Earthquakes and Seismic Waves — Quick Reference

Seismic Waves: Overview

• Seismic waves are waves of energy that travel through the Earth, generated by earthquakes, explosions, or other energetic processes. $ext{Seismic waves} \rightarrow \text{energy propagation through Earth}$
• Detected by seismometers and recorded as seismograms.
• Two main categories: body waves (travel through the interior) and surface waves (travel along the surface).

Types of Seismic Waves

• Body waves: travel through the Earth's interior.
• Surface waves: travel along or near the Earth\'s surface.
• Primary focus: distinguish P-waves, S-waves, Love waves, and Rayleigh waves.

Body Waves

• P-waves (primary):
  • Fastest seismic waves.
  • Compressional (longitudinal): motion in the same direction as the wave travels.
  • Pass through solids, liquids, and gases; are refracted/bent when crossing internal boundaries.
• S-waves (secondary):
  • Slower than P-waves; transverse (shear): motion perpendicular to the direction of travel.
  • Do not travel through liquids (e.g., outer core).
  • Velocity about $v<em>S \approx 0.60\text{ to }0.70\, v</em>P$ (i.e., roughly 60–70% slower than P-waves).

Surface Waves

• Travel near the Earth\'s surface; typically slower than body waves.
• Often cause the most damage in earthquakes.
• Two main types: Love waves and Rayleigh waves.

Shadow Zones and Earth\'s Interior

• P-wave shadow zone: absent or highly attenuated between about $103^\circ$ and $142^\circ$ from the epicenter due to refraction at the core-mantle boundary.
• S-wave shadow zone: absent beyond roughly $103^\circ$ because S-waves cannot travel through the liquid outer core.
• These observations helped reveal Earth\'s layered structure (crust, mantle, core).

Specific Surface-Wave Types

• Love waves:
  • Fastest of the surface waves.
  • Horizontal shear motion; no vertical motion.
  • Move side-to-side.
• Rayleigh waves:
  • Elliptical or circular ground motion (rolling, up-and-down plus forward-backward).
  • Can cause significant devastation, especially to large structures.

Seismographs and Seismograms

• Seismometer detects ground motion; seismogram records the signal.
• Instruments include vertical and horizontal seismographs with components such as a mass-spring frame fixed to Earth and inertial weights.
• Recording helps determine wave types, arrival times, and distances.

Earthquake Wave Sequence and Damage

• Initial jolt typically from the P-wave arrival.
• S-waves arrive next and often cause stronger ground shaking.
• Surface waves arrive later but can cause the greatest structural damage, especially to large buildings.

Practical Epistemology: How We Learn Earth\'s Layers

• Global patterns of seismic wave propagation from thousands of earthquakes reveal interior structure (crust, mantle, outer core, inner core).
• Surface and body wave behaviors constrain material properties and boundaries inside Earth.

Epicenter vs Hypocenter (Focus)

• Hypocenter (focus): the point within the Earth where an earthquake originates.
• Epicenter: the point on the Earth\'s surface directly above the hypocenter.

Earthquake Hazards: Faults and Active Regions

• Fault lines and active faults are locations where earthquakes originate.
• Movement along faults releases seismic energy as waves that propagate through the Earth.
• Understanding fault distributions informs hazard assessment and preparedness.

Disaster Preparedness (Summary)

• Before an earthquake:
  • Identify hazards, secure heavy furniture, anchor items to walls.
  • Check structural integrity of buildings; retrofit if necessary.
  • Prepare an emergency kit (water, food, first aid, flashlight, radio).
• During an earthquake:
  • Inside a sturdy building: DUCK, COVER, HOLD; stay away from glass and heavy objects; do not use elevators.
  • If outside: move to an open area away from trees, poles, and buildings.
  • If in a moving vehicle: stop safely and avoid bridges or overpasses.
• After an earthquake:
  • Expect aftershocks; evacuate safely if needed.
  • Check for injuries, hazards (gas leaks, electrical, spills).
  • Stay informed via battery-powered radio and official guidance.

Quick Practice Points

• Distinguish wave types by motion and media: P (compressional, through solids/liquids), S (shear, through solids only), Love (horizontal surface), Rayleigh (elliptical surface).
• Shadow zones reveal Earth\'s internal layering:
  • P-waves: shadow between $103^\circ$ and $142^\circ$.
  • S-waves: shadow beyond $103^\circ$.
• Surface waves generally cause the most damage to structures; body waves arrive first.
• Seismograms help locate earthquakes and characterize wave speeds and pathways.