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G 8 - Earthquakes


Introduction

An earthquake is sudden ground movement. This movement is caused by the sudden release of the energy stored in rocks. An earthquake happens when so much stress builds up in the rocks that the rocks break. An earthquake’s energy is transmitted by seismic waves. Each year, there are more than 150,000 earthquakes strong enough to be felt by people. An amazing 900,000 are recorded by seismometers.  

Geologic Structures
When plates are pushed or pulled, the rock is subjected to stress. Stress can cause a rock to change shape or to break. When a rock bends without breaking, it folds. When the rock breaks, it fractures. Mountain building and earthquakes are some of the responses rocks have to stress. 

Sedimentary rocks are formed in horizontal layers. This is magnificently displayed around the southwestern United States. The arid climate allows rock layers to be well exposed (Figure below). The lowest layers are the oldest and the higher layers are younger. 



Layers of different types of rocks are exposed in this photo from Grand Staircase-Escalante National Monument. White layers of limestone are hard and form cliffs. Red layers of shale are flakier and form slopes.

Folds

Deep within the Earth, as plates collide, rocks crumple into folds. You can model these folds by placing your hands on opposite edges of a piece of cloth and pushing your hands together. In sedimentary rocks, you can easily trace the folding of the layers (Figure below). Once rocks are folded, they do not return to their original shape.

There are three types of folds: monoclines, anticlines, and synclines. A monocline is a simple “one step“ bend in the rock layers (Figurebelow). In a monocline, the oldest rocks are still at the bottom and the youngest are at the top.

This is a geologic cross section of the Grand Staircase in Utah. The rock layers are no longer horizontal. They tilt downhill from right to left in a monocline. A small fold, called an anticline, is revealed at the left of the diagram.




An anticline is a fold that arches upward. The rocks dip away from the center of the fold (Figure below). The oldest rocks are found at the center of an anticline. The youngest rocks are draped over them at the top of the structure. When upward folding rocks form a circular structure, that structure is called a dome. If the top of the dome is eroded off, the oldest rocks are exposed at the center.




An anticline is a convex upward fold, as shown in (A). An anticline is well displayed in (B), which was taken at Capitol Reef National Park, Utah.

A syncline is a fold that bends downward (Figure below). In a syncline, the youngest rocks are at the center. The oldest rocks are at the outside edges. When rocks bend downward in a circular structure, it is called a basin. If the rocks are eroded, the youngest rocks are at the center. Basins can be enormous, like the Michigan Basin.



(A) A syncline is a concave downward fold. (B) This syncline is seen at Calico Ghost Town near Barstow, California.


Faults

With enough stress, a rock will fracture, or break. The fracture is called a joint if the rock breaks but doesn’t move, as shown in Figure below.



Joints in boulders in the Arizona desert. The rock on either side of the joints has not moved.

If the rocks on one or both sides of a fracture move, the fracture is called a fault (Figure below). Faults can occur alone or in clusters, creating a fault zone. Earthquakes happen when rocks break and move suddenly. The energy released causes an earthquake.



(A) This image shows at least one small fault. The white rock layer is not a line because a fault has broken it. Rock on each side of the fault has moved. (B) A large fault runs between the lighter colored rock on the left and the darker colored rock on the right.

Slip is the distance rocks move along a fault, as one block of rock moves past the other. The angle of a fault is called the fault’s “dip.“ If the fault dips at an angle, the fault is a dip-slip fault.

Imagine you are standing on a road looking at the fault. The hanging wall is the rock that overlies the fault, while the footwall is beneath the fault. If you are walking along a fault, the hanging wall is above you and the footwall is where your feet would be. Miners often extract mineral resources along faults. They used to hang their lanterns above their heads. That is why these layers were called the hanging wall.

In normal faults, the hanging wall drops down relative to the footwall. Normal faults are caused by tension that pulls the crust apart, causing the hanging wall to slide down. Normal faults can build huge mountain ranges in regions experiencing tension (Figure below).






The Teton Range in Wyoming rose up along a normal fault.

Reverse Fault. When compression squeezes the crust into a smaller space, the hanging wall pushes up relative to the footwall. This creates a reverse fault. Athrust fault is a type of reverse fault where the angle is nearly horizontal. Rocks can slip many miles along thrust faults (Figure below).


In this thrust fault, the rock on the left is thrust over the rock on the right.


Strike-Slip Fault

A strike-slip fault is a dip-slip fault where the dip of the fault plane is vertical. Strike-slip faults result from shear stresses. If you stand with one foot on each side of a strike-slip fault, one side will be moving toward you while the other side moves away from you. If your right foot moves toward you, the fault is known as a right-lateral strike-slip fault. If your left foot moves toward you, the fault is a left-lateral strike-slip fault (Figurebelow).




San Andreas Fault

The San Andreas Fault in California is a right-lateral strike slip fault (Figure below). It is also a transform fault because the San Andreas is a plate boundary. As you can see, California will not fall into the ocean someday. The land west of the San Andreas Fault is moving northeastward, while the North American plate moves southwest. Someday, millions of years from now, Los Angeles will be a suburb of San Francisco!


Causes of Earthquakes

Almost all earthquakes occur at plate boundaries. All types of plate boundaries have earthquakes. Convection within the Earth causes the plates to move. As the plates move, stresses build. When the stresses build too much, the rocks break. The break releases the energy that was stored in the rocks. The sudden release of energy creates an earthquake. During an earthquake the rocks usually move several centimeters or rarely as much as a few meters. Elastic rebound theory describes how earthquakes occur (Figure below).






Elastic rebound theory. Stresses build on both sides of a fault. The rocks deform plastically as seen in Time 2. When the stresses become too great, the rocks return to their original shape. To do this, the rocks move, as seen in Time 3. This movement releases energy, creating an earthquake.


Earthquake Focus and Epicenter

The point where the rock ruptures is the earthquake’s focus. The focus is below the Earth’s surface. A shallow earthquake has a focus less than 70 kilometers (45 miles). An intermediate-focus earthquake has a focus between 70 and 300 kilometers (45 to 200 miles). A deep-focus earthquake is greater than 300 kilometers (200 miles). About 75% of earthquakes have a focus in the top 10 to 15 kilometers (6 to 9 miles) of the crust. Shallow earthquakes cause the most damage. This is because the focus is near the Earth's surface, where people live.
Epicenter

The area just above the focus, on the land surface, is the earthquake’s epicenter (Figure below). The towns or cities near the epicenter will be strongly affected by the earthquake.

The focus of an earthquake is in the ground where the ground breaks. The epicenter is the point at the surface just above the focus.






Seismic Waves

Seismic waves are the energy from earthquakes. Seismic waves move outward in all directions away from their source. Each type of seismic wave travels at different speeds in different materials. All seismic waves travel through rock, but not all travel through liquid or gas. Geologists study seismic waves to learn about earthquakes and the Earth’s interior.

Types of Seismic Waves

There are two major types of seismic waves. Body waves travel through the Earth’s interior. Surface waves travel along the ground surface. In an earthquake, body waves are responsible for sharp jolts. Surface waves are responsible for rolling motions that do most of the damage in an earthquake.

Body Waves

Primary waves (P-waves) and secondary waves (S-waves) are the two types of body waves (Figure below). Body waves move at different speeds through different materials.

P-waves are faster. They travel at about 6 to 7 kilometers (about 4 miles) per second. Primary waves are so named because they are the first waves to reach a seismometer. P-waves squeeze and release rocks as they travel. The material returns to its original size and shape after the P-wave goes by. For this reason, P-waves are not the most damaging earthquake waves. P-waves travel through solids, liquids and gases.  These waves can be thought of as "Push-Pull" waves.

S-waves are slower than P-waves. They are the second waves to reach a seismometer. S-waves move up and down. These waves can be thought of as "Snake" waves They change the rock’s shape as they travel. S-waves are about half as fast as P-waves, at about 3.5 km (2 miles) per second. S-waves can only move through solids. This is because liquids and gases don’t resist changing shape.






Surface Waves

Surface waves travel along the ground outward from an earthquake’s epicenter. Surface waves are the slowest of all seismic waves. They travel at 2.5 km (1.5 miles) per second. There are two types of surface waves. Love waves move side-to-side. Rayleigh wavesproduce a rolling motion as they move up and backwards (Figure above). Surface waves cause objects to fall and rise, while they are also swaying back and forth. 
These motions cause damage to rigid structures during an earthquake.