8.1 Earthquakes and Seismic Waves

Earthquakes shake up our world, literally! These powerful events release pent-up energy in Earth's crust, causing seismic waves to ripple through the ground. Understanding how they work is key to staying safe and prepared.

From the deep rumble of P-waves to the destructive force of surface waves, earthquakes pack a punch. We'll explore their causes, how they're measured, and their impacts on our cities and lives. Get ready to dive into the shaky science of quakes!

Causes of Earthquakes

Elastic Strain Energy and Fault Rupture

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• Earthquakes caused by sudden release of stored elastic strain energy in Earth's lithosphere resulting in seismic waves
• Elastic strain energy builds up from stress of tectonic forces acting on rock
• Released when stress exceeds rock's strength causing rock to break or slip along a fault
• Elastic rebound theory: crust suddenly snaps back to original unstressed shape when strain exceeds rock strength
  • Releases energy in form of seismic waves causing an earthquake

Earthquake Focus and Epicenter

• Focus (hypocenter): point within earth where seismic waves originate during an earthquake
• Epicenter: point on Earth's surface directly above focus
• Foci usually concentrated in crust and upper mantle particularly along plate boundaries
• Majority of foci are shallow originating within a few tens of kilometers of surface
  • Deep focus earthquakes can occur up to depths of about 700 km but are more rare

Seismic Wave Types

Body Waves

• Can travel through earth's inner layers
• Two types: P-waves and S-waves
  • P-waves (primary waves): fastest seismic waves and first to arrive
    • Travel through solids, liquids, and gases and can pass through earth's core
    • Push and pull ground in direction wave is traveling
  • S-waves (secondary waves): second waves to arrive
    • Only travel through solids and cannot pass through earth's core
    • Move rock particles up and down or side-to-side perpendicular to direction wave is traveling

Surface Waves

• Can only move along surface of planet like ripples on water
• Two types: Love waves and Rayleigh waves
  • Love waves: fastest surface waves
    • Cause horizontal shearing of ground side-to-side perpendicular to direction of wave propagation
  • Rayleigh waves: slower than Love waves but tend to be larger and most destructive
    • Cause rock particles to move in elliptical motion with horizontal and vertical ground motion

Analyzing Seismograms

Seismology and Seismographs

• Seismology: study of earthquakes and seismic waves
• Seismologists use seismographs to record seismic waves and produce seismograms
• Seismograms: zig-zag trace recordings of ground motion detected during an earthquake
  • Seismographs record motion of ground as seismic waves pass a certain point

Determining Earthquake Location and Magnitude

• Time of arrival of different seismic waves (P-wave and S-wave) used to determine distance of seismograph from epicenter
  • Difference in arrival times between P and S waves used to determine distance
  • Longer time between P and S wave arrivals indicates earthquake occurred farther away
• Amplitude of seismic waves indicates amount of ground motion and energy released
  • Larger amplitudes generally indicate more powerful earthquake
• Seismologists calculate magnitude using amplitude information
  • Richter scale and moment magnitude scale used to quantify and compare seismic energy released by earthquakes

Earthquake Impacts

Damage to Buildings and Infrastructure

• Ground shaking: primary cause of earthquake damage to buildings and infrastructure
  • Intensity depends on magnitude, distance from epicenter, and local geology
• Liquefaction: strong shaking causes water-saturated sediments to temporarily lose strength and act as fluid
  • Can cause buildings to collapse and pipelines to rupture
• Poorly constructed buildings, particularly those not built to seismic safety codes, most vulnerable to damage and collapse
  • Unreinforced masonry buildings particularly at risk

Secondary Hazards and Consequences

• Landslides and avalanches: common secondary hazards triggered by ground shaking particularly in mountainous areas
• Tsunamis: large seismic sea waves triggered by earthquakes occurring under ocean
  • Can cause extensive damage and loss of life in coastal areas
• Fires: common result as gas lines may be damaged and electrical shorts can spark fires in damaged buildings
• Large earthquakes can cause substantial damage and loss of life particularly in populated areas with vulnerable infrastructure

Hazard Mitigation Strategies

• Adopting and enforcing seismic building codes
• Retrofitting older buildings to improve seismic resistance
• Preparing emergency response and recovery plans
• Educating public about earthquake safety and preparedness measures (securing heavy objects, identifying safe spots)