GEY 111-002 Ch 9 MA

Isostatic adjustment

-dam construction -mining

Pore water pressure

-groundwater extraction -wastewater injection

convergent boundary

-have the most earthquakes near coastlines -most earthquakes occur along this kind of boundary -most earthquakes in mountains or near mountains are associated with this kind of boundary

reverse fault

A reverse fault is formed when one slab of rock slides on top of another at an angle greater than 45°.

thrust fault

A thrust fault is formed when one slab of rock slides on top of another at an angle less than 45°.

source of friction between rock bodies

As shown in the diagram, rocks have rough surfaces that inhibit their ability to slide past each other. The rougher their surfaces, the more friction exists between them. Force also generates friction between rock bodies. If a giant hand were to push the boulder into the bedrock, then the rough surfaces of each rock body would push against each other even more, generating more friction. The forces along a fault plane are primarily responsible for the relative intensities of earthquakes they generate

Earthquakes

Earthquakes are vibrations in the ground caused by a sudden release of energy. For example, an explosion can cause the atoms in the ground to move in a particular direction. Since the atoms are held in place within minerals by chemical bonds, they will elastically rebound in the opposite direction and then vibrate back and forth around their crystal lattice position. The systematic vibration of countless numbers of atoms is responsible for the large-scale vibrations people call earthquakes. Explosions are actually a very rare cause of earthquakes. Most earthquakes are caused by the release of elastic energy in rocks sitting along fault planes. Bending the stick stretches the chemical bonds holding the stick's atoms together, which provides a source of elastic energy. The elastic energy is suddenly released when the stick breaks and the two ends straighten. The rocks near a fault plane behave similarly. Compressional, shear, and tensional stresses cause bodies of rock to move along fault planes. However, friction between the rocks prevents smooth motion along the fault plane. Instead, the rocks near the fault plane bend and store elastic energy. The elastic energy is released when the rock bodies suddenly move along the fault and the bending rocks straighten, which causes an earthquake.

Natural causes of earthquakes

Earthquakes have several natural causes. Plate tectonics cause earthquakes when rock bodies slide past each other along fault planes. Volcanic activity can cause earthquakes by fracturing the rock surrounding a magma chamber. Isostatic adjustments can also cause earthquakes when rock bodies slide past each other vertically. Finally, the motion of large landslides can cause small earthquakes.

Relationships Between Plate Tectonics and Earthquakes

Nearly all earthquakes occur along fractures in Earth's crust called faults. As shown in the figure below, friction prevents smooth crustal movement along the fault plane. Instead, motion along a fault plane occurs in a succession of violent bursts, which generate seismic waves that cause the ground to tremble. Earth's crust is divided into plates that move with respect to each other; unsurprisingly, most faults occur along the boundaries separating these plates. Geologists recognize three different types of plate boundaries: convergent, transform, and divergent. Each of these plate boundaries is associated with a different kind of fault.

Earthquakes caused by human activity

Sometimes people cause earthquakes. We have already discussed explosions in the introduction. In addition, people can induce earthquakes via isostatic adjustments, which are caused by changes in the weight of Earth's crust. Building structures that weigh down the crust will cause it to slide downward along faults. Conversely, removing earth can make the crust adjust upward. People have also induced earthquakes by changing the hydrostatic pressure in rock pores. Increasing the pore water pressure sometimes reduces friction between rock bodies along a fault, thereby inducing slippage along the fault. On the other hand, reducing pore water pressure can cause rock bodies to shrink, which will also reduce friction along the fault plane.

circum-Pacific belt

That is where most people live! Specifically, the zone of greatest seismic activity along coastlines. Encompasses the coastal regions of Chile, Central America, Indonesia, Japan, and Alaska.

Earthquake Severity along Different Kinds of Plate Boundaries

The amount of friction along a fault plane determines the severity of the earthquakes it produces. Therefore, the kinds of faults and the relative amount of friction generated at each type of fault, determine the relative severity of earthquakes produced at each kind of plate boundary.

Thrust faults and reverse faults

These faults are created by compression, which causes one slab of rock to push on top of another. The compressional forces generate friction along the faults.

Strike-slip faults

These faults are created when bodies of rock slide laterally past each other. The only force that generates friction along a strike-slip fault is the pressure of the rocks

Normal faults

These faults are created when one slab of rock pulls apart from another and slides downward with respect to the other slab. Gravity is the force that generates friction along normal faults

ranking plate boundary severity from highest to lowest

convergent, transform, & divergent

divergent boundary

most earthquakes in the middle of the oceans occur along this kind of boundary

Divergent plate boundaries

occur when two plates move away from each other. Divergent boundaries are associated with normal and strike-slip faults. A normal fault is formed when one slab of rock pulls apart from another and slides downward with respect to the other slab.

Transform plate boundaries

occur where one plate slides past another as shown below. Only strike-slip faults are associated with transform boundaries.

convergent plate boundary

occurs where two plates collide with each other, thrusting one plate on top of another. Convergent plate boundaries are associated with thrust faults, reverse faults, and strike-slip faults. A thrust fault is formed when one slab of rock slides on top of another at an angle less than 45°. A reverse fault is formed when one slab of rock slides on top of another at an angle greater than 45°. A strike-slip fault is formed when one slab of rock slides past another laterally. Crust shortens and thickens by brittle faulting and folding in upper crust and plastic flow in lower crust.

which are the four types of faults

thrust, reverse, strike-slip, and normal