EARTHQUAKES
Earthquakes
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.
Focus, Epicenter, and Fault
Slippage Along a Fault
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.
Elastic Rebound Hypothesis
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.
8.2 Measuring Earthquakes Earthquake Waves 8.2 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
Seismograph
Seismogram
8.2 Measuring Earthquakes Earthquake Waves 8.2 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)
Seismic Waves
8.2 Measuring Earthquakes Earthquake Waves 8.2 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)
Seismic Waves
8.2 Measuring Earthquakes Earthquake Waves 8.2 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
Seismic Waves
8.2 Measuring Earthquakes Earthquake Waves 8.2 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
Seismic Waves
Seismogram
8.2 Measuring Earthquakes Locating an Earthquake 8.2 Measuring Earthquakes Earthquake Distance • The epicenter is located using the difference in the arrival times between P and S wave recordings, 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.
Locating an Earthquake
Locating an Earthquake
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
8.2 Measuring Earthquakes Measuring Magnitude 8.2 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)
8.2 Measuring Earthquakes Measuring Intensity 8.2 Measuring Earthquakes Modified 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
8.2 Measuring Earthquakes Measuring Earthquakes- New Method 8.2 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
8.2 Measuring Earthquakes Measuring Earthquakes- New Method 8.2 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
Earthquake Magnitudes
Some Notable Earthquakes
Earthquake hazards in canada
8.3 Destruction from Earthquakes Damage can take several forms 8.3 Destruction from Earthquakes Ground motion: buildings and bridges collapse Fire: gas lines can break or oil tanks can ignite Landslides Displacement of land Aftershocks: can cause weakened structures to collapse Tsunamis: underwater earthquakes can cause massive waves that damage coastal areas
Earthquake Damage
8.3 Destruction from Earthquakes Seismic Vibrations 8.3 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
8.3 Destruction from Earthquakes Seismic Vibrations 8.3 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
8.3 Destruction from Earthquakes Seismic Vibrations 8.3 Destruction from Earthquakes Ground motion Building Design- base isolators As the ground moves back and forth, the base isolators distort and absorb the motion, significantly decreasing the motion of the building. Standard foundation Base isolator
8.3 Destruction from Earthquakes Seismic Vibrations 8.3 Destruction from Earthquakes base isolators
8.3 Destruction from Earthquakes Seismic Vibrations 8.3 Destruction from Earthquakes Building location Bedrock absorbs more wave energy and are the most stable Softer soils transfer more energy to the buildings causing more damage If softer soils have water in them, they can become a little like quicksand during an earthquake.
8.3 Destruction from Earthquakes Seismic Vibrations 8.3 Destruction from Earthquakes Resonant frequency If 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.
8.3 Destruction from Earthquakes Seismic Vibrations 8.3 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
Effects of Liquefaction
Effects of Liquefaction
Effects of Liquefaction Liquefaction during the 1964 Niigata, Japan earthquake (7.6)
Effects of Liquefaction
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.
Landslide Damage