Section 1: Earth’s Crust in Motion How Do Stress Forces Affect Rock? The movement of earth’s plates creates powerful forces that squeeze or pull the rock in the crust – these forces are examples of stress Stress – a force that acts on rock to change its shape or volume An earthquake is the shaking and trembling that results from the movement of rock beneath earth’s surface

How Does Stress Effect the Earth’s Crust? Deformation – any change in the volume or shape of earth’s crust Three kinds of stress in the crust: Shearing – stress that pushes a mass of rock in two opposite directions Tension – pulls on the crust, stretching rock so that it becomes thinner in the middle like warm bubble gum Compression – squeezes rock until it folds or breaks like a giant trash compactor

What Is a Fault? Fault – a break in earth’s crust where slabs of crust slip past each other; These usually occur at plate boundaries

What Kind of Faults Are There? Three Kinds: Strike-slip faults Normal Faults Reverse Faults

What Are Strike-slip Faults? Shearing forces cause rocks to slip past each other sideways with little up and down Motion; Ex. San Andreas fault in California

Tension forces cause the rocks to form the fault at an angle What Are Normal Faults? Normal faults Tension forces cause the rocks to form the fault at an angle One block is above the fault Hanging wall – the half of the fault that lies above Footwall – the half of the fault that lies below Ex. Rio Grande rift valley

What Are Reverse Faults? compression forces cause the rocks to move towards each other Same structure as normal fault but the blocks move in opposite direction; hanging wall move up Ex. Appalachian Mountains and Mount Gould in Glacier National Park

B A A miner walks on the foot wall and looks up at the hanging wall! Hanging wall moves down Hanging wall moves up What type of fault? What type of fault? Normal Fault Reverse Fault

How Are Mountains Effected by These Forces? Fault-block mountains – normal faults uplift a block of rock Folding – bends in the rock that form when compression shortens and thickens part of the earth’s crust. Ex. Himalayas

How Are Mountains Effected by These Forces? (Continued) Anticlines – a fold upward into an arch Syncline – a fold downward into an arch Plateaus – a large area of flat land elevated high above sea level

Earthquakes – most begin in the lithosphere Section 2: Measuring Quakes How Does the Energy of an Earthquake Travel Through Earth? Earthquakes – most begin in the lithosphere Focus – the point beneath the earth’s surface where rock that is under stress breaks, triggering an earthquake Epicenter – the point on the earth’s surface directly above the focus

What Are Seismic Waves? Seismic Waves – vibrations that travel through Earth carrying the energy released during an earthquake They move like ripples on a pond They carry the energy of an earthquake away from the focus, through Earth’s interior, and across the surface The energy is greatest at the Epicenter

What Are the Different Kinds of Seismic Waves? Three categories: P waves S waves Surface waves P waves and S waves are sent out from the focus; Surface waves develop when the waves reach the surface

What Are P Waves? P waves are primary waves The first waves to arrive Earthquake waves that compress and expand the ground like an accordion Cause buildings to contract and expand

Cannot move through liquids What Are S Waves? S waves are secondary waves Earthquake waves that vibrate from side to side as well as up and down These waves shake the ground back and forth Shake structures violently Cannot move through liquids

What Are Surface Waves? When P waves and S waves reach the surface some are transformed into surface waves Surface waves move more slowly than P waves and S waves Produce the most severe ground movements Can make the ground roll like ocean waves or shake buildings from side to side

How Do Scientists Detect Seismic Waves?                    How Do Scientists Detect Seismic Waves? Seismograph – records the ground movements caused by seismic waves as they move through the Earth

How Do Scientists Measure Earthquakes? There are at least 20 different measures for rating earthquakes, three are: Mercalli Richter Moment Magnitude Magnitude – a measurement of earthquake strength based on seismic waves

What Is the Mercalli Scale? Rated earthquakes according to their intensity Intensity: strength of ground motion in a given place Not a precise measurement Describes how earthquakes affect people, buildings, and the land surface

What Is the Richter Scale? A rating of the size of seismic waves as measured by a particular type of seismograph Accurate measurements for small, nearby earthquakes not large, distant earthquakes

What Is the Moment Magnitude? A rating system that estimates the total energy released by an earthquake Can be used to rate earthquakes of all sizes, near or far Below 5.0 – little damage Above 5.0 – great destruction

How Do Scientists Locate the Epicenter? Geologists use seismic waves P waves arrive first S waves arrive close behind Scientist measure the difference in arrival times The farther away an earthquake is the greater the time between their arrival Scientists draw three circles using data from seismographs set at different stations to see where they intersect – the epicenter