Earthquake Hazards

• An earthquake is caused by the sudden and rapid movement of large volumes of rock along fractures on the surface of the earth called faults
• Ground shaking during an earthquake is the disruptive up and down sideways motion experienced, which is the primary cause of earthquake damage to man-made structures
• There are two types of seismic waves:

  • Body waves: travel through the interior of the earth
  • Surface waves: trapped near the surface
• Body waves include:

  • Compression waves (P-waves): the first waves that reach the surface, moving the ground back and forth
  • Shear waves (S-waves): vibrate perpendicular to their propagation direction, producing an up and down motion
• Surface waves include:

  • Love waves: shake the ground in a rotational manner with no transverse motion
  • Rayleigh waves: have a horizontal motion perpendicular to the direction they are traveling
• Ground rupture is the visible breaking and displacement of the ground due to the movement of the fault, which can be vertical, lateral, or a combination of both
• Strike-slip faults are vertical or near-vertical faults that displace rock horizontally:

  • Sinistral (left-lateral) fault: block moves to the left
  • Dextral (right-lateral) fault: block moves to the right
• Dip-slip faults cause vertical displacement of the ground:

  • Normal faults: characterized by downward movement of the hanging wall
  • Reverse or thrust faults: move the hanging wall up
• Liquefaction occurs when the ground loses stiffness and behaves like liquid in response to earthquakes, causing sediments to compress and fluids to be squeezed out
• Landslides are the downslope movement of rocks or sediments under the influence of gravity, which can be induced by earthquakes along with extended periods of rainfall, lack of vegetation, and oversteepened slopes
• Tsunamis are series of waves generated by large-scale displacements of water triggered by earthquakes, which can be local or regional and cause damage depending on flooding, wave impact on structures, and erosion
• Signs of an approaching tsunami include prolonged severe ground shaking, receding sea level, loud roaring sounds, and a huge incoming wall of water. If in such a situation, evacuate to higher ground immediately and stay clear of low-lying or coastal areas even after the first wave has struck