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• An estimated 230,000 people lost their lives when a magnitude 7.0 earthquake struck the small Caribbean nation of Haiti, the poorest country in the Western Hemisphere
• Haiti earthquake


  • Originated only 25 kilometers (15 miles) from the country's densely populated capital city of Port-au-Prince
  • Occurred along a San Andreas-like fault at a shallow depth of just 10 kilometers (6 miles)
  • Ground shaking was exceptional for an event of this magnitude
• The city of Port-au-Prince is not built on solid bedrock, but rather on sediment, which is more susceptible to ground shaking by earthquake waves
• Inadequate or nonexistent building codes meant that buildings collapsed far more readily than they should have
• At least 52 aftershocks, measuring 4.5 or greater, jolted the area and added to the trauma survivors experienced for days after the original quake
• In addition to the staggering death toll, there were more than 300,000 injuries, and 250,000 residences were destroyed. Nearly a million people were left homeless
• Relief agencies from around the globe stepped in to distribute food and water and to provide for the enormous medical, security, and social needs of the injured and displaced
• For a lengthy period following the quake, inadequate infrastructure coupled with devastating damages combined to inhibit the timely delivery of essential services
• The rebuilding of Haiti will likely take a decade or more and billions of dollars in assistance
• Earthquake


  Natural geologic phenomena caused by the sudden and rapid movement of a large volume of rock
• Earthquakes


  • The violent shaking and destruction caused by earthquakes are the result of rupture and slippage along fractures in Earth's crust called faults
  • Larger quakes result from the rupture of larger fault segments
• Focus


  The origin of an earthquake, occurring at depths between 5 and 700 kilometers
• Epicenter


  The point at the surface directly above the focus
• Seismic waves


  A form of elastic energy that causes vibrations in the material that transmits them
• Thousands of earthquakes occur around the world every day, but most are so small that they can only be detected by sensitive instruments
• Only about 75 strong quakes are recorded each year, and many of these occur in remote regions
• When a quake occurs in a populated area, power and gas lines are often ruptured, causing numerous fires
• The energy released by atomic explosions or by the movement of magma in Earth's crust can generate earthquakelike waves, but these events are generally quite weak
• Elastic rebound


  The "springing back" of deformed (strained) rock to its original, stress-free shape, generating the vibrations of an earthquake
• Aftershocks are numerous smaller tremors that follow a strong earthquake and gradually diminish in frequency and intensity over a period of several months
• Foreshocks are small earthquakes that often precede a major earthquake by days or in some cases years
• Most of the displacement that occurs along faults can be explained by the plate tectonics theory, which states that large slabs of Earth's lithosphere are in continual slow motion
• Faults associated with plate boundaries are the source of most large earthquakes
• San Andreas Fault


  • Undoubtedly the most studied fault system in the world
  • Consists of numerous branches and smaller fractures that display kinks and offsets
  • Some sections exhibit slow, gradual displacement known as fault creep
  • Other segments slip at regular intervals, producing small to moderate earthquakes
  • Still other segments remain locked and store energy for a few hundred years before rupturing in great earthquakes
• Earthquakes that occur along locked segments of the San Andreas Fault tend to be repetitive, as the continuous motion of the plates begins building strain anew as soon as one is over
• Seismology


  The study of earthquake waves
• Seismographs


  • Instruments that record earthquake waves
  • Have a weight freely suspended from a support that is securely attached to bedrock
  • Designed to amplify ground motion to detect very weak earthquakes or great earthquakes that have occurred in another part of the world
• Seismograms


  The records obtained from seismographs, which provide useful information about the nature of seismic waves
• The first instrument to detect earthquakes was developed in about 132 AD in China by Chang Heng
• Chang Heng's instrument is thought to have detected unfelt earthquakes and estimated the direction to the epicenters
• Seismograph


  An instrument that detects and records earthquake vibrations
• Seismograph


  • It is securely attached to bedrock
  • The inertia of the weight keeps it relatively stationary while Earth and the support move
• Seismogram


  The record obtained from a seismograph
• Seismograms reveal that two main groups of seismic waves are generated by the slippage of a rock mass
• Surface waves


  Seismic waves whose motion is restricted to near Earth's surface
• Body waves


  Seismic waves that travel through Earth's interior
• P waves


  Body waves that momentarily push (squeeze) and pull (stretch) rocks in the direction the wave is traveling
• S waves


  Body waves that shake the particles at right angles to their direction of travel
• P waves can travel through solids, liquids, and gases, but S waves can only travel through solids
• Surface waves


  Cause the ground and anything resting upon it to move, similar to ocean swells tossing a ship