LESSON 10 GEOLOGY

• Earthquake


  Ground shaking caused by the sudden and rapid movement of one block of rock, slipping past another along fractures in Earth's crust called faults

  
• Earthquake


  An unpredictable event in which masses of rock shift below earth's surface, releasing enormous amounts of energy and sending out shock waves that sometimes cause the ground to shake dramatically

  
• Earthquake

  • Hypocenter/Focus – location where slippage begins
  • Faults – fractures along the earth's crust
  • Epicenter - the point on earth's surface directly above the hypocenter
  • Seismic waves – a form of energy that causes the materials that transmit them to shake

  
• Aftershocks


  Earthquakes of lesser magnitude that follow stronger earthquakes, which are the result of crust along the fault surface adjusting to the displacement caused by the main shock

  
• According to USGS, around 60 aftershocks of magnitudes 4.5 – 5.9 followed the major Haitian earthquake in 2010
• Aftershocks often trigger the destruction of already weakened structures
• Foreshocks


  Small earthquakes hat precede major earthquakes, usually by days or in some cases, by several years
• Unlike aftershocks, foreshocks are not often monitored and prediction to major earthquakes resulted only to limited successes
• Normal Faults


  • Associated by divergent plate boundaries, mainly seafloor spreading centers and continental rifting
• Reverse Faults


  • Associated with subduction zones and continental collisions

  
• Strike-slip Faults


  • May form transform plate boundaries

  
• Seismology


  The study of earthquake waves

  
• Seismology dates back in ancient China almost 2500 years ago to determine the direction from which these waves originated
• Seismographor Seismometer


  The instrument used to record earthquakes

  
• In a generalized earthquake observation, two types of seismographs are used: Horizontal seismograph and Vertical seismograph

  
• Seismogram


  The records obtained from seismographs
• Seismogram


  • Reveals two main types of surface waves generated by the slippage of a rock mass: Surface waves and Body waves

  
• Body Waves


  Seismic waves that travel through the earth's interior
• Body Waves

  • There are two types: Primary waves and Secondary waves
• Primary waves (P-waves)


  The "push and pull" waves, they momentarily push (compress) and pull (stretch) rocks in the direction of their movement
• Secondary waves (S-waves)


  "Shake" the particles at right angles to their direction of travel
• Surface Waves


  Waves that travels in the rock layers just below earth's surface
• Surface Waves

  • There are two types: Rayleigh waves and Love waves
• Rayleigh waves


  Cause the material and anything resting upon it to move in ellipses in planes normal to the surface and parallel to the direction of propagation
• Love waves


  Cause earth's materials to move side to side, particularly damaging to the foundations of structures
• Earthquake Intensity


  The measure of the amount of ground shaking at a particular location, based on observed property damage

  
• Modified Mercalli Intensity (MMI) scale


  Developed by Italian Giuseppe Mercalli in 1902 using California building standards
• Earthquake Magnitude


  Relies on data gathered from seismic records to estimate the amount of energy released at an earthquake's source

  
• Types of magnitude scales

  • Richter Magnitude Scale
  • Moment Magnitude Scale
• Richter Magnitude Scale


  Calculated by measuring the amplitude of the largest seismic wave (usually an S or surface wave) recorded on a seismogram. Developed by Charles Richter in 1935 of the California Institute of Technology

  
• Moment Magnitude Scale


  Measures the total energy released during an earthquake. Calculated by determining the average amount of slip on the fault, the area of the fault surface that slopped, and the strength of the faulted rock

  
• Earthquake Hazards

  • Destruction from Earthquake Vibration
  • Liquefaction
  • Landslides
  • Ground Subsidence
  • Fire
  • Tsunami

  
• Ground shaking


  1. wave and surface waves mainly causes horizontal damaging motion

  
• How to reduce effects of ground shaking

  • Building codes - First defense against earthquake damage, reduce damage and loss of life
  • Seismic zoning - Particular regions exist with different horizontal acceleration due to soil properties, base shear acceleration is expressed as a percentage of the acceleration due to gravity, a fraction of g in horizontal direction causes the buildings to separate from their foundations
  • Seismic joints and frames to foundation - Diagonal bracing and blocking provides shear resistance, bolting frames help to mitigate shearing force damage, L-shaped structures reduce damage at the joint

  
• Landslides


  Hundred of _____ may be triggered by an earthquake in a slide-prone area

  
• Fire


  Caused by ruptured gas lines or fallen electric power lines, water mains may also break due to earthquake, thus the difficulty accessing fire-fighting equipment
• Reducing effect of fire


  • Cutting off supply line immediately after an earthquake
• Tsunami


  Large ocean waves occasionally set in motion by major undersea earthquake events
• Tsunami came from the Japanese word for "harbor wave"
• Often mistaken to consist of moving water, but energy from a nearby tectonic activity or submarine landslide