Chapter 10: Stress in Earth's Crust

Tension

Rocks that are pulled apart are under tension; rocks under tension lengthen or break apart. Tension is the major type of stress at divergent plate boundaries.

Compression

squeezes rock together causing rocks to fold or fracture(break) Compression is the most common stress at convergent boundaries.

fracture

the rock breaks

plastic deformation

the rock does not return to its original shape when the stress is removed

Elastic deformation

the rock returns to its original shape when the stress is removed

Earthquakes at convergent plate boundaries mark the motions of subducting lithosphere as it plunges through the mantle. Eventually as the plate heats up enough deform plastically and earthquakes stop. The cross section of earthquake epicenters with depth outlines the subducting plate with shallow, intermediate and deep earthquakes. Convergent plate boundaries produce earthquakes all around the Pacific Ocean. The Pacific Northwest of the United States is at risk from a potentially massive earthquake that could strike any time. Subduction of the Juan de Fuca plate beneath North America produces active volcanoes but large earthquakes only hit every 300 to 600 years. massive earthquakes are the hallmark of the thrust faulting and folding when two continental plates converge.

Earthquakes a mid- ocean ridges are smallest and shallow because the plates are young,thin and hot . On land where continents split apart, earthquakes are larger and stronger. Intraplate earthquakes are the result of stresses caused by plate motions acting in solid slabs of lithosphere. Tsunami are deadly ocean waves from an earthquake. The sharp jolt of an undersea quake causes displacement of tectonic plates which forms a set of waves that travel throught the sea entirely unnoticed.

In an earthquake, the initial point where the rocks rupture in the crust is called the focus. The epicenter is the point on the land surface that is above the focus. Shallow earthquakes cause the most damage because the focus is near where people live. However, it is the epicenter of an earthquake that is reported by scientist and the media

Energy is transmitted in waves. Every wave has a high point called a crest and a low point called a trough. The height of a wave from the center line to its crest is its amplitude. The distance between waves from crest to crest (trough or trough) is its wavelength. The study of seismic waves is known as seismology. Seismologists use seismic waves to learn about earthquakes and also to learn abut the Earth's interior

Shear stress

when forces are parallel but moving in opposite directions the stress is called shear. Shear stress is the most common stress at transform plate boundaries.

Strain or deformation

when stress causes a material to change shape it has undergone strain or deformation. Deformed rocks are common in geologically active areas.

Notes

At the Earth's surface, rocks usually break quite quickly, but deeper in the crust, where temperatures and pressures are higher, rocks are more likely to deform plastically. Sudden stress such as a hit with a hammer, is more likely to make a rock break. Stress applied over time often leads to plastic deformation. 1.Sedimentary rocks are formed with the oldest layers on the bottom and the youngest on top 2. Sediments are deposited horizontally, so sedimentary rock layers are originally horizontal, as are some volcanic rocks, such as ash falls. 3. Sedimentary rock layers that are not horizontal deformed. You can trace the deformation a rock has experienced by seeing how it differs from its original horizontal, oldest on bottom position. this deformation produces geologic structures such as folds, joints and faults that are caused by stresses.

Why is this important? The amplitude of the largest wave increases ten times from one integer to the next. An increase in one integer means that thirty times more energy was released. These two scales often give very similar measurements. An increase in two integers on the moment magnitude scale equals 900- fold increase in released energy. The moment magnitude scale more accurately reflects the energy released and the damage caused, which is what most seismologists now use. In a single year, on average, more than 900,000 earthquakes are recorded and 150,000 of them are strong enough to be felt. Each year about 19 earthquakes are major with a Richter magnitude of 7.0 to 7.9, and on average one earthquake has a magnitude of 9.0 to 8.9. There have been 5 9.0 earthquakes since 1900; all but the Great Indian Ocean earthquakes of 2004 occurred somewhere around the Pacific Ocean basin. Areas along a fault line that have not experienced an earthquake in a long time are known as Seismic Gap and are thought of as are as that can experience an earthquake at any moment.

Finding the epicenter Scientists first determine the epicenter distance from these different seismographs. The longer the time between the arrival of the P-wave and S-wave the farther away is the epicenter. So the difference in the P and S wave arrival times determines the distance between the epicenter and seismometer. The scientist then draws a circle with a radius equal to the distance from the epicenter for that seismograph. The epicenter is somewhere along that circle. This is done for three locations. Using data from two seismographs, the two circles at a single point; This point is the earthquake epicenter. This technique has been digital calculations. Circles are drawn with radii representing the distance from each seismic station to the earthquake's epicenter. The intersection is the earthquake's epicenter. When an earthquake will occur is much more difficult to predict. Since stress on a fault builds up at the same rate over time, earthquakes should occur at the same rate over time, earthquakes should occur at regular intervals. Signs sometimes come before a large earthquake. Small quakes called foreshocks, sometimes occur a few weeks before a major quake.

Over the past century, scientists have developed several ways of measuring earthquake intensity. The currently accepted method is the moment magnitude scale, which measures the total amount of energy released by the earthquake. Seismologists have not found a reliable method for predicting earthquakes. Seismograph produces a graph like representation of the seismic waves it receives and records them onto a seismogram. Seismograms contain information that can be used to determine strong an earthquake was, how long it lasted, and how far it was. Modern seismometers record ground motions using electronic motion detectors. The data are then kept digitally on a computer. Seismograms show the arrival of P and S waves. The surface waves just after the S waves and are difficult to distinguish. Time is indicated on the horizontal portion or x axis of the graph.

If a seismogram records P waves and surface waves but not S waves but not S waves , the seismograph was on the other side of the Earth from the earthquake. The amplitude of the waves can be used to determine the magnitude of the earthquake. Mercali Intensity Scale: Earthquakes are described in terms of what nearby residents felt and the damage that was done to nearby structures. Richter magnitude scale: Developed in 1935 by Charles Richter, this scale uses a seismometer to measure the magnitude of the largest to measure the magnitude of the largest jolt of energy released by an earthquake. Moment magnitude scale: Measures the total energy released by an earthquake. Moment magnitude is calculated from the distance ground moved the fault. The Richter Scale and the moment magnitude scale are logarithmic. The logarithmic of a number is the exponent to which another fixed value, the base, must be raised to produce that number.

Faults lie at an angle to the horizontal surface of the Earth. That angle is called the fault's dip. The dip defines which of two basic types a fault is

If the faults dip is inclined relative to the horizontal, the fault is a dip-slip fault. The are two types of dip slip faults. In normal faults, the hanging wall drops down relative to the footwall. In reverse faults, the footwall drops down relative to the hanging wall. A thrust fault is a type of reverse fault in which the fault plane angle is nearly horizontal. Rocks can slip many miles along thrust faults. Normal faults can be huge. They are responsible for uplifting mountain ranges in regions experiencing tensional stress. A strike slip fault is a dip-slip fault in which the dip of the fault plane is vertical. Strike slip faults result from shear stresses.

What makes an earthquake deadly? Population density. The magnitude 9.2 Great Alaska Earthquake, near Anchorage of 1964 resulted in only 131 deaths. At the time few people lived in the area. Solid bedrock vibrates less than soft sediments so there is less damage in bedrock. Sediments that are saturated with water undergo liquefaction and become like quicksand. Liquefaction can be more prevalent in areas that are closer to large bodies of water, such as islands or peninsulas. Loose soil that sits on a hillside has a large chance of turning into a landslide. A few simple Earthquake Safety Guidelines are: Skyscrapers and other large structures built on soft ground must be anchored to be bedrock, even if it lies hundreds of meters below the ground surface. The correct building materials must be used. Houses should bend and sway. Wood and steel are better than brick, stone and adobe, which are brittle and will break.

Larger buildings must sway, but not so much that they touch nearby buildings. Counterweights and diagnol steel beams are used to hold down sway. To make older buildings more earthquake safe, retrofitting with steel or wood can reinforce a buildings structure and its connections. Large buildings can be placed on rollers so that they move with the ground. Buildings may be placed on layers of steel and rubber to absorb the shock of the waves . These rubber layers are known as Base. Connections, such as where the walls meet the foundation, must be made strong. In a multi-story building, the first story must be well supported. Fires start because seismic waves rupture gas and electrical lines, and breaks in water mains make it difficult to fight the fires.

P waves and S waves are known as body waves because they move through the solid body of the Earth. Surface waves travel along the ground, outward from an earthquake's epicenter. Surface waves travel along the ground, outward from an earthquake's epicenter. Surface waves are the slowest of all seismic waves traveling at 2.5km(1.5 miles) per second they are surface waves that are earthquakes waves that travel along earth surface.

Love waves cause particles of material to move from side to side, in a direction perpindicular to the waves' direction of travel. Rayleigh waves travel more slowly than love causes and cause particles of materials to move in elliptical patterns. The Rayleigh wave pattern is similar to the movement of particles in the ripples that appear on the surface of the lake into which a pebble has been tossed. In an earthquake, body waves produce sharp jolts. The rolling motions of surface waves do most of the seismic damage in an earthquake.

An earthquake is sudden ground movement caused by the sudden release of energy stored in rocks. Earthquakes happen when so much stress builds up in the rocks that the rocks rupture. The energy is transmitted by seismic waves.

The description of how earthquakes occur is called elastic rebound theory. As stress is built up on both sides of a fault,the rocks begin to deform plastically. When the stresses become too great, the rocks break and end up in a different location.This releases the built up energy and creates an earthquake.

P waves move material forward and backward in the direction they are traveling. The material returns to its original size and shape after the P wave goes by S waves move up and down, perpindicular to the direction the wave is traveling. This motion produces shear stresses. Transform faults have shallow focus earthquakes. Deadly occur at transform plate boundaries.

Nearly 95% of all earthquakes take along one of three types of plate boundaries, but earthquakes do occur along all three types of plate boundaries. The remaining 5% are scattered around other plate boundaries or are intraplate boundaries. About 80% of all earthquakes strike around the Pacific Ocean Basin because it is lined with convergent and transform boundaries. About15% take place in the Mediterranean-Asiatic Belt where convergence is causing the Indian Plate to run into the Eurasian Plate creating Mountain Building. The earthquake depth shows that most large quakes are shallow focus, but some subductedplates cause deep focus quakes.

If the blocks of rock on one or both sides of a fracture move, the fracture is called a fault. Sudden motions along faults cause rocks to break and move suddenly The energy released is an earthquake.

Slip is the distance rocks move along a fault . Slip can be up or down the fault plane. Slip is relative because there is usually no way to know whether both sides moved or only one.

Stress

Stress is defined as the f0rce applied to an object; force per unit area that is placed on a rock. There are four rock types: Confined stress, Compression, tension, and shear stress.

Rocks deforming plastically under compressive stresses crumble into folds; they do not return to their original shape. If the rocks experience more stress, they may undergo more folding or even fracture

Three types of folds are seen; Monocline: is a simple bend in the rock layers so that they are no longer horizontal. Anticline: an anticline is a fold that arches upward. The rocks dip away from the center of the fold. The oldest rocks are at the center of an anticline and the youngest are dropped over them. Synicline:is a fold that bends downwards the youngest rocks are at the center and the oldest are outside. When rocks arch upward to form a circular structure is called a dome. When rocks bend downward in a circular structure, that structure is called a basin. A rock under enough stress will fracture; if there is no movement on either side of a fracture, the fracture is called a joint.

Imagine placing one foot on either side of a strike-slip fault. One block moves toward you. If that block moves toward your right foot, the fault is a right-lateral strike slip fault; if that block moves toward your left foot, the fault is a left-lateral strike slip fault.

Two converging continental plates smash upwards to create mountain ranges. Stresses from this uplift cause folds, reverse faults and thrust faults, which allow the crust to rise upwards. The world's highest mountain range, the Himalayas is growing from the collision between the Indian and Eurasian plates. The crumpling of the Indian and Eurasian plates of continental crust creates the Himalayas. Subduction of oceanic lithosphere at convergent plate boundaries also builds mountain ranges.. When tensional stresses pull crust apart, it breaks into blocks that slide up and drop down along normal faults. The result is alternating mountains and valleys, known as a basin and range.

Confined stress

a deeply buried rock is pushed down by the weight of all the material above it. Since the rock cannot move it cannot deform.

Notes

a rock's response to stress depends on the rock type, the surrounding temperature and pressure conditions the rock is under, the length of time the rock is under stress and the type of stress. With increasing stress, the rock undergoes elastic deformation, plastic deformation and fracture