3 8.1 What Is an Earthquake? Earthquakes An earthquake is the vibration of Earth produced by the rapid release of energy Focus and Epicenter• Focus is the point within Earth where the earthquake starts.• Epicenter is the location on the surface directly above the focus. Faults• Faults are fractures in Earth where movement can occur. Usually where two plates meet.
6 8.1 What Is an Earthquake? Cause of Earthquakes Elastic Rebound Hypothesis• rocks are deformed at faults due to pressure as tectonic plates move past, subduct or converge. Eventually the rocks ‘slip’ releasing energy via the process of elastic rebound.• Most earthquakes are produced by the rapid release of elastic energy stored in rock that is under great amounts of pressure• When the strength of the rock is exceeded, it suddenly breaks, allowing the release of energy and causing the vibrations of an earthquake.
8 8.1 What Is an Earthquake? Aftershocks and Foreshocks • An aftershock is a small earthquake that follows the main earthquake. The plates or rocks readjust themselves after the ‘slip’ or elastic rebound has occurred. Can cause further damage to buildings weakened by main quake.• A foreshock is a small earthquake that often precedes a major earthquake. Can serve as a warning, but may happen years before a major earthquake.
9 8.2 Measuring Earthquakes Earthquake Waves8.2 Measuring Earthquakes Seismometer: used to measure seismic waves. Measure waves along the x,y, and z axis. Seismographs are instruments that record earthquake waves, use information from the seismometer. Seismograms are the paper record of the earth’s movement
12 8.2 Measuring Earthquakes Earthquake Waves8.2 Measuring Earthquakes Body Waves• Identified as P waves or S waves• P waves (primary wave- first to arrive)- Are push-pull waves that push (compress) and pull (expand) in the direction that the waves travel (like pushing a stretched slinky)- Travel through solids, liquids, and gases- Have the greatest velocity of all earthquake waves (4-7km/s)
14 8.2 Measuring Earthquakes Earthquake Waves8.2 Measuring Earthquakes Body Waves• S waves (secondary waves- second to arrive)Seismic waves that travel along Earth’s outer layer- Shake particles at right angles to the direction that they travel (like a shaking a stretched rope)- Travel only through solids- Slower velocity than P waves (2-5km/s)
16 8.2 Measuring Earthquakes Earthquake Waves8.2 Measuring Earthquakes Surface waves (the slowest and most destructive• Identified as Love waves or Rayleigh waves• Love Waves- Move from side to side perpendicular to the direction of the wave- Travel through solids- Tend to knock buildings off their foundations and highways bridges off their supports due to side to side movement
18 8.2 Measuring Earthquakes Earthquake Waves8.2 Measuring Earthquakes Surface waves (the slowest and most destructive• Identified as Love waves or Rayleigh waves• Rayleigh waves- Behave like rolling ocean waves- Incredibly destructive as they produce more ground movement and take longer to pass
21 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.
24 8.2 Measuring Earthquakes Historically, scientists have used two different types of measurements to describe the size of an earthquake —intensity and magnitude. Intensity• a measure of the earthquakes effect on people and buildings Magnitude• a measure of the energy released during the earthquake, calculated by height of wave
25 8.2 Measuring Earthquakes Measuring Magnitude8.2 Measuring Earthquakes Richter Scale• 10 base logarithmic scale based on the amplitude of the largest seismic wave• each whole number jump (ie 56) is a 10 fold increase.• so a jump from 13 is 100 fold increase, 14 is 1000 fold increase• Does not estimate adequately the size of very large earthquakes (above 6)
26 8.2 Measuring Earthquakes Measuring Intensity8.2 Measuring EarthquakesModified Mercalli Intensity Scale• intensities are expressed as roman numerals• higher number equals higher damage to buildings and infrastructure• Drawbacks are that intensity lessens with distance, different distances will report different intensities• As well, building damage can change depending on the geologic structure of area
27 8.2 Measuring Earthquakes Measuring Earthquakes- New Method8.2 Measuring Earthquakes Momentum Magnitude• Moment magnitude is the most widely used measurement for earthquakes because it is the only magnitude scale that estimates the energy released by earthquakes.• Measures very large earthquakes accurately• Derived from information about the strength of rock, surface area of the ‘slip’ and the amount of rock displaced
28 8.2 Measuring Earthquakes Measuring Earthquakes- New Method8.2 Measuring Earthquakes Momentum Magnitude• take a look at your text book, pg 83, notice that there are differences between the Richter magnitude and moment magnitude• scientists use readings from different seismograms when reporting earthquake magnitudes• despite the differences, magnitude is always a better measurement because it can determine earthquake strength even when no buildings or people are present
32 8.3 Destruction from Earthquakes Damage can take several forms8.3 Destruction from EarthquakesGround motion: buildings and bridges collapseFire: gas lines can break or oil tanks can igniteLandslidesDisplacement of landAftershocks: can cause weakened structures to collapseTsunamis: underwater earthquakes can cause massive waves that damage coastal areas
34 8.3 Destruction from Earthquakes Seismic Vibrations8.3 Destruction from Earthquakes ground motion• Factors that determine structural damage- Intensity of the earthquake- Materials used to build it; stone or brick buildings are the most dangerous- Nature of the material upon which the structure rests- The design of the structure
35 8.3 Destruction from Earthquakes Seismic Vibrations8.3 Destruction from Earthquakes ground motion• seismic waves cause buildings to sway with the energy of the waves- Tall buildings are safer than short ones- The taller the building, the more flexible it is- Shorter buildings are ‘stiffer’ and resist swaying making them more susceptible to fracturing
36 8.3 Destruction from Earthquakes Seismic Vibrations8.3 Destruction from Earthquakes Ground motionBuilding Design- base isolatorsAs the ground movesback and forth, the base isolatorsdistort and absorb the motion,significantly decreasing the motionof the building.StandardfoundationBase isolator
37 8.3 Destruction from Earthquakes Seismic Vibrations8.3 Destruction from Earthquakes base isolators
38 8.3 Destruction from Earthquakes Seismic Vibrations8.3 Destruction from Earthquakes Building locationBedrock absorbs more wave energy and are the most stableSofter soils transfer more energy to the buildings causing more damageIf softer soils have water in them, they can become a little like quicksand during an earthquake.
39 8.3 Destruction from Earthquakes Seismic Vibrations8.3 Destruction from Earthquakes Resonant frequencyIf a seismic wave hits a building with a frequency that matches that structure's natural sway then damage increases!In physics terms, the building has the same resonant frequency as the wave.When this happens, multiple waves at the resonant frequency pass through the structure, amplifying each other. Making a very destructive force.
40 8.3 Destruction from Earthquakes Seismic Vibrations8.3 Destruction from Earthquakes Liquefaction• seismic waves can squeeze saturated soil causing it to behave like a liquid• buildings may sink• Underground objects (rocks or water) may float to surface
45 8.3 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.