2 Types of StressTension – stretches rock, they become thin in the middle2. Compression – pushes rock together, squeezes rock until it folds or breaks3. Shearing – masses of rock slip, they move past in opposite directions
3 8.1 What Is an Earthquake? Earthquakes An earthquake is the shaking & trembling of rock produced by the rapid release of energy Focus and Epicenter• Focus is the point within Earth where the rock breaks.• Epicenter is the location on the surface directly above the focus. Faults• Faults are fractures in Earth where movement has occurred.
4 Kinds of faultsNormal – due to tension, fault is at an angle, at diverging boundaries, hanging wall slips down footwallReverse or thrust – due to compression, fault is at an angle, hanging wall slides up and over footwallStrike-slip or transform – due to shearing, 2 plates slip past each other sideways, at transform boundaries
5 Normal Fault: The hanging wall has slipped down in comparison to the foot wall. Gravity causes the hanging wall to slip down. Normal Faults are from layers being pulled apart. Also known as a GRAVITY FAULT.
6 ReverseFault: The hanging wall has slipped up in comparison to the foot wall. When layers are pushed together this is the kind of fault that occurs.Also known as a THRUST FAULT.
7 Strike Slip Fault: Two layers of rock are shifted horizontally or parallel to the fault plane.
11 Fault-block mountains – uplifted block of rock createdbetween 2 normal faultsAnticlines – upwardfold = ridgesSynclines – downward fold= valleys
12 Fault-block mountains in Juniper Canyon and Yosemite
13 8.1 What Is an Earthquake? Cause of Earthquakes Elastic Rebound Hypothesis• Most earthquakes are produced by the rapid release of elastic energy stored in rock that has been subjected to great forces.• When the strength of the rock is exceeded, it suddenly breaks, causing the vibrations of an earthquake.
15 8.2 Measuring Earthquakes Earthquake Waves8.2 Measuring Earthquakes• P waves- Are push-pull waves that push (compress) and pull (expand) in the direction that the waves travel- Travel through solids, liquids, and gasesHave the greatest velocity of all earthquake wavesa.k.a. primary waves
16 8.2 Measuring Earthquakes Earthquake Waves8.2 Measuring Earthquakes• S wavesSeismic waves that travel along Earth’s outer layera.k.a. secondary waves- Shake particles at right angles to the direction that they travel or back & forth- Travel only through solids- Slower velocity than P waves
17 Surface waves are seismic waves that travel along Earth’s outer layer. -Make most severe ground movements.
22 8.2 Measuring Earthquakes Mercalli ScaleRates damage at a given placeUses Roman numerals 1-12 Richter Scale• Based on the amplitude of the largest seismic wave• Each unit equates to roughly a 32-fold energy increase• Does not estimate adequately the size of very large earthquakes
23 Modified Mercalli Scale IntensityVerbal DescriptionMagnitudeWitness ObservationsIInstrumental1 to 2Detected only by seismographsIIFeeble2 to 3Noticed only by sensitive peopleIIISlight3 to 4Resembling vibrations caused by heavy trafficIVModerate4Felt by people walking; rocking of free standing objectsVRather Strong4 to 5Sleepers awakened and bells ringVIStrong5 to 6Trees sway, some damage from overturning and falling objectVIIVery Strong6General alarm, cracking of wallsVIIIDestructive6 to 7Chimneys fall and there is some damage to buildingsIXRuinous7Ground begins to crack, houses begin to collapse and pipes breakXDisasterous7 to 8Ground badly cracked and many buildings are destroyed.There are some landslidesXIVery Disasterous8Few buildings remain standing; bridges and railways destroyed;water, gas, electricity and telephones out of action.XIICatastrophic8 or greaterTotal destruction; objects are thrown into the air,much heaving,shaking and distortion of the ground
24 No. of earthquakes per year Richter scale no.No. of earthquakes per yearTypical effects of this magnitude< 3.4Detected only by seismometers30 000Just about noticeable indoors4 800Most people notice them, windows rattle.1400Everyone notices them, dishes may break, open doors swing.500Slight damage to buildings, plaster cracks, bricks fall.6.2 6.9100Much damage to buildings: chimneys fall, houses move on foundations.15Serious damage: bridges twist, walls fracture, buildings may collapse.4Great damage, most buildings collapse.> 8.0One every 5 to 10 yearsTotal damage, surface waves seen, objects thrown in the air.
25 8.2 Measuring Earthquakes Moment Magnitude Scale• Derived from the amount of displacement that occurs along the fault zone• most widely used measurement for earthquakes because it estimates the energy released by earthquakes.Scale ranges from 0-10+• Measures very large earthquakes
34 8.2 Measuring Earthquakes Locating an Earthquake8.2 Measuring Earthquakes Earthquake Distance• The epicenter is located using the difference in the arrival times between P and S waverecordings, which are related to distance. Earthquake Direction• Travel-time graphs from three or more seismographs can be used to find the exact location of an earthquake epicenter. Earthquake Zones• About 95 percent of the major earthquakes occur in a few narrow zones.
36 8.3 Destruction from Earthquakes Seismic Vibrations8.3 Destruction from Earthquakes The damage to buildings and other structures from earthquake waves depends on several factors. These factors include the intensity and duration of the vibrations, the nature of the material on which the structure is built, and the design of the structure.
38 8.3 Destruction from Earthquakes Seismic Vibrations8.3 Destruction from Earthquakes Building Design• Factors that determine structural damage- Intensity of the earthquake- Unreinforced stone or brick buildings are the most serious safety threats- Nature of the material upon which the structure rests- The design of the structure
39 8.3 Destruction from Earthquakes Seismic Vibrations8.3 Destruction from Earthquakes Liquefaction• Saturated material turns fluid• Underground objects may float to surface
41 8.3 Destruction from Earthquakes Tsunamis8.3 Destruction from Earthquakes Cause of Tsunamis• A tsunami triggered by an earthquake occurs where a slab of the ocean floor is displaced vertically along a fault.• A tsunami also can occur when the vibration of a quake sets an underwater landslide into motion.• Tsunami is the Japanese word for “seismic sea wave.”
43 8.3 Destruction from Earthquakes Tsunamis8.3 Destruction from Earthquakes Tsunami Warning System• Large earthquakes are reported to Hawaii from Pacific seismic stations.• Although tsunamis travel quickly, there is sufficient time to evacuate all but the area closest to the epicenter.
44 8.3 Destruction from Earthquakes Other Dangers8.3 Destruction from Earthquakes Landslides• With many earthquakes, the greatest damage to structures is from landslides and ground subsidence, or the sinking of the ground triggered by vibrations. Fire• In the San Francisco earthquake of 1906, most of the destruction was caused by fires that started when gas and electrical lines were cut.
46 8.4 Earth’s Layered Structure Discovering Earth’s Layers8.4 Earth’s Layered Structure Mohoˇ• Velocity of seismic waves increases abruptly below 50 km of depth• Separates crust from underlying mantle Shadow Zone• Absence of P waves from about 105 degrees to 140 degrees around the globe from an earthquake• Can be explained if Earth contains a core composed of materials unlike the overlying mantle