where the earthquake started. It is usually many miles/kilometers below the surface of the Earth. Scientists also look to see where the epicenter was. The epicenter is the point on the surface directly above the focus. When you think about the epicenter, try to remember there is actually a crack in the crust many kilometers below. Elastic Rebound Theory
 | Here we have a landscape with a road, a fence, and a line of trees crossing a fault. As the crust moves, the rocks adjacent to the fault are deformed out of shape (in reality the deformation is spread across many kilometers - if it were this obvious, earthquake prediction would be easy). Eventually the rocks are so stretched out of shape that they cannot bear the stress any longer. The fault slips, and the stage is set for the next cycle of strain buildup and release. |
Epicenter and Focus
TYPE OF EARTHQUAKE WAVESPrimary Waves (P Waves)
•A
type of seismic wave that compresses and expands the ground
•The
first wave to arrive at an earthquake
Secondary
Waves (S Waves)
How do scientists calculate how far a location is from the epicenter of an earthquake?
Scientists calculate the difference between arrival times of the P waves and S waves
•The further away an earthquake is, the
greater the time between the arrival of the P waves and the S waves
How are Earthquakes Measured?
Richter
Scale
MODIFIED MERCALLI SCALE INTERACTIVE #1 - click on the graphic
EARTHQUAKE WAVES
Identify the P & S waves, Focus
Seismic Waves in the Earth
After an earthquake, you can turn on the television and see all sorts of footage showing buildings that didn't quite survive. Buildings are built for up and down forces. If you jump on your floor a bunch of times your house will stay up. Earthquakes have forces that move side to side. Houses aren't built for that direction of energy. When the Earth moves side to side, the house can't wobble and it snaps. The walls collapse and roof falls in. Really tall skyscrapers are well designed for earthquakes. They are built to withstand strong winds. When strong winds happen, they can move side to side and sway if needed.
EARTHQUAKE - INTERACTIVE #2
- complete the module to learn more about Earthquakes
- complete the Introduction & Simulation
| SAVAGE EARTH Animated Graphics: click on highlighted words & draw in your notes. Normal Fault Blind Thrust Fault Strike-Slip-Fault Dip-Slip Fault Mid-Ocean Ridge Subduction Primary Wave Secondary Wave Surface Wave Tsunami Spread |
WORDS TO KNOW:
focus
epicenter
amplification
divergent boundary
elastic rebound
foreshocks
liquefaction
normal fault
strike-slip fault
thrust fault
thrust fault
fault
focus
landslide
mantle
P wave
ocean spreading ridge
Richter scale
ring of fire
S wave
seismic wave
strike slip
subduction
transform fault
focus
landslide
mantle
P wave
ocean spreading ridge
Richter scale
ring of fire
S wave
seismic wave
strike slip
subduction
transform fault
Use the Earthquake glossary for terms & definitions
- some of terms may be explained in the notes, check the words in glossary
